88 TERMINOLOGY. . 45. 46. 



Draw from the angles of this form, perpendicular 

 lines to a horizontal plane, and join all the points 

 thus determined by straight lines. The greatest 

 plane figure obtained by this proceeding is the Ho- 

 rizontal Projection of the form. 



The horizontal projection belongs to the vertical axis, 

 and is homologous with the sections to which the axis be- 

 longs (. 40.), and to which the projection is parallel. 



A form possesses as many different horizontal projections 

 As it has kinds of principal axes. 



The Side of the horizontal projection is the unity of most 

 of the subsequent calculations referring to the dimensions 

 of crystalline forms. 



. 45. REGULARITY. 



The Regularity of simple forms is their greater 

 or lesser agreement with the regular solids of geo- 

 metry. 



Regularity refers only to simple forms. The regularity 

 of the simultaneous existence of these in the compound, is 

 termed the Symmetry of combinations, which will be con- 

 sidered more at large in . 141. 



The irregularities so frequently occurring in crystals, 

 must be abstracted, in our consideration of them, and the 

 forms reduced to their peculiar regularity. 



, 46. DEGREES OP REGULARITY. 



The regularity of simple forms allows of being 

 arranged in Several Degrees. 



Geometry considers solids whose angles are not altogether 

 situated in the surface of one sphere, to be less regular than 

 those whose angles are all touched by the surface of a single 



