78 OUTLINES OF NATURAL PHILOSOPHY. 



78. If these computations are to be made with 

 accuracy, the spheroidal figure must be taken 

 into account, as affecting the observations in 

 two different ways, viz. by making the semidia- 

 meters of the. earth unequal, under different 

 parallels, and by making the zenith different 

 from the point in which the semidiameter of the 

 earth, if produced would meet the heavens. 



If the radius of the equator be called , as in the last 

 section, and if r and r f be radii of theearth^ com- 

 puted for A and B, and if the horizontal parallax 

 at the equator = n, at A the horizontal parallax 



r r f 



will be n x -, and at B, n X ; and therefore, 

 a a 



supposing the zenith distances pand <?' to be found 

 as before, but to be corrected for the angles be- 

 tween the vertical and the semidiameter at each 



station, then n= 



79- The parallax affects the position of a bo- 

 dy, only by depressing it in the direction of a 

 vertical plane, and therefore does not change 

 its place in azimuth, or in the direction of any 

 plane parallel to the horizon ; but it changes 

 the place in respect to circles, which cut the 

 vertical obliquely, such as the circles of decli- 



nation 



