denser, revolve with the two pistons, he makes them in a part of the fixed case, and the passage of the piston over them in each revolution is accomplished by the momentum of a fly.

Mr. Richard Witty has published a description of two engines moved by steam, the second being an improvement on his first. His scheme is peculiar: "he combines the reciprocating rectilinear motion with the rotative in such a manner, that the steam cylinders with pistons moving in them in a rectilineal direction, do at the same time turn round on a horizontal axle, and partly form a fly-wheel." The second patent describes that the further improvement consists in "making the piston draw or force the machinery to be worked by it, whilst itself moves both in a rectilinear and rotatory direction in a cylinder or steam vessel, which also revolves on its axis placed in any position. †

The first Steam-boat in America was launched at New York in October 1807, and began to ply between that city and Albany. +

Mr. Samuel Clegg's rotative piston makes a complete revolution in a channel at a distance from the centre of motion. The details are very different from those of any that have been described, and much more likely to succeed in practice. On this principle Mr. Clegg constructed several engines of a good size; and he informs us that he found them to answer all the ends he intended, by their introduction; they took

*

Repertory of Arts, vol. XVII. p. 130. Second Series. + Idem, vol. XX. p. 258.

Buchanan on Steam-boats, p, 7.

up very little room, made no noise, and could be ma nufactured at about half the price of condensing engines.

In the Forty-fourth Figure the bottom plate of this machine (supposed to be turned up) is made perfectly smooth and flat. A series of blocks are placed in a groove concentric to the outer rim of the plate; these blocks are of considerable weight, and occupy the entire circular groove excepting the space a', which is fitted with adjusting springs and screws to keep the series of blocks close together; the sides which apply to each other are ground plain, so as to make them fit, and be as nearly steam-tight as possible; the under surfaces are also made flat, and the whole series forms one flat horizontal surface, except the space x, which has the screws, and these are sunk so far as to allow a flat bar to pass clear over them. i, is the axis which communicates motion to the machinery. f is a bar, to which the revolving steam piston is attached, having a small wheel g, fixed upon it. The movable blocks are enclosed on their two sides and top by an iron box e, d, in which they slide upward and downward. The piston moves round in a semicircular channel or chamber, having a valve or flap at n, and the steam is admitted at v. The segments are described as being perfectly flat on their under-side, but in fact a part of their under-surface is formed with a small curve, which appears from the shading on the Figure A.

When the piston is placed as in the Figure, and steam introduced between it and the valve or door at n,

N

it will recede from the pressure, and be carried round; and the air which is in the chamber, on the opposite side of the piston, will be pressed out through the apertures x, x, into the atmosphere, or into a condenser. During the progress of the piston, the segments being in fair bearing on the flat rim of the piston chamber, will keep the space into which the steam is admitted steam-tight. The roller attached to the bar or piston rod, (being made to precede it) presses upon the under-side of each segment in its revolution and raises it just sufficient to let the thin bar (about ths of an inch thick for the largest engine), or piston rod, slide between its under surface and that of the plate or rim. The weight of each segment, after the bar has passed under it, makes it again fall into its place; or another roller may be made to follow the piston and depress the block into its situation. The passage of the bar under each block is completed before any communication can possibly be made between the external air and the front of the piston. When the steam piston arrives at the valve or door, which hangs from the flat plate covering the chamber, and moves only in one direction, the piston presses it up into a recess, and passes it. The valve, admitting steam to the piston, being also at this moment shut, the piston is carried beyond the position of the hanging valve by the momentum of a fly wheel. When it has moved so much forward as to allow the valve to fall, the steam is again admitted between the valve and the piston, which makes another revolution. The steam which has propelled the piston being now in com