OBSERVATORY AT WEST POINT. 197 
whence we obtain, by taking the second from the first, 
nm—7' 
Anm=—y- +++ ++ + (8) 
by which the verniers were corrected. 
THE LINE OF COLLIMATION. 
The declination circle was placed at zero, after correcting the verniers, (the instrument 
reads declinations,) and the telescope directed to some distant point to the east or west, 
in the direct and reversed position of the instrument. In equation (1), we had 
ee 
and because the graduation of the hour circle is numbered from 0° to 360°, or from 0 to 
Q4%, 
s= o+Ac-», 
$= 0'+Ao—vr; 
subtracting the second from the first we get 
o—o’ 
This error was applied with its proper sign to the last reading, to which the instrument 
was then set, and the line of collimation brought to bear on the object by the screws which 
attach the cradle to the declination axis. ’ 
THE DECLINATION AXIS PERPENDICULAR TO THE POLE. 
The telescope was directed to a point directly north or south, to get the greatest decli- 
nation, in the direct and reverse position; and because by the last adjustment, »=0, 
equation (1) gave 
s=0+Ao-+A. sin (@—s). tan S+u. tan d. 
s=o'+Ao+A. sin (?—s), tan d—u. tan 8. 
Taking the difference and reducing, we had 
o—o’ 
Setting the instrument to the last reading corrected by 752, the line of collimation was 
brought back to the object by the adjusting screw (21), connected with one end of the 
declination axis. 
VOL. 1X.—53 
en 
