The Journal of the United Service Institution 1872: Vol 15 Iss 66
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+ Ss ~ a > 8 N N S < s indicate the ntal sott wr Une by inductor trom the NOYLLONLAL FRAGA force CSSIVE AXTINUANS AF the | | ENDWAYS pole of 12 inch bar magnet 15" needle » Black continua = E o z IRY CYLYNDERS } | ' ' / lal deviatior fan tron Shap m end. of needles,or ur same position as adopted. tar correcting the Quadri ) hemasphervcal ercds 12 ches, diameter 3 Inches rd. Compass card. 7.6 inches - Black Curve, Single Bar Needle ¥ 15 trches long RESPECTING THE MAGNETISM OF IRON SHIPS. 91 Applying these principles to the observed facts, it is found, without exception in every iron ship, at the ordinary place of the compass, that the quadrantal deviation is positive, or follows the sign-+ — + — in the successive quadrants NH., SE., SW., NW., from whence we may infer that it is caused by the co-existence of the arrangements of masses of soft iron represented by + @ and — e, as shown in Fig. 4, or the excess of — e over — a, as shown in Fig. 5, The investigations of the observed values of D and X give this result, that the positive value of D arises from the transverse iron con- tinued from side to side, such as beams, decks, bulkheads, and the ship’s bottom, producing a large — e, and overcoming the force of the fore and aft iron, or a; the latter is sometimes, but rarely +, but more generally —, and when — always less than e (see tabulated co-efficients); and hence that it is caused by the excess of the thrust on the north end of the compass needle from the north side of the ship, over the thrust from the north exd of the ship. In this explanation of the causes of the quadrantal deviation, I have somewhat gone over the same ground as in my paper of 1865, but the facts given in that paper on the subject appear by more recent pro- posals and writings in reference to the magnetism of iron ships, to be entirely overlooked ; a serious omission, which I now endeavour to rectify by a reiteration of the principles. At the standard compass of ordinary iron ships the quadrantal de- viation is usually from 43° to 5°, in armour-plated iron ships 6° to 7°. On the main deck of the latter class it rises to 8° and 10°, while in the batteries and between decks of the turret ships it reaches the enormous amount of 15° and 2U°. The diminution of the directive force, and the amount of the quad- rantal deviation, are nearly the same at the same level in different parts of the ship. They increase in descending from the position of the standsrd compass to the steering and main deck compasses, par- ticularly in ships having a thick iron deck. The diminution of the directive force, and the quadrantal deviation, both diminish with the lapse of time, there are also grounds for believing that X is affected by temperature, diminishing by increased temperature, but this considera- tion does not affect navigation. I 1 I R specting the Fee ling Error. I now arrive at the question of the “‘heeling error” in iron ships, or that amount of deviation which is superposed on the ordinary deviation of the compass when the ship heels over to port or to starboard from an upright position, and which, for theoretical and practical purposes is in its value indicated for every degree of heel of the ship. Theoretically considered, the heeling error is a complicated though not an obscure problem; fortunately practically considered, it does not baffle the careful navigator, nor is it without amelioration by partial correction through the aid of magnets, and especially by a proper 92 ON THE PRESENT STATE OF OUR KNOWLEDGE position being selected for the compass. In all cases we know that iis maximum amount occurs when the ship’s head is north or south by the disturbed compass, and that it vanishes when the ship’s head is east or west by that compass. So forewarned, the navigator knows when he can ascertain the extreme amount of error from actual observation at sea, and when to use precaution. The worst possible condition for producing heeling error is when an iron ship is built head north, and the compass placed near the stern,* as that part is strongly magnetised, and all the additional forces de- veloped by the ship’s heeling conspire to produce the error. These forces are— 1. Vertical induction in transverse iron, such as the beams, &c. 2. The vertical force from the permanent magnetism of the ship. 3. The vertical induction in all vertical iron situated under the compass, such as the sternpost, stanchions, &c., and each force pulls the north end of the needle downwards and to windward.t Conversely, the best possible condition for the reduction of the hee!