James Watt (1736-1819)

       Mr Watt’s Description of his Perspective Machine.

261

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 5 inches wide, and
hinged so, that when opened it formed a flat board, A, B, Plate
VII. fíg. 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 for an 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