vessel, must, during the whole time the engine is at work, be kept as hot as the steam which enters it; first, by enclosing it in a case of wood, or any other materials that transmit heat slowly; secondly, by surrounding it with steam or other heated bodies; and thirdly, by suffering neither water, or other substance colder than the steam, to enter or touch it during that time. Secondly, in engines that are to be worked wholly or partially by condensation of steam, the steam is to be condensed in vessels distinct from the steam vessel or cylinders, though occasionally communicating with them. These vessels I call condensers, and whilst the engines are working, these condensers ought at least to be kept as cold as the air in the neighbourhood of the engines, by application of water or other cold bodies. Thirdly, whatever air or other elastic vapour is not condensed by the cold of the condenser, and may impede the working of the engine, is to be drawn out of the steam vessels or condensers by means of pumps, wrought by the engines themselves, or otherwise. Fourthly, I intend in many cases to employ the expansive force of steam to press on the pistons, or whatever may be used instead of them, in the same manner as the pressure of the atmosphere is now employed in common fire-engines. In cases where cold water cannot be had in plenty, the engines may be wrought by this force of steam only by discharging the steam into the open air, after it has done its office Fifthly, where motions round an axis * are required, I make

* "A Steam Wheel moved by the force of steam acting in a circular channel against a valve on one side, and against a column

the steam vessels in form of hollow rings, or circular channels, with proper inlets and outlets for the steam, mounted on horizontal axles like the wheels of a water-mill. Within them are placed a number of valves, that suffer any body to go round the channel in one direction only: in these steam vessels are placed weights, so fitted to them as entirely to fill up a part or portion of their channels, yet rendered capable of moving freely in them by the means hereinafter mentioned or specified. When the steam is admitted in these engines between these weights and the valves, it acts equally on both, so as to raise the weight to one side of the wheel, and by the reaction on the valves successively to give a circular motion to the wheel; the valves opening in the direction in which the weights are pressed, but not in the contrary. As the steam vessel moves round, it is supplied with steam from the boiler, and that which has performed its office may either be discharged, by means of condensers, or into the open air. Sixthly, I intend in some cases to apply a degree of cold not capable of reducing the steam to water, but of contracting it considerably, so that the engines shall be worked by the alternate expansion and contraction of the steam. Lastly, instead of using water to render the piston or other parts of the engines air or steamtight, I employ oils, wax, resinous bodies, fat of animals, quicksilver, and other metals in their fluid state.”*

of mercury or other fluid metal on the other side, was executed at Soho upon a scale of six feet and tried repeatedly, but was given up, as several objections were found against it." Mr. Watt in Robison, vol. II. p. 133. Specification of Patent, 1796.

These improvements were combined in a very masterly manner, in what were called his single reciprocating Engines. The lever-beam, boiler, pumprod, and the use of a plug-frame to act by its pins or tappets on the hand-gear, or contrivances for opening and shutting the valves, with some improvements in their arrangement, were retained; the valves were, however, on a different and a better construction.

In the Twenty-sixth Figure the pipe d, conducts steam from a boiler (which is omitted in order to have the principal parts of the apparatus on a larger scale); e, is the nozle, or square box, containing a valve, which in its rise or fall opens or shuts the passage between one side of the piston and the boiler, and also between the pipe o, l, and the cylinder a. Y Y is the interstice between the casing and its cylinder: the casing was called the jacket. This interval was sometimes filled with charcoal, or some other slow conducting substance; or steam from the boiler might be admitted into it: by any of these means the radiation of heat from the steam cylinder, and its conduction by the air, was very perfectly prevented. b, is the steam piston attached to the lever-beam by the rod x. The pins or tappets n, n, fixed on the plug-frame (or tappet rod,) which in our Figure also serves for the rod of the pump, attached to the condensing apparatus: at the ascent or descent of these pins, they strike on the ends of the levers or spanners m, m, m, connected with the valves, e, f, c, and open or shut them, as they may be adjusted. The condenser h, is connected with the steam cylinder by the pipes u, and