404 Short Method of expreffing 
as they prefent themfelves to the eye, and then to proceed to 
the faces of the fummit, or of the pyramid. This will be il- 
luftrated by the different examples we fhall mention in the 
courfe of this article. 
Let sv (fig. 5.) be the variety of feld-fpar, named /imi- 
laiye, the primitive form of which is reprefented fig. 1. 
In this variety the plane gar (fig. 5.) refults from a de- 
erement by two ranges on the edge G (fig. 1.), proceeding 
towards H; the plane rés pu (fig. 5.) is parallel to the 
plane M (jig. 1.), which is concealed only in part by the 
effect of the decrement. The plane s «m p is parallel to the 
plane T; the pentagon £zy xs arifes from a decrement by 
two ranges on the angle I, parallel to the diagonal which 
proceeds from A to O; and, in the laft place, as this decre- 
ment does not reach its limits, the fummit bears a fecond 
pentagon z/2rk parallel to the bafe P. All this defeription 
may be thus expreffed by five letters, G? M TIP. 
18, Let us next proceed to parallelopipedons of a. more 
reoular form ; and let us firft confider the cafe in which 
they differ from the rhomboid. We fhall fuppofe that the 
parallelopipedon is the fame as that of fig, 1, the form of 
which has varied, fo as to become more fymmetrical. In 
confequence of this variation, certain folid or falient angles, 
which were different in the firft parallelopipedon, have be- 
come equal: whatever takes place in the one is repeated in 
the other; and confequently they muft be marked with the 
fame letter, Thus, in algebra, certain folutions are fimplified 
in the particular cafes in which a quantity at firft fuppofed 
to be different from another becomes equal to it. 
19. Let us conceive, for example, that the primitive form is 
a right prifm, the bafe of which is the oblique-angled paral- 
Jelograms (fig..6.), and we fhall have O= A, I=E, &c. 
‘We muft then fubftitute on both fides the fecond letter for 
_ the firft, as feen in the figure. By continuing to run over 
the different modifications of the parallelopipedon, we fhall 
fee them pafs throngh different degrees of fimplicity, ana- 
logous 
