OBSERVATIONS WITH THE ZENITH TELESCOPE. 



79 



but these values were revised from time to time as new material gave promise of improve- 

 ment. 



The values used were as follows: 



1894, January 19-April 26, A 1 ".280 



April 80 September 15, 1''.276 



October 2 December 6, 1".292 



1894 December 88-1896, August 19, 1".384 



P, 1".028 

 1".008 

 l".0O8 

 1».024 



The Reticle. 



This is Btted with three vertical threads mid one horizontal. The latter is for the 

 purpose of marking the middle of the Held, but it is not required for this. Moreover, it 

 involves the ineon venienee that, where the difference of zenith distance is small, one com- 

 ponent is found on this thread with surprising frequency, thus interfering seriously with 

 its bisection. 



In practice, the stars were bisected at the time of passing each vertical thread. The 

 equatorial intervals of the side threads from the middle one were 13'.46 and 13\61. 



The correction to I he latitude for the interval t is 



(15t)» 



sin 1" tan S. 



This is computed for each pair employed and is practically constant for the entire 

 time embraced by the series. 



The Micrometer. 



The movable frame carries live threads at intervals of approximately ten revolutions 

 of the screw. There are small pieces of brass near one end of the threads marked with 

 1, 1, 3, I ami 5 points respectively, to avoid mistakes in identifying the threads. When 

 the difference Of zenith distance is not greater than twenty revolutions, both stars were 

 generally bisected will. Ill; for greater differences II and IV were used to avoid turning 

 the screw through so great a, number of revolutions. I and V were not used. 



It was necessary to determine carefully the distance between threads II and IV, and 

 as Ibis did „ ( ,t remain constant, the operation must he frequently repeated. After some 

 experimenting it was found that this could be most satisfactorily done by pointing the 

 telescope toward the sky during daylight and bringing the threads in succession up to the 

 fixed thread before mentioned, moving the screw until the line of light between the two 

 vanished. 



In the pages which follow, giving details of the latitude determinations, the column 

 headed 1) contains this correction where required, combined with a, second correction to 

 the micrometer, I lie explanation of which will be given presently. 



A. I'. S. VOI,. XX. K. 