- ing error is in the ship built head south (or from SW. to SE.), when the compass is placed in the after part of the ship; as then force 2, from the reversed polarity of the stern of the ship, operates against forces 1 and 3, and may exceed them in intensity ;-—if so, a small heeling error to leeward would result. Hence the difference in the heeling errors of individual ships: it is, applying a term I did when speaking of the semi- circular deviation, a constitutional error, and one ship affords no guide to another unless all the conditions of build are the same. The numerical values of the heeling error in the ships of the Royal Navy are about as follows:—At the standard compass of the armour- plated ships, when in England, the worst type does not exceed 1}° for each degree of heel, and it is always to windward; 1° and under would, perhaps, be the average: in all these ships about #° is found to arise from force 1, or that from vertical induction in transverse iron, the remainder from forces 2 and 3. On the main decks of these ships there is a strong upward force producing a heeling error to leeward. In ordinary iron ships in England the heeling error from transverse iron averages about 3° to windward, an error it must be remembered which changes with the changes of magnetic dip, and would, therefore, in the southern hemisphere be an error to leeward. The forces 2 and 3 are mixed together in such an uncertain way, depending on the build of the ship and arrangements of iron, that their proportions cannot be estimated. Fortunately, by a simple class of observations with a dipping needle, * See paper on the loss of the iron ship “@lenorchy.” “Royal Society’s Pro- ceedings,” 1869. + See Fig. 3 in illustration, when (1) the weather or upper end of the beams and all transverse iron acquire south polarity (s) by induction from the earth (in northern magnetic latitudes) ; and similarly from induction in vertical iron (3) the south poles are brought nearer the north end of the needle as the ship heels over. SCI ay al RESPECTING THE MAGNETISM OF IRON SHIPS. and with the knowledge of the co-efficients D and \, we can approxi- mate very closely to the error that would be found if the ship were actually heeled, and thus a formidable amount of labour is avoided. The details are fully described in the manuals* on the subject. When it is considered necessary to correct the heeling error, the practice in the Royal Navy is to employ a magnet a few inches long, which is inserted vertically in the supporting pillar of the standard compass, exactly beneath the centre of the compass needle, a hole being accurately bored for the purpose. The simple class of observations for finding the heeling error, with- out heeling the ship; together with those when a horizontal needle is employed to determine the co-eflicients B and C, and thus to form a table of deviations, without swinging the ship, are, from my point of view, among the real triumphs in this branch of science. The time and labour that by these means are saved in acquiring a competent know- ledge of the ship’s magnetic characteristics are important considerations, with the advantages that a secure and desirable position for the com- pass may be selected while the ship is even on the stocks, and certainly objectionable positions be avoided. The degree of approximation to correct results in these methods just brought to your notice, depends on the approach to accuracy with which the values of D and X are assumed, if they are not known. From the numerous records we have now of so many types of ships, it is not difficult to estimate a value for any individual ship, even when new. As a consequence, nearly all the preliminary work of our ships in the Royal Navy is thus effected, nothing is left to chance even when the architect will scarcely § give space and place for selection. With the coefficients determined, the correcting magnets are prepares land placed in accordance, and the ship is ready “for the thorough “swinging” to form a table of residual errors—errors which are ‘generally small in amount and not embarrassing to the navigator. It is well to record here that the results obtained by these several methods, approximate closely to those determined by the complete pro- cesses of swinging and heeling, and are confidently adopted by the Admiralty Compass Department. Notes on the Mechanical Correction of Compass Deviation. After the investigation of the several characteristics of an iron ship’s magnetism, the most important consideration is by what means can we correct or reduce the errors so produced on the action of the mariners’ compass ? The now universally adopted method of mechanically correcting the cleviations of the compass, was proposed by the Astronomer Royal more than 30 years ago, after a series of remarkable experiments and inves- tigations on two of the earliest built iron merchant vessels; and con- sists, as is well known, in correcting the semicircular deviation produced * Sce “ Admiralty Compass Manual,” 3rd edition, pp. 82—89 ; and “ Elementary Manual,” companion to the foregoing, pp. 89—104. { { i | | 94 ON THE PRESENT STATE OF OUR KNOWLEDGE by the ship’s polar magnetic force, by the application of permanent magnet-bars; and in correcting the quadrantal deviation arising from the induction in the soft iron of the ship by the earth’s horizontal magnetism, by the application of soft iron in masses of diverse shapes. In past years, as you will have gathered from my opening remarks on the labours of Scoresby, much diversity of opinion prevailed as to the necessity for, and the freedom from danger in the employment of, permanent magnets ; the limits of these diverse opinions are now, from circumstances which I will hereafter explain, very much narrowed. Still among, it is to be hoped, a limited few, erroneous opinions exist on this subject, errors too of a nature that can be disproved by simple experiment.” The general question of the mechanical correction of the deviation of the compass has, on other grounds, materially changed its aspect of late years ; the cause arises from the vast increase in the employment of iron in the interior as well as the exterior fabric of the iron ship. Ships of the mercantile marine, no less than ships-of-war, have been affected by these changes