REDUCTION OF LATITUDE OBSERVATIONS. 



225 



for the argument i difference of zenith-distance, or i difference of micrometer-reading 

 on the side, and the argument "Zenith-distance" on the top. The sign of the cor- 

 rection is the same as that of the micrometer difference. 



Reduction to the meridian. — First, when the line of collimation of the telescope is 

 off the meridian, the instrument having been revolved in azimuth and the star observed 

 at the hour-angle r, near the middle thread, then 



_2 sin- ir cos <p cos 8 



sin 1' 



sin Q 



and the correction to the latitude, if the two stars are observed off the meridian 

 = h (m'—m). The value of 



2 sin 2 £r 

 sin 1" 



for every second of time up to two minutes (a star being rarely observed at a greater 

 distance than this from the meridian in zenith-telescope observations^ is given in the 

 following table: 



-1.-- 



