Mr Watt’s Dcscrvjptlon oflm Perspective Maclime. 2(51 
^embarrassing to the hand, and the instrument itself also was 
heavy and too bulky. 
I wished to make a machine more portable and easier in its 
use, and at the suggestion of my friend Mr John Robison, (af- 
terwards Professor Robison), I turned my thoughts to the 
double-parallel ruler, an instrument then very little known, , and, 
I believe, not at all used. After some reflection, I contrived 
the means of applying it to this purpose, and of making the ma- 
chine extremely light and portable. 
The machine consisted of a box, about an inch and a half 
deep on the outside, 13 inches long, and inches wide, and 
hinged so, that when opened it formed a flat board, A, B, Plate 
VII. fig. 7. and 8., of 13 inches long and 10 inches wide, on 
which the paper was stretched. It was kept open by means of 
three legs, C, C, C, which were fastened to the back part of it, and 
served to support it at a proper height. From the right hand 
upper corner of this box or board, a jointed arm, D E, project- 
ed forward to carry the eye-piece' or sight, F, which could, 
by means of the joints, be adjusted higher or lower, nearer to or 
farther from the board, as might be required. To the lower 
edge of the board were attached two thin slips of wood, G, G, 
10 inches long and 10 inches apart, and to the lower ends of 
these slips was attached the lower side of a double parallel 
ruler, H, I, K, every member of which was 10 inches long 
between centres ; so that when fully open, it formed two 
squares, joined by one side of each, and in other states formed 
lozenges, or rhombuses of different degrees of obliquity. This 
double parallelogram was formed partly of thin slips of wood, 
H, I, K, and partly of brass much hammer-hardened, and all 
very light. 
To the middle, K, of the upper side of the higher parallelo- 
gram was fixed, at right angles upwards, another slip of wood, 
L, about 11 inches long, and ending in a brass point, which 
served foi* '^ index, which, by the construction, could be mov- 
ed equally easily in every direction, and with very little friction, 
and at the same time all the positions of the rulers were always 
parallel to each other. A pencil, pressed upon by a spring, was 
fixed in the junction of the perpendicular slip or index at K. 
A paper being stretched upon the board, and the sight being 
moved to a proper distance from the board, generally about 
