ASTRONOMY. 





e = s (A D) X -; 

 2 v cos* A 



\ 



Ml 



a = w A - (A D) tan* A ; 

 2 



c A D 



r nearly. 

 * 



a 2 A cos 



Thus the figure of the earth is determined by the de->- 

 gree of the meridian in any latitude, compared with 

 the degree perpendicular to it. 



The degree of the section perpendicular to the meri- 

 dian, is to the degree of the circle parallel to the 

 equator, that is, to the length of the degree of 

 longitude, as 1 to cos A. 



The degree of longitude is therefore A cos A. Edin. 

 Trans, vol. v. p. 26. 



The manner in which the amplitude of the celestial 

 arch is measured in the case of a perpendicular to 

 the meridian, is not so direct as that which is fol- 

 lowed in the case of an arch of the meridian itself. 

 It is best done by azimuths or by determining the 

 convergency of the meridians. If P be thepole, (fig. 5.), 

 AP an arch of the meridian, AD an arch at right 

 angles to it, or nearly so : Let the latitude of A, and 

 also the angle PAD be found, and again at D the an- 

 gle PDA ; then in the spherical triangle PAD, the 

 angles at A and D are given, and also the side AP, 

 from which the arch AD may be computed in de- 

 grees and minutes j and its length also having been 



measured, 



