300 APPENDIX CALCULATION OF THE 



We are supposed to know the inclination of the 

 primary planes to each other, and let 

 P on M be called I- 

 P T . . . /, 

 M T . . . J 3 



We may from these readily deduce the plane angles 

 at t by means of a spherical triangle ; and having 

 measured the inclination of the plane a b c on P, 

 M, and T, we may discover the plane angles at , #, 

 and c, by means of the three spherical triangles 

 marked on fig. 330. 



Fig. 330. 



In these triangles we known only the angles, which 

 are those at which the primary planes incline to each 

 other, and the supplements of those at which the 

 secondary plane inclines on the adjacent primary 

 planes. 



The plane angles at 0, b, and e, being found, we 

 may readily discover the ratios of ib : i c, and ib : ia, 

 which will give the law of decrement by which this 

 modifying plane has been produced. 



Let us still suppose, i k : i d : : m : n y 

 and i k : i h :: m : o, 



our units of comparison here would be - and 



n <> 



and let i b : i c : : p m : q n 

 i b : i a :: p m : r o. 