in naval architecture. Before this period of change the deviation of a standard compass, properly placed in an iron ship, seldom exceeded 20° at the maximum, and the directive: force which acted on it as the ship was swung, was comprised within two- thirds and four-thirds: of the .mean force. In the present day it is generally impossible to find a position for this compass at which the deviation and variations of directive force do not greatly exceed these limits. Hence this change in the conditions experie meed has produced a corresponding change in the practice of the Royal Navy. Magnet correction is absolutely necessary in nearly every ship-of-war afloat ; and broadly I may here state that the results of the system adopted, a system suited to the magnetic knowledge of the day, are satisfactory both as to efficacy and security. The process recomme nded by the Astronomer Royal for the mer- cantile marine and generally adopted, was, as is well known, a tentative one, or an operation simply ‘ ‘to the making the compasses act cor- rectly :” it took no account of the amount or distribution of the mag- netic forces at the pos ition of the compass, due to the hull of the ship, or the disturbing action of individual masses of iron masking the mag- netic force of the hull of the ship. But a second method was described by Mr. Airy, in his well-known paper on the magnetism of iron-built ships (Phil. Trans. 1859, p. 196), which did require the knowledge of these important details, and it is this method which is generally employ ed in the Royal Navy. It may be thus explained. * Tt has been stated, for e xample, within the last five years in this theatre, by once naval authority, that the employment of correcting magnets decreases the directive force of the nee dle, and that ‘‘common sense dictated at once the sweeping them away ;” by another, that they lessen the tendency of the seaman’s “guide, philo- sopher, and friend” (an eupkuism for the compass needle) to point truly. These misconceptions will doubtless pass away with the spread of sound knowledge, for in fact, magnets cannot affect the mean directive force, while soft iron correctors in- crease it. eR RESPECTING THE MAGNETISM OF IRON SHIPS. 95 The co-efficients B, C, \ being found by observation, or when necessary, B and C being found by observation and 2d estimated ; we have ./b*+C? the tangent of the semicircular deviation when the ‘ polar force acts to the east or west of the compass, and B the tangent 2 of the starboard angele. If we desire to correct the semicircular devia- tion completely, a magnet of suitable size, adequate power, and proved permanence, is selected. The distance above or below the card at which this magnet, when placed cast and west, will produce on shere a devia- tion of which the tangent is \ / B? + C? is ascertained by trial. The magnet is then inserted into the pedestal of the standard compass at the ascertained distance immediately below the centre of the card, and in the direction of the starboard angle, the pole being so placed as to counteract the polar magnetism of the ship. In newly built ships, the polar force, as before mentioned, is generally undergoing a process of gradual diminution, it is therefore generally considered best not to correct entir ly the semicircular deviation, but to under correct it. In general, no attempt is made to correct the quandrantal deviation, that deviation and the residual semicircular deviation being the subject of the tabular correction ;—a correction it may be observed now thoroughly grafted in the practice of the Royal Navy, and which practice is certainly productive of security, as its foundation implies that the compass needle must never be considered to point correctly, whatever mechanical correction may be employed. The question of quadrantal correction by soft iron is a difficult one ; it is true that by its neglect a small amount of directive force is lost, and a further small correction of heeling error; the danger, however, that may arise from the large masses of iron necessary for the correc- tion of modern ships taking up polar or permanent magnetism, must be placed as a set off, as also the further consideration that after the correction is effected, all record is lost of the ship’s magnetic state. When the quadrantal deviation is small, as in the ordinary iron ship, it may be, I consider, safely allowed as a residual error ; and the omis- sion in practice has this advantage, that it acts as a valuable check on the correctness of the deviation tables constructed by the navigator from the circumstance of the coefficient D remaining constant, or nearly so, under all changes of time and place. With the large quadrantal deviations at the between-deck compasses of our armour-plated ships, two compasses brought together sufficiently close to produce a negative quadrantal deviation equal in amount to the positive value of the ship, and then mutually corrected by one system of magnets for semicircular deviation, have proved useful, but the loss of directive power at the place is not remedied. This is a subject now engaging the attention of the Compass Department. It may be here naturally asked why the practice of compass-correc- tion, which is now considered efficacious and secure, should in the earlier period of the iron ship’s history, have been deemed untrust- worthy and indeed dangerous? The reply is not difficult ; the magnetic 96 ON THE PRESENT STATE OF OUR KNOWLEDGE conditions of the few iron ships of the Royal Navy at that period did not as a matter of necessity, require magnet correction—their compass deviations rarely, if ever, exceeded 20°~-tabular corrections were readily obtained, and gave ample security. The practice of compass correction was thus confined to the mercantile marine; the process, as recommended by the Astronomer Royal and generally adopted, being purely a tentative one, was easily carried out in practice and involved little or no knowledge of principles: it soon became a branch of ordi- nary trade business in connection with compass manufacture, and the professional ‘* compass adjuster’ has been long established at all our chief mercantile ports.