precise disposition of the parts as they are usually put to gether but in fig. 1. of the plate, this deviation is very little, and represents a single engine. A the cylinder in which the piston moves, B the steam-pipe, D the conden ser, E the discharging-pump, F a bottom common to the pump and condenser, in which is an occasional commu, nication by a hanging valve at F: g is a valve to be lifted by the engine at every stroke, for the purpose of conden. sing the steam; h is a valve placed outside the cistern (of which x x x x is a section on purpose to shew the contents), but communicates only with the condenser by a pipe passing through the side of the cistern, and is inserted at the side of the condenser; i is a valve to be lifted by the engine, and opens a communication between the cylinder and the condenser; k is a valve to be lifted by the engine, and opens a communication between the lower part of the cylinder and the steam-pipe; and is also a valve of the same kind, opening a passage from the boiler to all parts of the engine.

The piston-rod, which is here broken off at m, is connected by a chain to the lever or beam, which is supported on a wall of good masonry, with proper pivot-blocks for support to the gudgeons; and as this kind of engine is usually employed for pumping water, another set of chains at the other end of the beam is appropriated to connect to the pump-rods.

Then to set the engine to work, the first thing is to lift the three valves i, k, and I (for which there are apparatus too minute to lay down on this scale); these being opened, the steam occupies every cavity and crevice of the engine, and in a little while displaces all the air in the cylinder, condenser, &c. which is discharged at the valve h: this valve is always covered with water in a small cistern attached to the side of the large one; for it is hardly in the power of art to fit it to that degree of accuracy as to

ensure its tightness: but here the air is discharged at first setting the engine to work; and this valve is called the blowing valve. When the cylinder and other vessels are properly heated and the air discharged, which is well known by a very smart crackling noise at that valve, like a violent decrepitation of salt in a fire:* these valves k and i are to be shut; and after waiting a few seconds, gently open the valve i: and if the engine does not move, the injection valve g must be opened a little ;† and if the engine does not move, then the operation of blowing must be performed again, though but for a few seconds, and the engine in general will go off smartly.

In all engines on this principle, it is necessary that the parts appropriated to condensation of the steam, should be kept as cold as possible, and that those parts intended for the operation or passage of the steam be kept as hot as possible: hence the discharging pump and condenser are placed in a cistern of cold water kept constantly full, and a little running away; and if the injection-valve is placed low in this cistern, it will take the water in the coldest

state.

As the condenser is immersed in water to be kept cold, so the cylinder should if possible be immersed in steam to be kept hot: for which purpose Mr. Watt formerly used a casing round the cylinder, and at the top and bottom; and this would have been attended with very beneficial effects if it did not enlarge the steam surface, and expose it to a more rapid condensation when it ought to

This noise is occasioned by the air being all gone, and the water producing a sudden and rapid condensation of the steam.

† The valve i being opened, there is a passage made from the cylinder to the condenser; but on account of long blowing, the sides of the condenser become hot, and the water in the cistern hot likewise: so that the condensation must needs be very slow, even at the first injection of the water into the condenser.

be preserved; for to have the vacuum as perfect as pos sible, it is necessary that the cylinder be kept up to such a temperature as to prevent the least condensation on the internal surface either above or below the piston: because, if the sides of the cylinder were to be wet, as in the common atmospherical engine, the vacuum would be vitiated, as it is there occasioned by this wetness or moisture gradually forming to steam, which the outside casing prevents, being filled with steam from the boiler. But if it were possible to cover this outward case with any sort of substance which would entirely prevent the transmission of heat for that casing, it would supercede the use of the casing altogether, and would apply with more advantage to the cylinder itself. But we do not know of any substance which will not admit this transmission more or less. They who wish for information on this subject may find it in count Rumford's Essays.

The first circumstance of importance to the proportion and disposition of the several parts, is the solidity of the vessels and the perfection of the joints. Copper tubes are apt to be unsound at the seams, and other parts which are required to be bent out of a right line; and iron castings, which require any particular sort of stay in the moulding to keep the core from the outside, as these stays are made mostly of wrought iron, they contract more in cooling than the cast-iron does about them, even so as to become loose sometimes; in such cases, it is unutterably perplexing to find out the places or cause of this defection; the joints are suspected for the most part, but even remaking them sometimes proves no amendment; and this must be the cause in general why one engine from the same patterns is better or worse than others. And we have reason to fear that this matter of complaint is on the increase; for self-interest has so powerful a preponderancy, especially in the metropolis, that