In filling the cylinder with steam, it will require a much more copious supply of steam than merely to fill up the space left by the ascent of the piston ; for as the descent of the piston was only in consequence of the vacuum occasioned by the interior of the cylinder being sufficiently cooled to condense the steam, this cooled surface must be again presented to the steam during the rise of the piston, and must condense steam a second time. The piston cannot rise another inch till that part of the cylinder which the piston has already quitted has been warmed up to the boiling point, and much steam must be expended in this warm ing, for the inner surface of the cylinder must not only be raised to the heat of boiling water while the piston rises, but must also be made perfectly dry ; and the film of water left on it by the ascending piston must be completely evapo rated, otherwise it will continue to condense steam.
On this account, although the counter-weight is not necessary to suck in the steam, the moving force during the ascent of the piston must be considered as resulting chiefly, if not solely, from the preponderating weight of the great pump-rods ; and this force is expended partly in returning the steam piston to the top of the cylinder, where it may be again pressed down by the air, and make soother working stroke by raising the pump-rods ; and partly in return ing the pump-buckets into their places at the bottom of their respective working barrels, in order that they may also make another working stroke. This latter operation requires force independent of the friction and inertia of the moving parts; for each bucket must be pushed down through the water in the barrel, which must lift up and rise through the valves in the bucket, with a velocity proportioned !e the velocity of the bucket in the same degree as the area of the pump-barrel /8 proportioned to the opening of the valves through which the water must nass.
From this general consideration of the ascent of the piston, we may see that the motion differs greatly from the descent ; it can hardly be supposed to accele rate, even if the steam were supplied to the cylinder in ever such quantity ; for the resistance to the descent of the pump bucket is the same with the weight of the column of water, which would cause water to flow through the valves of the buckets with the velocity with, which it really rises through them ; and this resistance must therefore increase, as the square of that velocity increases ; that is, as the square of the velocity with which the bucket descends. Independent of the force of friction, and the weight of the valves, the velocity of descent through the water must soon become a maximum, and the motion will become uniform. Accordingly, any one who observes with attention the working of a steam-engine, will see that the rise of the piston, and descent of the pump-rods, are extremely uniform; whereas the working stroke is very sensibly accelerated. These two motions complete the period of the operation, and the whole may be repeated by shutting the regulator, and opening the injection cock,'whenever the piston has attained the proper height. For the first two or three strokes, the
opening and shutting of the cocks are performed by the attendant ; but when he has thus ascertained that all parts are in order?he puts pins, into the holes of the ping-frame, and the motion of the engine will then actuate its own machinery, and perform its reciprocations with greater regularity than can be done by hand..
Mr. KEANE FITZGERALD proposed, in 1759, a contrivance to work the venti lator by the fire-engine for the benefit of those who work in mines, where it is employed to draw off water by making the engine turn a wheel constantly one way. This gentleman, and some others associated with him, obtained a patent excluding all others from employing the steam-engine for turning a crank ; but they appear to have received little or no encouragement from the public.
in 1766 Mr. BLAKEY obtained a patent for some improvements on Savery's engine.
To avoid the condensation that took place when the vapour came in contact with the surface of the water in the receivers, he to introduce oil or air on the surface ; but failure attended all his endeavours to carry the project into practical effect. His scheme of a boiler attracted more attention among the scientific men of his day, and it elicited much commendation. The annexed figure will explain Into the furnace a are placed the tubes b eat, connected by small pipes ; f a flumel for supplying the generator with water. e a cock for the purpose of cleaning the appa ratus by allowing a passage of water through it. The contrivance is simple and inge nious, and may have been the origin of the numerous variety of tubular boilers of more recent invention (see article BOILER); but an accident occurring in the inventor's experi ment, he failed to secure the support he was led to expect.
In 1769 a patent was taken out by a gen tleman named Stewart for an engine which produced a rotative motion by a chain going round a pulley, and also round two barrels, furnished with ratchet-wheels with a weight suspended to the free end of the chain, which served to continue the motion during the return of the engine. Mr. Matthew Was borough, at a later date, also obtained a patent for communicating a rotative motion to an engine.
It may be imagined that the progress which the steam-engine had now made stimulated speculators and machinists to bestow their studies on a subject of so much interest, and presenting the prospect of a reward far less visionary than that dreamed of at any previous period. Plans and adaptations and hypotheses sprung up on all sides, if not with a fertility, at least with a rapidity proportioned to its importance. The multiplied devices of ingenious men on matters of comparatively minor consequence, which, though valuable items in the general fund of improvement then, must be passed over, inasmuch as that they would not at this day be considered of sufficient magnitude to trench upon the reader's attention. We therefore pass at once to the next great era in its career; namely, its progress to perfection under the directing genius of Watt.