334 APPENDIX CALCULATION OF THE 



OCTAHEDRON, RECTANGULAR BASE. 



C) produced to a and c. These lines are to each 

 other as tang. A t : tang. A 2 . 



When the decrement proceeds along the plane P, 



the unit of comparison is ^1 L, but when it is 



tang. A, 



conceived to proceed along the plane M, the unit 



becomes ?! - . 



tang. A , 



Knowing the elements of the primary form, the 

 unit of comparison, and the symbol of any plane 

 whose law of decrement we require, we are to mea- 

 sure the inclination of that plane on one or more of 

 the primary planes, and then to adapt such formulae 

 to the particular case, as will give the particular 

 values of the general indices of the plane in question. 





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