282 Ona Method of fixing @ Transit Instrument 
by the method, above mentioned, of observing the transit of twa 
stars, differing cousiderably from each other in declination, and 
whose right ascensions are wel] ascertained. The principles of 
this method have been treated on by M. Lalande in his Astro- 
nomie, vol. ii. page 715; by M. Delambre in his Astronomie, 
vol. i. page 421; and by M. Biot in his Traité d’ Astronomie, 
vol. ili, Additions, p. 130: with one or other of which I shall 
presume the reader to be previously acquainted. 
The stars which should be chosen for the purpose, are those 
which differ at least 50 degrees from each other in declination : 
but the nearer that difference approaches to 90 degrees, the 
more correct will be the results. Their right ascensions, on the 
contrary, must be as near as possible to each other; a circum- 
stance which will moreover prevent the possibility of any error 
arising from a variation in the rate of the clock during the in- 
terval of the observations. And here it may be proper to remark 
that the time, used in these computations, is sidereal time: if 
therefore a clock or watch, which marks solar time, be made 
use of, it must be corrected in the manner hereafter mentioned. 
This being premised, it will be readily seen that, in this pa- 
rallel of latitude, one of the stars will have north declination, and 
the other sou¢h declination: and, in order to avoid repetition, 
1 shall call the former the northern star, and the latter the 
southern star. Their declinations I shall denote by N and S re- 
spectively : and it may be useful to know that they may be taken 
out to the nearest minute only, as great accuracy is not required 
in this respect. The right ascensions, however, of the two stars 
(which must be expressed in ¢ime) should be taken out from the 
most approved tables, and corrected for aberration and nutation® ; 
in order that their apparent positions in right ascension may be 
exactly stated: on which indeed the accuracy of the method de- 
pends. ‘The apparent right ascension of the northern star I shall 
denote by At", and the time of its observed passage, as shown 
by the clock, I shall denote by T*: the apparent right ascension 
of the southern star I shall denote by As, and the time of its 
observed passage by 1s. The latitude of the place I shall denote 
by L; and the quantity sought (or the deviation of the instru- 
ment in azimuth) by A. 
Now, in order to determine A, we must first take the differ- 
ence of ‘the apparent right ascensions of the two stars, and also 
the difference of the time of their observed transits; that is, we 
* When the two stars are at equal distances from the equator, and differing 
but little fron each other in right ascension, their mean places on the given 
day may be taken; as they will be nearly equally affected by aberration and , 
nutation.” Many pairs of stars, situated in this manner, may be mentioned: 
such as 8 Geminorum and + Navis, » Corone Borealis and « Regule, &c. 
must 
