A sufficient quantity of steam being first formed in the boiler, the attendant pushes the handle or lever which he holds down to j, which, by the wheels and and, opens the cock k, and allows the steam to enter the cylinder. The steam being only sufficient to equal the pressure of the atmosphere, will not of itself lift the piston and loggerhead; it is therefore necessary that some means should be adopted to aid its ascent. This is done by means of the weight or counterpoise/; so that by the force of the steam and gravity of the counterpoise, the piston is elevated to the top of the cylinder, and forces down the pump-rod m into the pump below. When this is effected, the attendant returns the handle to its original position (shown in the cut), which prevents the admission of more steam from the boiler, and, at the same time, opens the cock n, so as to admit a small quantity of cold water from the reservoir g into the cylinder; this, by dispersing itself among the steam, almost instantly condenses it, so that a void is at once obtained; and the pressure of the atmosphere meeting no longer with resistance, presses upon the external surface of the piston, and, by its descent to the bottom of the cylinder, raises the pump bucket in the mine.
The handle is again depressed to j, which allows fresh steam to enter the cylinder and elevate the piston as before. To prevent the accumulation of water in the cylinder, the eduction pipe o is of such length that the weight of a column of water Newcomens Engine, 1705 within it exceeds that of a column of the atmosphere; so that it runs off by its own gravity.
From the superiority of the principle of Newcomen's engine over Savery's it soon superseded the latter, and it came gradually into very general use for draining mines, receiving from time to time various important improvements, in the mechanical details of the arrangement, from various ingenious persons, particularly the celebrated Smeaton. Our limits will not allow us to notice each of these as they occurred, we shall therefore proceed at once to the inventions of the great Watt, which completely altered the character of the engine, rendering it almost universally applicable, as a prime mover of machinery.
Prior to the introduction of Mr. Watt's improvements, the condensation of the steam in even the most improved arrangement of the atmospheric engine took place in the cylinder in which the steam operated, as it had done in its predecessor Savery's engine. The waste of fuel from this cause we have already pointed out, but in addition to this evil there was also a direct loss of power, owing to the vapour which was given out by the injection water, and which materially affected the vacuum. The attention of Mr. Watt was drawn to the subject about the year 1763. He was at that time a young man recently established in business at Glasgow, as an optician, and holding likewise the appointment of mathematical instrument maker to the University of Glasgow,' and, having to repair a small model of a steam engine belonging to the Institution, upon setting the machine to work after completing the repairs, he was struck by the great quantity of steam consumed by the engine: upon reflection he perceived that this was owing to the heat abstracted from the cylinder and piston at each stroke, during the formation of the vacuum.
This led him to institute a series of experiments, in the course of which he became acquainted with the leading facts upon which the theory of latent heat is founded, and at length, after much patient investigation, he hit upon the grand idea which forms the basis of his fame - the separate condenser, whereby he was enabled to produce the desideratum he had been in search of, viz., a perfect vacuum with a hot cylinder.
The detached condenser consisted of a close vessel, in which a jet of water could be produced, and connected with the bottom of the cylinder by a pipe closed by a valve. During the ascent of the piston by the operation of the counterpoise, this valve remained closed, and the steam was admitted to flow through a valve from the boiler to the cylinder below the piston. When the piston had attained its highest point, the communication with the boiler was closed, and that with the condenser opened, and the steam rushing into the condenser was quickly condensed by a jet of cold water, and a vacuum established in the condenser and cylinder, causing the piston to descend by the pressure of the atmosphere. The air and injection water which collected in the condenser at each stroke were removed by an air-pump worked by the engine beam.
The success of this improvement realised his most sanguine expectations, and confirmed his opinion of the importance of maintaining the temperature of the cylinder as high as possible. With the view therefore of preventing the loss of heat from the exposure of the interior of the cylinder to the air, his next improvement was to exclude the air from the cylinder, by an air-tight cover, having an aperture for the passage of the piston rod, which worked in an air-tight manner, by means of a contrivance which he called the "stuffing box." In this arrangement, the upper side of the piston was continually exposed to the pressure of the steam during the working of the engine, and a valve called the "equilibrium" valve, was introduced, by which the steam could be admitted to the under side of the piston. During the ascent of the piston, the eduction valve was closed, and the equilibrium valve was kept open, and the pressure upon each side of the piston being thereby rendered equal, the piston was raised to the top of the cylinder by the action of the counterpoise.
The equilibrium valve was then closed, and a vacuum being established within the cylinder by opening the passage to the condenser, the pressure of the steam upon the upper side of the piston carried the latter to the bottom of the cylinder. By this arrangement the engine became, in the strictest sense of the term, a "steam engine," depending "in no way upon the pressure of the atmosphere, and constituting in fact, in its leading features, the pumping engine employed to this day in the mining districts.