24 DEFINITIONS. 



abstracted in the direction d, will always be equal to 

 the number abstracted in the direction e. Thus if it 

 be a simple decrement by one row in breadth, one 

 molecule will apparently be omitted on each of the 

 edges cf, and e, as in fig 43. But in an intermediary 

 decrement, the numbers are obviously unequal in the 

 direction of those edges, and the number in height 

 will also differ from both the numbers in the direction 

 of the edges, as in fig. 48 and 49. 



The new planes produced by decrements are deno- 

 minated secondary planes, and the primary form, when 

 altered in shape by the interference of secondary 

 planes, is said to be modified on the edges or angles 

 on which the secondary planes have been produced. 

 And such edges or angles are sometimes also said to 

 be replaced by the secondary planes. 



The law of a decrement is a term used to express 

 the number of molecules in height, and breadth, 

 abstracted from each of the successively superimposed 

 plates, in the production of a secondary plane. 



When an edge, or solid angle, is replaced by one 

 plane, it is said to be truncated. When an edge is 

 replaced by two planes, which respectively incline on 

 the adjacent primary planes at equal angles, it is 

 bemlled. 



If any secondary plane replacing an edge, and being 

 parallel to it, incline equally on the two adjacent primary 

 planes, or if replacing a solid angle, it incline equally 

 on all the adjacent primary planes, it is called a tangent 

 plane. 



