t 347 ] 

Fig. 2 . (g i k l o o x a;) the lid of the box, whole 
rim {ox o x), is a quarter of an inch deeper than 
the box (op Fig i), that the air-holes (0) may be 
pierced in its upper-part ; and the lower-part is 
fcolloped with wide fcollops, for the air to pafs 
through the holes (p p Fig. 1.) 

Fig. 3. {a b) the milk-boiler, with the broad rim 
(0 d), and perpendicular rim {c e df) foldered to 
the horizontal rim; the perpendicular rim to en¬ 
ter the circular groove ( ef) four inches deep full 
of fand, thereby to prevent the afcent of the 
imoke from the fire-ftove. 


LVII. ExtraEl of a Letter of Thomas Bar¬ 
ker, Efq\ to the Reveretid James Bradley, 
D. D. Ajlronomer Royal , and F. R. S. con¬ 
cerning the Return of the Comet , expeSled 
in 1757, or 1758. 


S I R, Lyndon, near Uppingham, Rutland, Dec. 17, 1754. 

Read March 20, A s we expedt the comet of 1731, 
17 xJl 1<5o 7> an d 1682, to return in 
1/57 or 175-8, it is proper to be aware where to 
look for it. But that will be very different, ac-* 
cording to the time of the year it comes j and its 
period is not fufficiently known to fix the month 
•of its next perihelion, which Ihould be July 25, 
1757, according to its laft period; but the length of 
that before would make it 0£t. 2f, 1758. I have 
therefore, in 12 Ihort tables, given the apparent path 
of the comet, fuppofing its perihelion any month in 

Y y 2 the 


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[ 348 ] 

the year, with its curtate distance from the earth; and 
the two firit articles of each are the places which it 
would probably begin to appear in. Thefe will Ihew 
in general the courfe of the comet, efpeciaily at its 
firft appearance, which is mod: wanted; but cannot 
be depended on where its motion is fwift, and may 
be 40 ° in a day, the beginning of May, or middle of 
October. From thefe tables, compared with the 
fcheme, I have made another, where the comet 
would begin to be feen any month in the year. 

To conftruCt the places, on a large Iheet of pafte- 
board, I divided the circumference of a circle ten 
inches radius into degrees, for the magnus orbis. 
On the right point of the ecliptic and focal length I 
drew a parabola like that obferved in J682, round 
the fun, the center of the circle, and marked every 
fourth day’s motion from the perihelion, and the 
line of its nodes. The co-fine of the comet’s incli¬ 
nation fet 6fF on perpendiculars to this, towards the 
feveral points of the parabola, forms the projection 
of it, or points in the plane of the ecliptic, over 
which the comet is at any time perpendicular. 

To find the comet’s place at any time, count how 
long it is before or after its perihelion, and mark the 
place in the projection of the parabola: lay one edge 
of a parallel-ruler through that point, and the place 
the earth is therj in, and the other edge palling thro’ 
the fun, will cut the magnus orbis at the geocentric 
longitude of'the comet: The tangent of the comet’s 
inclination making the perpendicular from the co¬ 
met’s projected place to the line of nodes, the ra¬ 
dius is the tangent of its apparent latitude, making 
the curtate diftance of the comet from the earth the 

radius. 



JPbitoj. Trans. VolJOlX. TAB. XL.f>. 34 $. 



(Ulterior Part^ 

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s'*"* -—- v -^' */ r *v 

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C 349 ] 

radius. For expedition thus; draw two lines, make- 
ing an angle of ij° $ 6 ': on one of them fet off the 
perpendicular from the comet’s projected place, and 
raife a perpendicular to the other; or, which is the 
fame, from the comet’s real place in the parabola; 
and let fall a perpendicular, that is the tangent of 
the geocentric latitude. 

One obfervation of a known comet will, on fuch 
ar fcheme, determine in fome meafure its whole 
courfe; for, from the earth’s place, draw the ob- 
ferved longitude of the comet, where that cuts the 
projection of the parabola is the comet’s place; to 
which if the obferved latitude agrees, it confirms it: 
Then the other data being already known, and one 
place given, its whole courfe may be traced. Such 
a fcheme may be alfo of ufe to find the periods of 
comets, where the defcription of one is not good 
enough to find its orbit by; for if an old comet was 
feen in Auguft, in «, or in s, with fouth latitude, 
or very bright in January, it cannot be the comet of 
1682 ; but if in November in , near the ecliptic, 
it may. It then remains to fee, whether the reft of 
the defcription will agree with the courfe it would 
in that cafe take; if it does, then, as the account is 
more or lefs perfect, there is a greater or lefs proba¬ 
bility of its being the fame. (See Plate XL). 


A Table 



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