* Unfortunately, too, the practice of furnishing the seamen with a tab‘e of residual errors was not at the same time adopted or its necessity r-cognized, but on the faith of the professional adjuster, the compass was considered correct. It was further a prevailing custom at this time in the mercantile marine to use the steering or binnacle compass alone for the navigation of the ship. This compass was usually not more than two feet above the deck beams, and frequently placed in the after part of the vessel near such vertical masses as the stern-post and rudder. Large and irregular deviations were sure to obtain at this position, the large errors requiring to be corrected by large and powerful magnets; the result was that the slightest change in the magnetism of the ship produced a large compass error, and not unfrequently in foreign-going ships proceeding to the southern hemisphere, on reaching the latitude of the Cape of Good Hope, the compass was entirely useless: this arose from the induced magnetism of the rudder and other adjoining masses then acting in unison with those correcting magnets, which originally at the port of departure had been purposely placed to act in direct antagonism. This indiscriminate use or rather abuse of the method of correction by magnets, was certainly not intended or contemplated by the Astro- nomer Royal when he freely gave seamen the issue of his valuable labours; but the prejudices arising from the numerous recorded failures, the result of an ignorant application of a sound scientific principle, operated for many years against a proper spirit of enquiry into the causes of failure. In 1855, however, the Liverpool Compass Committee,t under the auspices of the Board of Trade and with a large number of iron ships of the mercantile marine of that port placed at their disposal, com- * Tam here nearly quoting the words of Mr. Rundell, many years Secretary to the Liverpool Compass Committee, and perhaps the best authority on this point in the country. In a paper read before the Institution of Naval Architects in 1866, “On Compass Equipment in Iron Ships, and Government Supervision,” he remarks :— “ Experience shows that compass adjusters rarely know more than the mechanical “ practice of adjustment, as explained by the Astronomer Royal more than twenty- “ six years since—a practice so simple that any person of ordinary ability could, ‘* under good instructions, acquire it in a few hours.” + The formation of this Committee originated from the discussions that took place at the meeting of the British Association for the Advancement of Science, held at Liverpool, in 1854, on a paper by Dr. Scoresby, ‘‘On the Loss of the sTayleur,’ aad the Changes in the Compasses in Iron Ships.’ ’ i RESPECTING THE MAGNETISM OF IRON SHIPS. 97 menced a vigorous inquiry into the general question of their magnetism, and the correction of their compasses ; in their Third Report, embracing the period 1857—1860, and dated "February , 1861,* they state with reference to the latter— ‘“‘ The compass errors occasioned by the more permanent part of a “ ship’s magnetism may be successfully compensated, and that this “‘ compensation equalises the directive power of the compass needle ‘“‘ on the several courses on which a ship may be placed.” It will of course be understood that this general statement, is subject to the qualification, that magnet correction in a newly-built iron ship must not be strictly depended on, that we ought not to disregard the pagan as nite ) magnitude of the original deviation, as also that when the correction is @ applied to compasses having large deviations and placed near large a vertical masses of iron, as a rudder, there must always be great uncer- tainty as to the correction on a change of magnetic latitude. About the same time that the Liverpool Compass Committee were engaged on their Third Report, I was engaged in the reduction and dis- cussion of the deviations observed on board all the iron-built ships of : : the Royal Navy, numbering among them about fifteen vessels of ordi- . A nary types that had been “employed on distant stations over several , Bs 5 ‘ years. t 4 In a report I made to the Admiralty on this discussion, the following Pe passage will be found: ‘“‘ On examination of the Tables, it is at once 5 : ‘* observed that in the majority of examples therein given, a permanency f ‘‘ of magnetism exists so little affected by changes of geographical posi- 1 & “tion as materially to confirm the views entertained by the Astrono- on § “mer Royal in his earliest discussion (1838), that the effect of f “ transient induced magnetism in iron-built ships is very small com- “ paratively.” + n These results were opposed to the then generally received opinion, . that the magnetism of an iron ship arose chiefly from transient induc- e tion ; this opinion being chiefly formed on the reports of the action of d compasses frequently improperly placed and corrected by those unac- c : quainted with the principles: they were nevertheless conclusive as to y the security of magnet correction for a compass placed with due care, were in unison with the experiences of the Liverpool Compass Com- ‘ mittee, and gave confidence when the changes in naval architecture "4 necessitated its employment. 