transit instrument at the Cambridge Observatory. 425 
Hence for determining the deviations of the transit instru- 
ment we have, respectively, the following equations : 
By Nautical Almanack. 
— 4.38 + s = .5^57 Z 
5.77 — e = 1.48 Z 
By Schumacher’s Tables. 
— 4.58 -f- 6 = .5657 ^ ^ o 
^ , Z = 0 '. 98 . 
5.48 — 6 = 1.48 Z 
The value of the deviation (Z) is uncertain then to the 
amount, and more, of half a second of time. From such kind 
of uncertainty, the method of circumpolar stars is entirely 
free; its characteristic excellence, as it has been already said, 
consists in its being independent of the results of astronomical 
science. 
In what I have said, I must be supposed to speak of the 
exact adjustment of large instruments. The method of high 
and low stars is very convenient, and easily practised ; it in- 
forms us, in the space of a few hours, of the nature and 
degree of the deviation of the instrument ; and in some cases, 
when the transit instrument is prevented by its situation from 
being directed to stars beneath the pole, it is almost an indis- 
pensable method. 
I wish to add a few words respecting tlie adjustment of the 
line of collimation by means of the reversion of the transit 
instrument during the passage of the pole star. This method 
has indeed the air of being philosophical ; but, according to 
my opinion, is neither so easily practised, nor so certain as 
the old method. It is liable to the uncertainty of the times 
of the pole star’s passages over the wires ; and always re- 
quires, before and after the observation, the examination of 
