( 23® )

Prob. III. Fig. 6. Plate XV.
repref ent a Square funding upright on tbe Ground, but parallel
to dbe PiPlure.
The ground line and horizontal line, 8cc. remaining as in
the preceding problem, proceed to the operation. Draw A DBC
a geometrical fquare on the ground line. Draw the vifuals A r,
D r, B r, C s; then lay on a fpace A N on the ground line equal
to the diftance which the fquare is fuppofed to be from the
picture. Draw N d cutting the vifual A j in I. From 1 draw
IM parallel to A C. Draw I K, L M, perpendicular to A D, C B.
And, laftly, draw K L parallel to D B; then will the fquare I K,
L M, be the reprefentation of the original fquare A D B C, as
propofed.

Obfervations.—The vifual rays Ar, Dr, BC t, form a
pyramid*, whofe bafe is a geometrical fquare ADBC, and
whofe vertex is r, the center of the pidhire. If this pyramid
have a fedion parallel to its bafe, it muft be evident to every
one, that the fedtion will produce a geometrical fquare. The
* See its definition page 97, and its figure Plate VI.
reprefentation