4 The introduction of magnet correction for the Royal Navy, you will have now gathered was not “made until the necessity arose, and a thorough tollUdie investigation of the causes of past failures and past successes in its appli- ee cation had cleared aw ay the doubts that prevailed: and I may here ven- a ture the opinion that had magnet correction been as indiscriminately a applied in the Royal Navy as it had been in the mercantile marine prior - to these investigations, we should not have had those ample records of d * In my former papers I have borne testimony to the valuable Reports of the ok Committee ; indeed, without a careful study of these Reperts, no one could presume °C, to pronounce authoritatively on the subjects therein reviewed. + Printed in the “ Philosophical Transactions of the Royal Society,” 1860, page 346. VOL. XVI. 98 ON THE PRESENT STATE OF OUR KNOWLEDGE the real magnetic history of individual ships, the discussion of which has so greatly conduced to clearing away the obscurity which hung over the subject. ‘ : , er ee Observations on the Treatment of an Iron Siip’s Compasses. In examining the numerous patent specifications relating to the mariner’s compass, and which extend back to the year 1776, one is struck with the few real advantages that have been gained by the labours of inventors in this field. The earlier patents relate chiefly to arrangements and contrivances for making observations at sea of the variation, and for modifying the motion of the ship. In 1815 the liquid compass, a real gain to the sea- man, appears to have been first introduced.* In 1818 we have the first patent of an invention “to guard” or “protect” the compass needle from all action arising from iron in its neighbourhood. From about this date the patents increase in number, and of late years, notably since 1850, have somewhat rapidly multiplied. The designs of the inventors may be thus generally classed :— a. Mechanical arrangements applied to compass bow] or its binnacle to prevent vibration of the end, or friction on the pivot. b. An arrangement of metals, or some presumed non-conducting substance, surrounding the compass, to prevent or cut off local attrac- tion. c. Self acting apparatus for registering the ship’s course by compass. d. Arrangements of auxiliary needles to detect or register the devia- tion. e. Arrangements of magnets outside the compass to isolate the needle or prevent action of neighbouring iron. f. Arrangements of magnets on the compass card to cause it to point at all times to the true north. g. Special arrangements of cards for visibility at elevated positions, and for adjustments to meet the deviation caused by the ship’s iron. h. Methods for depolarizing or demagnetizing the iron ship’s hall, in order to prevent the deviation of the compass. i. Methods for polarizing such parts of the iron ship as may be deemed requisite to correct or prevent the deviation of the compass. With the exception of a limited number of the more simple arrange- ments necessary to the mariner’s compass, scarcely one of these numerous inventions are to be seen employed at the present time. The mechanical contrivances for registering the ship’s course, for example, are generally remarkable for ingenuity, but fail from this cause for every day use; they give, asa rule, work for the needle to do which detracts from its directive power and consequent accuracy, at a time when these con- ditions are most required. * This description of compass has been improved in later. years by many well known makers, and is a necessary part of every ship’s equipment. * meat iwars coioasebtedlle. oy pita Di 5s Sh i is Suat as Sie atari ER ES iad Seis aelareane Seis. Bini Sea Bia Dogon RESPECTING THE MAGNETISM OF IRON SHIPS. 99 A number of those patents relating to the “‘ prevention or cutting off “the local attraction” have evidently emanated from minds unac- quainted with the rudimentary principles of magnetism; the projectors especially overlooking the fundamental law, that the interposition of a body between the magnet and the needle on which it acts, can as little intercept the action of the magnet as the interposition of a body between the earth and another body would intercept the action of the gravitation of the earth, and also further overlooking a very obvious consideration, that if any body could intercept the action of the ship’s iron it would also intercept the action of the earth’s magnetic force on the needle. In other cases, where intelligence has been displayed, the inventor has overlooked, or tried to evade—and believed he had done so—some simple but obvious law, the neglect of which produced failure. Passing from patented inventions to the adaptations suggested by progress in scientific research, we find embodied in the Admiralty Standard-Compass many sound and elegant improvements over the compasses of olden times. In this instrument, which resulted from the labours of a Committee of well known names in magnetic science, working at the request of the Admiralty between the years 1837 and 