OF COPPER-PYRITES. 
The second is identical with the more acute of the 
second variety. The last is not a member of the 
principal series ; in the seventh variety, its relations 
to the members of that series will be developed. The 
axis of this pyramid is 
2«=V17.4708; 
terminal edge ==100^ 44' ; 
lateral edge z=z 128° 51'. 
Locality : the mines of Nassau- Siegen. 
a p— OD. p. p+i. %. 11. 
T P— 2. P. ^-^P. g;^2P+l. fig. IS. The first, second, 
and fourth, of these pyramids, are already contained 
in some of the foregoing combinations. Their posi- 
tion to each other is the parallel one. We shall make 
use of fig. 15. for the devdopement of the relations 
in which the third and the fourth of these pyramids 
are to the members of the principal series. 
The intersection of the face h (fig. 12.) of P — % 
and the face a on one side, is parallel to that of the 
same face 6, and the face opposite to c, on the / 
other. In fig. 13., both the faces a (LQKHF) and 
h (GKH) pass through the point H, and their inter- 
sections with c' (NAE) coincide therefore in the hne 
HK. 
GI is half the axis of the more obtuse pyramid, 
one of whose faces is ^, if AO is half the axis of the 
more acute pyramid one of whose faces is d ; because, 
in this case, GH is the side of their common hori» 
zontal projection. 
