LAWS OF DECREMENT. 373 



RHOMBOID. 



to 4, the line o d, which we have called g, must be 

 greater than 2; for as the line b c is equally divided 

 at the pointj^ if d o z= d />, the line d f would be 

 parallel to o c. 



Therefore when p' zz: 4, we must have q zz: 3, and 

 consequently p zn 6. 



And as r = . , -if we suppose the value of q to be 



successively increased to 4, 5, u, &c. we shall have 

 the following series of indices to represent the series 

 of planes of class /. 





From what has been stated in the preceding pages, 

 it will be readily perceived, that when, in addition to 

 the inclination of the primary planes to each other, 

 we know the unit of comparison, and the inclination 

 of the secondary plane to the primary plane along 

 which the decrement is conceived to proceed, we 

 may immediately determine the law of decrement. 

 For we can from these data directly deduce the ratio 

 of the lines of the defect corresponding with those 

 from whence we derive our unit; and if we divide this 

 ratio by our assumed unit, we obtain, as we have 

 before observed, the law of decrement producing the 

 plane we have measured. 



