6 Description of a view Transit Instrument. 
their transits is elapsed ; the instrument will then be extreniely 
near its true position; and, by repeating the observation once 
more, will be brought to a perfect exactness. Or, if another 
star, following the pole-star in its passage, be observed on the 
same evening, if the times elapsed between their transits are equal 
to the tabular difference or their right ascensions, which will pro- 
bably be the case, the accuracy of the first placing the instru- 
ment will be immediately ascertained. Other stars near the pole 
may be made use of in the same manner as is here described for 
the pole-star, but with proportionally less advantage as the polar 
distance is increased. 
It is also obvious from the figure, that the transit of the pole- 
star above the pole may be also used, and that with nearly, 
though not quite, the same advantage as the transit below the 
pole. 
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 of ten seconds in 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 
ef time in the passage of, stars near the equator. ‘ 
Example of the Computalion with the Numbers given above, 
Star A. Pole Star. 
Sin. IK  8-241855 Sin. IK 8241855 
Sin. ZA 9°998844 _ Sin. Z3  9°810316 
Sin. AC 8240199 Sin. 3B 8:052171 
Sin. PA—9°860562 Sin. P3 —8-484848 
Sin. APC 8379636 Sin. 3PB 9°567328 
APC 1° 22’ 20” 38PB 21°4(/.10% ~ 
In time 5™ 291s, Intime J* 26™ 402°, 
The error of a degree, therefore, in the position of the transit 
telescope at the horizon, causes the star A to pass through it 
5° 29'% in time later than it ought; whereas, the same error 
causes the transit of the pole-star to be 1° 26™ 402° later than 
it aught; and the difference between these two times, viz. 1" 21™ 
1145, will be the difference of the observed time of their transits, 
owing to the error of the position of the transit telescope, their 
real right ascension being supposed the same. 
To Messrs. Nicholson and Tilloch. - y 
, H. Ow 
