198 OBSERVATORY AT WEST POINT. 
THE POLAR AXIS OF THE INSTRUMENT PARALLEL TO THAT OF THE HEAVENS, 
A well known star was observed in quick succession on the meridian, in the direct and 
reversed position of the instrument. The declination reading, corrected for refraction, 
gave in equation (2) 
p=n+A 1+A. Cos >, 
p= —An+2. cos 9; 
whence 
mn , 
cos oap—=s~ . betas ane slovia(®) 
The first member, being the projection of the are 4 on the meridian, is the are by 
which the pole of the instrument is too high or too low. The foot of the polar axis being 
moved through this distance by estimation, the declination circle was then set to the 
declination, corrected for refraction, of a second star soon to come to the meridian, and 
the telescope directed to it, the clock put in motion, and as the star culminated, as indi- 
cated by a chronometer, the adjusting screws (8), which are in the meridian at the foot of 
the polar axis, were turned till the line of collimation was brought to the star. 
Another star was observed in quick succession in the direct and reverse position of the 
instrument, when six hours to the east or west of the meridian, in which case 
$s = 90°, 
and formula (2) gave 
pH=nt+An+a. sin >, 
p=w—A +A. sin 9; 
whence 
a. sin p=p— ~~ ; poe Bre es ee A) 
the first member, being the projection of the arc 4 on a declination circle at right angles 
to the meridian, is the displacement of the pole of the instrument from this latter plane, 
and was hence known. This error being treated in manner similar to the preceding by 
the screws (8) which are perpendicular to the meridian, the adjustment of the polar axis 
was completed. 
The instrument when first put together being very much out of adjustment, terres- 
trial objects, as already remarked, were taken as being more convenient for the first 
approximations. Celestial objects in similar positions were afterwards used for all subse- 
quent ones. 
VALUE OF THE MICROMETER REVOLUTION. 
This was obtained by the usual process of taking the transits of stars near the equator 
and the distances of well known double stars. But the process which gave results 
agreeing with each other as well as any, is one which occurred to me while observing 
