360 APPENDIX CALCULATION OF THE 



THE RHOMBOID. 



In this fig. df is the oblique diagonal, 



a b . . . horizontal diagonal, 

 d i ... axis. 



If we imagine a b, a c, b c. to be edges of a plane 

 passing through the solid, that plane would be per- 

 pendicular to the axis, and the line en e drawn upon 

 it,, would consequently be perpendicular to the axis. 



But the point e at which this perpendicular touches 

 the plane d afb, is the middle of the diagonal d f. 

 If therefore we draw/o parallel to e n, we shall have 

 o n HZ H d. ButjTo zz c w, and/ i d c ; therefore 

 i o n d ~ on. Hence perpendiculars upon the 

 axis of a rhomboid, drawn from two adjacent lateral 

 solid angles, divide the axis into three equal parts. 



Its elements. 



The inclination of the adjacent planes at the su- 

 perior edges will be different in different 

 minerals, but may be designated generally by 



