424 Mr. WHEWELL on the Classification 
The edges of the pyramid 5 are truncated by the faces a, the edges 
of combination being parallel. Hence, by C, 6 is 2Qr. 
The edges of combination of @ and ¢ are parallel to the terminal edges 
of a. Hence, by B, c is derived, by the second law, from a. Therefore, 
q = P, gQn = Qn. 
If the faces a were removed, the combination b, c, would agree with 
c, a, in II. 2. Hence 
m—1lg=2, orn=2+1=3: and c is Q3. 
The combination c, f,; supposing the other faces absent, would agree 
with xz, r, Il. 9. Hence, p” =3. 
Therefore, the expression for the form is 
Q, 2Qr, 3Q, Q3, ~Q, «Qr 
a bit of) nie d e. 
Fig. 5, (Mohs I. 72.) is a form belonging to the Oblong-Pyramidal 
system. It is obvious that it may be thus represented 
pP, p’P, qPn, p’Pr, p”P’r, oPr, Pm, Pm’ 
be d c a h We g. 
o 
Since we have horizontal edges of combination between 5, e, f, we 
will suppose the pyramid e to be the fundamental form. Therefore, as in 
I.1, Pm is oP. 
Since c would truncate the edges of e, by III. 1, ¢ is Pr, and p’'=1. 
The faces a would make rhombs with c and e, as in III. 3. Hence, a is 
iPr. 
The faces a truncate the edges of b, asin III. 2. Hence, d is iP. 
The pyramid d would have its edges truncated by e and a respectively. 
Hence its edges make the same angles with the axis as the faces of those 
prisms do. 