1842 ; we find the application of pure copper of substantial thickness for the bowl, as being the metal next to silver known as the best con- ductor of electricity for quieting the vibrations of the magnetic needle —an application mainly due to the late Snow Harris. We further find the needles constructed of such a kind of steel, and on such a system as will afford the greatest directive power with little weight, a system the product of the researches of Kater, Christie, and Scoresby. Again, the pivots and caps—an all-important detail of the compass—-are constructed of such materials and in such forms as had stood the best test in an extended series of experiments made by the late Captain Ki. Johnson, my predecessor in office, and a member of the Admiralty Compass committee; and, lastly, a remarkable arrangement, whether considered mechanically or magnetically, is to be found in the dispo- sition of the needles on the compass card. This arrangement and its bearings I will briefly describe. By the substitution of two or more parallel bars or needles of the compound structure adopted in the Admiralty Standard-Compass, instead of the single bar needle which was formerly in universal use, a much greater directive power was found to be obtained with the same weight. A problem then to be solved was the relative distances of the parallel bars on the card, in order that the “moments of inertia” about all hori- zontal axes should be equal, for if this condition wes not secured, the oscillations of the card would be accompanied with a “ wabbling”’ motion detrimental to good work. The investigations of Archibald Smith resulted in this law, that two equal parallel bars of which the ends are 60°, or four bars of which the ends are 30° apart—i.e., 15° and 45° on each side of the diameter— secured the required conditions, and this arrangement was adopted, and for many years retained, on account of this property. H 2 100 ON THE PRESENT STATE OF OUR KNOWLEDGE But a remarkable, and at the time unlooked for property attended this arrangement of the needles on a compass card. In later years, when the employment of very long needles was getting into fashion, it had been observed that compasses in merchant ships, corrected by mag- nets and soft iron on the plan of the Astronomer Royal, were affected by anomalous errors; in one case, for example, which came under my own observation, these errors amounted to 5° and 6° on some points of the ship’s head. To test whether these errors were caused by the length of the needle and the proximity of the correctors, a series of experi- ments on the deviation produced on single-bar needles of different lengths by magnets and soft iron were made at the Admiralty Compass observatory. The following results were obtained :—* With 3-inch single needles deflected by magnets the deviations were nearly semicircular,t but with 6-inch needles, and much more strongly with 12-inch needles, a “sextantal” error of considerable magnitude was introduced. With soft iron correctors, deflecting a long single needle, in addi- tion to the “quadrantal”’ deviation, a considerable “ octantal”’ error was introduced. When the same experiments were made with an Admiralty card, con- structed as usual with four parallel needles, the extremities of which are 15° and 45° on each side of the extremities of the diameter to which they are parallel, there was no appreciable sextantal or octantal devia- tion. On Mr. Smith further investigating this subject on a mathe- matical basis, it appeared that this arrangement of needles, as well as the simpler arrangement of two needles each 30° on each side of the diameter, produces a complete compensation and correction of these errors. We see here the remarkable property adverted to, that the arrange- ment of needles which produces the equality in the moments of inertia, is by a happy coincidence, the same as that which prevents the sextantal deviation in the case of correcting magnets, and the octantal deviation in the case of soft iron correctors. The practical result is this, that by the employment of Admiralty standard compass cards, or of cards with two needles, each 30° from the central line, correcting magnets and soft iron correctors may be placed much nearer the compass than can be safely done with a single needle card, and that the large deviations existing in iron ships may be thus far more acurately corrected.f Before passing from the subject of compass-needles, I would wish to offer an opinion on the employment of those large compasses fitted * See Fig. 6, where the several curves are shown on Napier’s diagram. + i.e., followed a curve of sines. t To ‘those who may be interested in this branch of the subject, I would refer to the “ Philosophical Transactions ” for 1861, where will be found described the whole of the experiments, together with the mathematical investigations. I believe, as re- sulting from this communication to the Royal Society, that the employment of double needle cards, arranged as described, is now in nearly general use. Ces ee RA ee ARON MDa acaba ah Tete Gi oy IR TERRY RITE ee 101 RESPECTING THE MAGNETISM OF IRON SHIPS. with needles of extraordinary length, which would appear from modern practice to be considered suitable to ships of extraordinary size. | am aware that these large compasses commend themselves from the facility afforded of steering a course to degrees, instead of the old- fashioned quarter or half-quarter points; and for a certain steadiness due in