



Produced by Marius Masi, Don Kretz and the Online
Distributed Proofreading Team at http://www.pgdp.net







Transcriber's notes:

(1) Numbers following letters (without space) like C2 were originally
      printed in subscript. Letter subscripts are preceded by an
      underscore, like C_n.

(2) Characters following a carat (^) were printed in superscript.

(3) Side-notes were relocated to function as titles of their respective
      paragraphs.

(4) Macrons and breves above letters and dots below letters were not
      inserted.

(5) [root] stands for the root symbol; [alpha], [beta], etc. for greek
      letters.

(6) The following typographical errors have been corrected:

    ARTICLE HYDROMECHANICS: "... and [omega] the angular velocity
      about it generated by an impulse couple M, and M' is the couple
      required to set the surrounding medium in motion..." 'impulse'
      amended from 'impluse'.

    ARTICLE HYMENOPTERA: "... see P. Cameron's British Phytophagous
      Hymenoptera (4 vols., London, Roy. Soc., 1882-1893)." 'Roy'
      amended from 'Ray'.

    ARTICLE HYRCANUS: "During its later years his reign was much
      disturbed, however, by the contentions for ascendancy which arose
      between the Pharisees and Sadducees, the two rival sects or
      parties which then for the first time (under those names at least)
      came into prominence." 'disturbed' amended from 'distrubed'.

    ARTICLE ICELAND: "Iceland is emphatically a land of proverbs,
      while of folk-tales, those other keys to the people's heart, there
      is plentiful store." 'people's' amended from 'poeple's'.




          ENCYCLOPAEDIA BRITANNICA

  A DICTIONARY OF ARTS, SCIENCES, LITERATURE
           AND GENERAL INFORMATION

              ELEVENTH EDITION


            VOLUME XIV, SLICE II

        Hydromechanics to Ichnography




ARTICLES IN THIS SLICE:


  HYDROMECHANICS          HYTHE
  HYDROMEDUSAE            I
  HYDROMETER              IAMBIC
  HYDROPATHY              IAMBLICHUS (Greek philosopher)
  HYDROPHOBIA             IAMBLICHUS (Greek romance writer)
  HYDROSPHERE             IANNINA
  HYDROSTATICS            IAPETUS
  HYDROXYLAMINE           IAPYDES
  HYDROZOA                IATROCHEMISTRY
  HYENA                   IAZYGES
  HYERES                  IBADAN
  HYGIEIA                 IBAGUE
  HYGIENE                 IBARRA
  HYGINUS (eighth pope)   IBERIANS
  HYGINUS (Latin writer)  IBEX
  HYGINUS, GAIUS JULIUS   IBIS
  HYGROMETER              IBLIS
  HYKSOS                  IBN 'ABD RABBIHI
  HYLAS                   IBN 'ARABI
  HYLOZOISM               IBN ATHIR
  HYMEN                   IBN BATUTA
  HYMENOPTERA             IBN DURAID
  HYMETTUS                IBN FARADI
  HYMNS                   IBN FARID
  HYPAETHROS              IBN GABIROL
  HYPALLAGE               IBN HAUKAL
  HYPATIA                 IBN HAZM
  HYPERBATON              IBN HISHAM
  HYPERBOLA               IBN ISHAQ
  HYPERBOLE               IBN JUBAIR
  HYPERBOREANS            IBN KHALDUN
  HYPEREIDES              IBN KHALLIKAN
  HYPERION                IBN QUTAIBA
  HYPERSTHENE             IBN SA'D
  HYPERTROPHY             IBN TIBBON
  HYPNOTISM               IBN TUFAIL
  HYPOCAUST               IBN USAIBI'A
  HYPOCHONDRIASIS         IBO
  HYPOCRISY               IBRAHIM AL-MAUSILI
  HYPOSTASIS              IBRAHIM PASHA
  HYPOSTYLE               IBSEN, HENRIK
  HYPOSULPHITE OF SODA    IBYCUS
  HYPOTHEC                ICA
  HYPOTHESIS              ICE
  HYPOTRACHELIUM          ICEBERG
  HYPSOMETER              ICELAND
  HYRACOIDEA              ICELAND MOSS
  HYRCANIA                ICE-PLANT
  HYRCANUS                ICE-YACHTING
  HYSSOP                  I-CH'ANG
  HYSTASPES               ICHNEUMON
  HYSTERESIS              ICHNEUMON-FLY
  HYSTERIA                ICHNOGRAPHY
  HYSTERON-PROTERON




