matical-instrument-making and his surveying. His first perspective machine, like his first organ, was constructed for his friend Dr. Black.

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Perspective Machine.

"The perspective machine was invented about 1765, on the following occasion. My friend, Dr. James Lind, brought "from India a machine, invented by some English gentleman "there, I believe, a Mr. Hurst,-which consisted of a board, "fixed on three legs perpendicularly, upon which, close to "the bottom and near the ends, were fixed two small frictionwheels, upon which a horizontal ruler rested, and could be "moved endways horizontally, and this ruler was about twice "the length of the board. On the middle of this ruler was "fixed a perpendicular ruler, reaching a little above the upper edge of the board, and in this ruler there was a groove similar to that of the slip of a sliding rule, which "was also, like that, furnished with a slider which could freely move up and down in it, and the upper end of which, being pointed, served for an index. In the bottom of the "groove in the perpendicular ruler there was a slit cut quite through the ruler, nearly from one end to the other. In "the lower end of the slider was fixed a pencil, the point of "which reached quite through the ruler to the paper to be drawn upon, which was stretched upon the board. An arm projecting forward was fixed to one of the upper corners of "the board, (I do not remember how), and upon its end nearest "the draughtsman it carried a sight or eye-piece, consisting

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of a small piece of metal with a small hole in it, and this "eye-piece was elevated about half the width of the board "above its upper edge. The rulers, sliders, &c., were, I think, made of brass, consequently heavy; but moved easily "on the pulleys, or friction-wheels.

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"In using the machine, the board being placed at right "angles to a line supposed to be drawn from the middle of "the object which was to be delineated, and the sight adjusted so as to give a proper scale, the socket of the pencil was "taken in the hand, the eye applied to the sight, and the

"index or acute top of the slider was made to travel over the "lines of the object to be delineated, which it was enabled to "do by a composition of the horizontal motion of the lower "ruler on the wheels, and of the perpendicular motion of the "slider in the upright ruler: the pencil then described the "lines upon the paper. This instrument very readily described "perpendicular or horizontal lines, as these accorded best "with its natural motions. But in diagonal or curved lines "it was difficult to make the index follow them exactly, and "the whole motions were heavy and embarrassing to the "hand. Moreover, the instrument 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

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Robison, I turned my thoughts to the double parallel ruler, "an instrument then very little known, and not at all used “that I know of. After some meditation, I contrived the "means of applying it to this purpose, and of making the machine extremely light and portable.

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"The machine consisted of a box about an inch and a half deep on the outside, thirteen inches long, and five inches "wide, and hinged so that when opened it formed a flat "board (AB) of thirteen inches long and ten inches wide, on "which the paper was stretched. It was kept open by means "of three legs (CCC), 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 (DE) projected forward, to carry the eye-piece or sight (F), which could, by means of the joints, be adjusted higher "or lower, nearer to, or further from the board, as might be required. To the lower edge of the board were attached "two thin slips of wood (GG), ten inches long and ten inches "apart, and to the lower ends of these slips was attached the "lower side of a double parallel ruler (HJK), every member " of which was ten 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

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degrees of obliquity. This double parallelogram was formed partly of thin slips of wood, and partly of brass much

hammer-hardened, and all very light. To the middle (K) "of the upper side of the higher parallelogram, was fixed, at right angles upwards, another slip of wood (L), about eleven "inches long, and ending in a brass point which served for "an index, which, by the construction, could be moved 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.

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"A paper being stretched upon the board, and the sight being moved to a proper distance from the board, (generally "about eighteen inches), the hand being applied to the pencil "socket, (for the pencil was not pressed upon by the hand), "the upper point of the index was led along the lines of the

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objects intended to be delineated; and, when perpendicular "lines occurred, the index pointing to the upper end of them, "the finger of the left hand being applied to the board touching the perpendicular slip, and the pencil drawn down"wards, the line would be straight. In the same way, the "horizontal slip served as a guide for horizontal lines; all "others were drawn by the eye guiding the index, and, if the "paper was smooth, could be drawn very correctly.

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"The whole of the double parallelogram and its attached slips, (which latter were contrived to be easily separated "from the board), were made capable of being readily folded "up, so as to occupy only a small space in the box formed "by the board, when folded up. The sight-piece also folded up, and readily found its place in the box, which also con"tained screws for fixing on the legs of the instrument; and "the box, when shut, could be put into a great-coat pocket. "The three legs were made of tinned iron, tapering, and one a little smaller than another, so that they went into one another, and formed a walking-stick about four and a half "feet long.

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"I made many of these instruments about the time mentioned, perhaps from fifty to eighty. They went to various "parts of the world: among other places several went to