one sense to the extreme slowness of oscillation in long needles as compared with short ones. There are notwithstanding fallacies and insecurity attending their use, which appear to me borne out by the facts that have come within my own knowledge of fine and well- equipped steam-ships, navigated apparently with precision and care— the course being ostentatiously steered to degrees—and still running ashore in the finest weather in some unlooked-for manner. Now, it is well known that small magnet bars are proportionally much more powerful than large magnet bars, and that beyond a certain limit, an increase in the length of the magnet bar is not accompanied by an increase of directive power in the same proportion; so that if the thickness of the bar be preserved, its weight and consequently, its friction when supported as a compass needle is on the pivot, increases in a greater ratio than the directive power, and a direct loss of force results. Common experience embodying this principle, has heretofore limited compass needles for use afloat to about 6 inches (the Admiralty Com- pass Committee adopted about 7} inches as the maximum length), and we may well consider that when these lengths are doubled the in- creased weight of the card and appendages produce an amount of friction that far exceeds the increase of directive force derived from the added length of magnet bar; the friction further is operated on at a disadvantage from the sluggishness of movement incidental to the long needle. As a matter of opinion, I consider that the Admiralty Standard Card is as large as should be used for the purposes of navigation, as dis- tinguished from a steering or auxiliary compass; and that as regards safety, in the long, steady, and fast ship of modern days, the choice is really between the Admiralty card and a smaller one. Briefly to review the question of the mariners’ compass. For em- ployment in an iron ship, the chief points to be aimed at are, great directive power and susceptibility of motion ; directive power in a high degree to meet the prejudicial effect caused by the earth’s horizontal magnetic force acting on the soft iron of the ship, and thus seriously diminishing the mean directive force of the needle, and susceptibility of motion to overcome any tendency of the hanging up of the needle from undue friction or wear in the pivot and cap. A compass with these attributes is invaluable to the fast ship with little motion in smooth water—conditions, be it remembered, where errors in reckoning have often proved most serious—but such a com- pass would probably, from its sensibility, fail when in a heavy seaway, or when subject to the violent oscillations of some of our recent ships of war propelled at high rates of speed. I certainly know of no compass that is equally accurate and effective under these diverse conditions ; 102 ON THE PRESENT STATE OF OUR KNOWLEDGE and that instrament which shall be at the same time accurate and sensitively alive in smooth water, perfect in its action when pitching or rolling to the gale of wind, capable of resisting the concussion from the firing of heavy guns, and not responding to the vibrations produced by an abnormal rate of propulsion by steam, has yet, despite the long list of patents, to be invented. I may, however, I think, venture to assert that, for a combination of all that is effective under the greatest number of conditions, no compass yet projected equals the Admiralty Standard Compass. Its patient designers, with one excep- tion,* have passed away from us, and their labours are but little known : it fell to my lot for some years in my official capacity, to be the conser- vator of their admirable werk when exposed to the adverse influences of rash inventors and economical arguments. I am, however, now gratified to think that lengthened experience has confirmed its merits, and that, intact as I received the Admiralty Standard Compass, so will it remain for our successors, unless, indeed, some master-mind appears to give it those attributes which eager, but, I think, unreflecting minds consider it ought to possess. I have now passed in review the chief points connected with my subject, which appeared to me to deserve your attention ; there is, how- ever, an incident that may be considered to claim attention, from the circumstance that it created much discussion in this Institution. I allude to the “de-polarizing” or ‘ de-magnetizing”’ process of the late Mr. Evan Hopkins, for rendering correcting magnets or deviation tables unnecessary, by destroying the polarity acquired while the ship is building; and which was applied to Her Majesty’s ship ‘“* Northum- berland.” As I have already, in a paper read before the Royal Society, and published in their Transactions,t given my opinions on this proposal and the numerical data on which those adverse opinions are based, [ should not, as an incident of progress in magnetic science applied to navigation, have alluded to it here; but, as the proposal has been recently revived with a vigour of announcement and assertion of practi- cal utility scarcely surpassed by the clever but sanguine original pro- jector, I will detain you a few moments by reading brief extracts from the paper to which allusion has been made bearing on the failure of this unsound proposal, as also the condemnatory opinions expressed thereon. [ state, as proved by observations made in ample detail, of the kind [I have brought to your notice, and very thoroughly discussed numerically and graphically, “that the process’ —(which, departing from the terms of the patent specifications resolved itself ultimately into rubbing the beams and iron stanchions near the compasses with powerful electro-magnets),—‘‘is not one of general de-magnetization, “but of partial counter-magnetization, and results in an irregular dis- “tribution of magnetism of very variable intensity, necessarily very * Gencral.Sir Edward Sabine, K.C.B., late President of the Royal Society, and Chairman of the present meeting. + For 1868, pages 487-508. RES ea RESPECTING THE MAGNETISM OF IRON SHIPS. 