HYDROMECHANICS ([Greek: hydromechanika]), the science of the mechanics
of water and fluids in general, including _hydrostatics_ or the
mathematical theory of fluids in equilibrium, and _hydromechanics_, the
theory of fluids in motion. The practical application of hydromechanics
forms the province of hydraulics (q.v.).

  _Historical._--The fundamental principles of hydrostatics were first
  given by Archimedes in his work [Greek: Peri ton ochoumenon], or _De
  iis quae vehuntur in humido_, about 250 B.C., and were afterwards
  applied to experiments by Marino Ghetaldi (1566-1627) in his _Promotus
  Archimedes_ (1603). Archimedes maintained that each particle of a
  fluid mass, when in equilibrium, is equally pressed in every
  direction; and he inquired into the conditions according to which a
  solid body floating in a fluid should assume and preserve a position
  of equilibrium.

  In the Greek school at Alexandria, which flourished under the auspices
  of the Ptolemies, the first attempts were made at the construction of
  hydraulic machinery, and about 120 B.C. the fountain of compression,
  the siphon, and the forcing-pump were invented by Ctesibius and Hero.
  The siphon is a simple instrument; but the forcing-pump is a
  complicated invention, which could scarcely have been expected in the
  infancy of hydraulics. It was probably suggested to Ctesibius by the
  _Egyptian Wheel_ or _Noria_, which was common at that time, and which
  was a kind of chain pump, consisting of a number of earthen pots
  carried round by a wheel. In some of these machines the pots have a
  valve in the bottom which enables them to descend without much
  resistance, and diminishes greatly the load upon the wheel; and, if we
  suppose that this valve was introduced so early as the time of
  Ctesibius, it is not difficult to perceive how such a machine might
  have led to the invention of the forcing-pump.

  Notwithstanding these inventions of the Alexandrian school, its
  attention does not seem to have been directed to the motion of fluids;
  and the first attempt to investigate this subject was made by Sextus
  Julius Frontinus, inspector of the public fountains at Rome in the
  reigns of Nerva and Trajan. In his work _De aquaeductibus urbis Romae
  commentarius_, he considers the methods which were at that time
  employed for ascertaining the quantity of water discharged from
  ajutages, and the mode of distributing the waters of an aqueduct or a
  fountain. He remarked that the flow of water from an orifice depends
  not only on the magnitude of the orifice itself, but also on the
  height of the water in the reservoir; and that a pipe employed to
  carry off a portion of water from an aqueduct should, as circumstances
  required, have a position more or less inclined to the original
  direction of the current. But as he was unacquainted with the law of
  the velocities of running water as depending upon the depth of the
  orifice, the want of precision which appears in his results is not
  surprising.

  Benedetto Castelli (1577-1644), and Evangelista Torricelli
  (1608-1647), two of the disciples of Galileo, applied the discoveries
  of their master to the science of hydrodynamics. In 1628 Castelli
  published a small work, _Della misura dell' acque correnti_, in which
  he satisfactorily explained several phenomena in the motion of fluids
  in rivers and canals; but he committed a great paralogism in supposing
  the velocity of the water proportional to the depth of the orifice
  below the surface of the vessel. Torricelli, observing that in a jet
  where the water rushed through a small ajutage it rose to nearly the
  same height with the reservoir from which it was supplied, imagined
  that it ought to move with the