500 
ON DRAWING CRYSTALS 
with the former determine the situation of the angles of 
the rhombohedron, as it appears from the inspection of the 
6th figure. 
Problem XI. To draw a Scalene Six-sided Pyramid^ 
Jhr instance, (Py of rhombohedral Lime-haloide^ or 
the " variete metastatique'''' of Haiiy. 
Having completed the projection MABCDEFM^ 
(Fig. 7.) of the rhombohedron R, take N M =r M 
— j^i^ because here the number of derivation is = 3, 
which requires N to be = 3 M M^. Join the ends of 
the lengthened axis with the lateral angles of the rhombo- 
hedron by straight lines; the result will be the projection 
of the required scalene six-sided pyramid. 
Problem XII. To draw an Isosceles Six-sided Pyramid^ 
for instance^ P, or that which belongs to the Rhombo- 
hedron R of the same species. 
Project (Fig. 8.) the rhombohedron ABB^B^C C^C^X, 
and take A M =: X^ X. Draw C C"^, C^^ C" pa- 
rallel to the axis A X, and lengthen the lines B B^ M, 
B" M, till they intersect the former in C^^, C^, C"^. Join 
the points B, C"i, B^ C^^ B", with each other, and 
with the points A and X^ equidistant from M, the result 
will be the projection of the isosceles six-sided pyramid P, 
as it is represented in Fig. 9. 
The preceding cases will suffice for obtaining the pro- 
jection of any simple form we may desire, if the rules laid 
down in this respect be properly seconded by a knowledge 