103 “ unstable, and producing, wherever effective, a rapidly varying field “ of force. “That so far from the process being in any sense of the word one of “ ¢ depolarization,’ either of the whole ship or any part of it, it was, on “the contrary, the ‘ polarization’ to a high degree of intensity of a “ particular portion of the iron in the neighbonrhood of three of the * compasses.”” That ‘the iron so magnetized was iron capable of re- “ ceiving only sub-permanent magnetism, and which, from its forming “ part of the structure of the vessel, was subject to strains and concus- “ sions from which detached magnets are wholly free. The magnetism “s90 communicated was, therefore, necessarily unstable and transient, “and, from its liability to change suddenly and unexpectedly, was a “ source of danger to the vessel. “ Nor would the effect produced, even if it had been much more “ permanent than it proved to be, be considered an advantage. In two “ out of the three compasses to which it was applied . . . there- “ quisite reduction might have been effected with infinitely greater ease “and certainty, as well as permanency, by the application of a single magnet to each compass.” Let us, however, in conclusion, pass from the consideration of matters unpractical and unsound to those based on secure foundations. On a review of what in this lecture I have brought under your notice I fear in a somewhat fragmentary and imperfect manner—the question may be asked, how far has this knowledge extended, and how far has it aided the navigator? The reply, so far as 1 am aware, is generally satisfactory. Foreign nations interested in maritime affairs have unhesitatingly adopted the principles and the practice of the Royal Navy; their handbooks are all based on our own Admiralty Com- pass Manual. America, with her iron monitor fleet, having felt the necessity of establishing a Compass Committee of Inquiry some few years back, her Bureau of Navigation at Washington has collected in two volumes, in the interests of the “ naval service, and, indeed, of all intelligent navigators,” the various theoretical and practical investiga- tions embodied in papers contributed by our countrymen to various learned societies from the time of Poisson’s Memoir, to 1866. The preface to the first of these volumes states that ‘the English investiga- *‘ tions on compass deviation, while eminently practical, nave steadily “advanced towards establishing a comprehensive and available theory “in all that relates to this subject.” In our own mercantile marine an impetus to extended knowledge has been lately given by the Board of Trade. Masters, in order to obtain an extra certificate of competency, must pass an examination in our sub- ject—the opportanity has been afforded me of inspecting the examina- tion papers of a few successful candidates, and I found them stamped by a sound practical knowledge of principles and details—highly favourable to a more perfect system of “ compass-adjustment” in our merchant ships of the future. The ordinary examination of masters and mates also with the present year embraces the methods of ascer- 104 ON THE PRESENT STATE OF OUR KNOWLEDGE, ETC. taining and applying the deviation of the compass, the knowledge of which is equally necessary with the calculation of a day’s work. In the Royal Navy, though magnetism as a branch of nautical education has scarcely yet taken root, we have a few Officers well trained in its principles and prectice as applied to navigation. This small band of experts, it is to be hoped, will be steadily kept up in the interests of the service. The fact is also assuring, and speaks well for the care and skill of our navigating Officers, that within the last twenty years, so far as I am informed, no misadventure has happened to a British ship-of-war through default of her compasses. In the interests of navigation—and I say this with pleasure in the presence of one of our greatest masters in magnetic science, and to whom the Royal Navy-and myself personally owe many debts of gratitude, I allude to our eminent Chairman—I confidently look for- ward at no distant date to magnetism in its several divisions occupying a fair share of the attention of naval Officers, not only as a necessary branch of education, but as an intellectual study that carries with it an indescribable charm, and is of a high order for estimating and grasping the more secret workings of nature. HEIRS sagen } (Rae wie SoA as Aarne aS ee. EChening Meeting. Monday, February 5th, 1872. GexeraL Sir WILLIAM J. CODRINGTON, G.C.B., in the Chair. NAMES of MEMBERS who joined the Institution between the 30th January and the 5th February, 1872. LIFE. Warre, Rev. Edward, Captain, 8th Bucks Rifle Volunteers. Spratt, Fredk. T. N., Lieut., R.E. Moreton, Augs. H. M., Captain, Coldstream Guards. Charteris, Hon. Alfred W., Lieut., Coldstream Guards. ANNUAL. Bailey, Vincent, Captain, Herts Militia. 