APPENDIX. 



CALCULATION OF THE LAWS OF 

 DECREMENT. 



IN the preceding sections, some general rules have 

 been given for determining the class of primary forms 

 to which any given secondary crystal belongs, and 

 for describing the secondary crystal by means of the 

 position of its secondary planes, and of the angles at 

 which those planes respectively incline on the pri- 

 mary. 



The following is an outline of the method of apply- 

 ing the theory of decrements to determine the rela- 

 tions between the secondary and primary forms of 

 crystals.* 



The application of this theory will embrace the 

 following problems. 



First, To determine the law of decrement by which 

 any secondary plane is produced, the elements 

 of the primary form being known, and the angles 

 at which the secondary plane inclines on the 

 adjacent primary planes, being also known. 



* The reader of this appendix is supposed to be acquainted with th* 

 elements of plane and spherical trigonometry, and with the use of the 

 tables of logarithms. 



