The atmospheric engine of Newcomen (for it is usually designated by his name alone) was, perhaps, lees a matter of original discovery than of a combi nation of the inventions of others ; still, much fertility of thought was displayed in applying the suggestions of his predecessors to practical purposes. A fact connected with this 'part of jhe history of the steam-engine deserves notice. Newcomen, in the course of his experiments, applied to Dr. Hooke, a man of great eminence in his day, and well acquainted with the labours of Papin ; the Doctor, in a letter to Newcomen, dissuaded him from wasting his time and labour in any attempt to produce motion on Papin's plan ; and the letter contains this very remarkable expression, "could he (meaning Papin,) make a speedy va cuum under your piston, your work is done." Now, this expression betrays either great ignorance of rapidity with which steam was condensed by con tact with a cold body, or a conviction that Papin had been incapable of effec tuating the suggestions he claimed as his own. At any rate, it proves that the " discoveries" up to this time promulgated, were so irreducible to practice, by the statements given of them, that a learned theorist doubted the possibility, and acute experimentalists had been unable to accomplish the means of making those discoveries available to any efficacious extent.
This discouraging opinion, though given by so great a mechanic, had not the effect of damping the ardour of Newcomen and Cawley, who, still acting upon the same leading idea, arrived at an application and result totally different. Instead of deriving their power from the force of steam under high pressure, their object was to make the property of steam—contraction—subservient to the power of atmospheric pressure, to act under more approved mechanical arrange ments. To raise the piston by steam, and against the atmospheric pressure on its upper surface, the vapour, of course, required to be of a temperature con siderably above that which was necessary to balance the atmospheric column ; and, in this case, the elastic force of the agent so employed became the power, and the property of condensation was employed as a simple means to restore the piston to its first position, to be similarly operated upon afresh. But sup posing the space under the piston to be filled with steam at 2120, and cold water applied to the sides of the vessel, the vapour becomes condensed, and the column of air resting on the piston, presses it to the bottom of the now empty cylinder. If the piston be attached to one end of a lever, resting on a fulcrum in the centre, it is clear that the fall of the piston, depressing that end of the lever, must elevate the opposite end, and draw up ihe weight suspended from it. Behold, then, the outline of the atmospheric engine!
Note. — Thi. famous OTTO GUERICKE, in his Experts:oda Magdeburgica, (1672), first Opposes to obtain the power of atmospheric pressure, by producing a vacuum beneath a cylinder ; the plan lie devised was by means of an air pump ; but the labour of effecting the exhaustion would be at least equal to that of any work which could be performed by the subsequent descent of the piston.
The figure on the following page (an ideal construction), will perhaps assist the reader's imagination.
a represents the boiler • b the safety-valve ; c the cylinder, open at the top, but closed at the bottom, in which there are three holes, d ef ; e the passage for steam from the boiler ; d admits a jet of cold water from the reservoir g, for the condensation of the steam ; / the exit passage for condensed steam and water; h the piston, working air-tight in the cylinder, by packing ; i the beam or loggerhead, fur the purpose of transmitting the motion of the piston to the pumps in the mine.
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 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 counter poise 1; 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 resist ance, presses upon the external surface of tho 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 tresh 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 within it exceeds that of a column of the atmosphere ; so that it runs off by its own gravity.