238 ON THE USE OF SYMBOLS. 



would be denoted by O p , and be read p on the right 

 ofO. 



In either of the preceding cases, the intersection of 

 the new plane with the primary plane along which 

 the decrement is conceived to proceed, will, as we 

 have already seen, be parallel to the diagonal of that 

 plane. 



Let us now suppose an intermediary decrement to 

 have taken place on the angle O, of such a nature, 

 that the mass of molecules abstracted should belong 

 to a double plate, or be two molecules in height, or 

 as it might be otherwise expressed, 2 molecules in 

 the direction of the edge H, 3 in the direction of the 

 edge D, and 4 in the direction of the edge F. 



The appropriate symbol to denote such a decre- 

 ment, ought obviously to represent this threefold 

 character ; which it does by combining the indices 

 expressive of the particular law of decrement, with 

 the letters which represent the edges and angles af- 

 fected by it, in this manner, (D3 H2 F4). This 

 symbol is placed in a parenthesis to distinguish it 

 from a combination of three simple or mixed decre- 

 ments, and it would be read thus, 3 on the edge D, 

 2 on the edge H, 4 on the edge F.* 



Jf instead of the angle marked by O, we no\v 

 imagine the solid angle on which A is placed to be 

 modified simlarly to that denoted by O ; before we 

 describe the modifications of A, the crystal is con- 

 ceived, to be turned round, until the angle on which 



* This mode of representing intermediary decrements differs from that 

 adopted by the Abbe Haiiy, in referring the decrement to the adjacent 

 edges ; whereas he refers them to two edges and the angle they include. 

 But the form of the symbol here given will best accord with the results 

 obtained by the methods of calculating the laws of decrement, which 

 will be given in the Appendix. 



