TRANSIT IN^tRUMENT. Q.Q3 



served on the same evening, if the times elapsed between their Adjustment of 

 transits are equal to the tabular difference or' their right ascen- s''^umenT by"' 

 sions, which will probably be the case, the accuracy of the levelling its 



first placing the instrument will be immediately ascertained, ^^'s, and ob- 

 ^ ° c ■ x. serving the dif- 



Other stars near the pole may be made use ot in the same fej-ence of lime 

 manner as is here described for the pole-star, but with propor- between the 

 tibnally less advantage, as the polar distance in increased. pole-star and of 



It is also obvious, from the figure, that the transit of the another more 

 pole-star above the pole, may be also used, and that with fhTpole^rthe 

 nearly, though not quite, the same advantage as the transit be- R. A. being 

 low the pole. S'ven. 



The same method may also be applied with equal ease, if 

 the second star A pass the southern meridian instead of the 

 northern. 



The slowness of the pole-star's motion, though it renders its 

 transit uncertain to a few seconds, cannot materially affect the 

 accuracy of this method, as an error often seconds of time in 

 the estimation of its passage^ which is certainly more than can 

 be committed, would not cause an error of a third of a second 

 of time in the passage of stars near the equator. 



Example of the Cofriputation wilh the Numbers given above. 



Star A. Pole-Star. 



Sin. IK 8 241855 Sin. JK 8.241855 



Sin. ZA 9.998844 Sin. Z3 9.810316 



Sin. AC 8.240199 Sin. 3B 8.052171 



Sin. PA — 9.860562 Sin. P 3 —8.484848 



Sin. APC 8.379037 Sin. 3 PB 9.567323 



APC 1°22'20" 3PB21°40'10" 



In time 5'^ 29|s In time 1^ 2d'" 40|5 



The error of a degree, therefore, in the position of the tran- 

 sit telescope at the horizon, causes the star A to pass through 

 it 5h. 29yS. in time later than it ought, whereas, the same 

 error causes the transit of the pole-star to be ]h. 26m. 40yS. 

 later than it ought; and the dijference between these two 

 times, viz. Ih. 21m. llys. will be the difference of the ob- 

 served time of their transits, owing to the error of the position 

 of the transit telescope, their real right ascensions being sup- 

 posed the same. 



VI, 



