COMSTOCK STUDIES IN ASTRONOMY 99 



where A is the angular distance of the star from the east 

 point. If in the quadrantal triangle formed by the star, 

 the zenith and the east point we represent the angle at the 

 star by ^ we shall have for the effect of the diurnal aber- 

 ration upon the time of the star's transit over an almu- 

 c an tar 



s. 

 H = 0.021 cos (p sin A . cos tp . sec <p cosec A 



which reduces to 



s. 

 = 0.021 cos z 



or for an observation made near the almucantar passing 

 through the pole 



s. 

 K = 0.021 sin (p 



Since the effect of the diurnal aberration is thus shown 

 to be constant for all stars observed at the same zenith 

 distance, it will be most readily taken into account by ap- 

 plying to the clock correction derived from the uncor- 

 rected observations the correction +. 



The application of the preceding formulae may be illus- 

 trated by the reduction of the following observations of 

 four pairs of stars, made with a very small universal in- 

 strument mounted upon a portable wooden tripod. The 

 aperture of the telescope was 33 mm, the magnifying power 

 27 diameters, the value of a level division 7". 4: 



