SAVERY'S ENGINE. BOILERS AND THEIR APPENDAGES.- WORKING APPARATUS.-MODE OF OPERATION. DEFECTS OF THE ENGINE. NEWCOMEN AND CAWLEY. ATMOSPHERIC ENGINE.- ACCIDENTAL DISCOVERY OF CONDENSATION BY INJECTION. HUMPHREY POTTER MAKES THE ENGINE WORK ITSELF.- ADVANTAGES OF THE ATMOSPHERIC ENGINE OVER THAT OF SAVERY. IT CONTAINED NO NEW PRINCIPLE. -ITS PRACTICAL SUPERIORITY.

(31.) THE steam engine contrived by Savery, like every other which has since been constructed, consists of two parts, essentially distinct. The first is that which is employed to

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generate the steam, which is called the boiler; and the second, that in which the steam is applied as a moving power.

The former apparatus in Savery's engine consists of two strong boilers, sections of which are represented at D and E in

turning on a centre c, so that by moving a lever applied to the axis c on the outside of the top, the sliding plate R can be brought from the mouth of the one tube to the mouth of the other alternately. This sliding valve is called the regulator, since it is by it that the communications between the boiler and two steam vessels (hereafter described) are alternately opened and closed, the lever which effects this being moved at intervals by the hand of the attendant.

Two gauge cocks are represented at G, G', the use of which is to determine the depth of water in the boiler. One, G, has its lower aperture a little above the proper depth; and the other, G', a little below it. Cocks are attached to the upper ends G, G', which can be opened or closed at pleasure. The steam collected in the top of the boiler pressing on the surface of the water, forces it up in the tubes G, G', if their lower ends be immersed. Upon opening the cocks G, G', if water be forced from both, there is too much water in the boiler, since the mouth of G is below its level. If steam issue from both, there is too little water in the boiler, since the mouth of G' is above its level. But if steam issue from G, and water from G', the water in the boiler is at its proper level. This ingenious contrivance for determining the level of the water in the boiler is the invention of Savery, and is used in many instances at the present day.

The mouth of the pipe & should be at a level of a little less

than one third of the whole depth, and the mouth of G' at a a level little lower than one third; for it is requisite that about two thirds of the boiler should be kept filled with water. The tube I forms a communication between the greater boiler D and the lesser or feeding boiler E, descending nearly to the bottom of it. This communication can be opened and closed at pleasure by the cock K. A gauge pipe, is inserted similar to G, G', but extending nearly to the bottom. From this boiler a tube F extends, which is continued to a cistern c (fig. 12.), and a cock is placed at м, which, when opened, allows the water from the cistern to flow into the feeding boiler E, and which is closed when that boiler is filled. The manner in which this cistern is supplied will be described hereafter.

Let us now suppose that the principal boiler is filled to the level between the gauge pipes, and that the subsidiary boiler is nearly full of water, the cock K and the gauge cocks G G being all closed. The fire being lighted beneath D, and the water boiled, steam is produced, and is transmitted through one or other of the tubes T, T', to the working apparatus. When evaporation has reduced the water in D below the level of G', it will be necessary to replenish the boiler D. This is effected thus: A fire being lighted beneath the feeding boiler E, steam is produced in it above the surface of the water, which, having no escape, presses on the surface so as to force it up in the pipe 1. The cock K being then opened, the boiling water is forced into the principal boiler D, into which it is allowed to flow until water issues from the gauge cock G'. When this takes place, the cock K is closed, and the fire removed from E until the great boiler again wants replenishing. When the feeding boiler E has been exhausted, it is replenished from the cistern c (fig. 12.), through the pipe F, by opening the cock M.

(32.) We shall now describe the working apparatus in which the steam is used as a moving power.

Let v v' (fig. 12.) be two steam vessels communicating by the tubes TT (marked by the same letters in fig. 11.) with the greater boiler D.

Let s be a pipe, called the suction pipe, descending into

the well or reservoir from which the water is to be raised, and

communicating with each of the steam vessels through tubes D D', by valves A A', which open upwards. Let F be a pipe continued from the level of the engine to whatever higher level it is intended to elevate the water. The steam vessels v v' communicate with the force-pipe F by valves B B', which open upwards, through the tubes E E'. Over the steam vessels and on the force-pipe is placed a small cistern c, already mentioned, which is kept

filled with cold water from the force-pipe, and from the bottom of which proceeds a pipe terminated with a cock G. This is called the condensing pipe, and can be brought alternately over each steam vessel. From this cistern another pipe communicates with the feeding boiler (fig. 11.), by the cock M.*

The communication of the pipes T T' with the boiler can be opened and closed alternately, by the regulator R (fig. 11.), already described.

Now suppose the steam vessels and tubes to be all filled with common atmospheric air, and that the regulator be placed so that the communication between the tube T and the boiler be opened, the communication between the other tube T' and the boiler being closed, steam will flow into v through At first, while the vessel v is cold, the steam will be condensed, and will fall in drops of water on the bottom and sides of the vessel. The continued supply of steam from the boiler will at length impart such a degree of heat to the vessel v, that it will cease to condense it. Mixed with the heated air

T.

This pipe is represented as proceeding from the force-pipe above the cistern c, in the perspective view of Savery's engine at the head of this chapter.

contained in the vessel v, it will have an elastic force greater than the atmospheric pressure, and will therefore force open the valve B, through which a mixture of air and steam will be driven until all the air in the vessel v will have passed out, and it will contain nothing but the pure vapour of water.

When this has taken place, suppose the regulator be moved so as to close the communication between the tube T and the boiler, and to stop the further supply of steam to the vessel v; and at the same time let the condensing pipe & be brought over the vessel v, and the cock opened so as to let a stream of cold water flow upon it. This will cool the vessel v, and the steam with which it is filled will be condensed and fall in a few drops of water, leaving the interior of the vessel a vacuum. The valve B will be kept closed by the atmospheric pressure. But the elastic force of the air between the valve A and the surface of the water in the well, or reservoir, will open A, so that a part of this air will rush in, and occupy the vessel v. The air in the suction pipe s, being thus allowed an increased space, will be proportionably diminished in its elastic force, and its pressure will no longer balance that of the atmosphere acting on the external surface of the water in the reservoir. This pressure will, therefore, force water up in the tube s until its weight, together with the elastic force of the air above it, balances the atmospheric pressure. When this has taken place, the water will cease to ascend.

Let us now suppose that, by shifting the regulator, the communication is opened between T and the boiler, so that steam flows again into v. The condensing cock & being removed, the vessel will be again heated as before, the air expelled, and its place filled by the steam. The condensing pipe being again allowed to play upon the vessel v, and the further supply of steam being stopped, a vacuum will be produced in v, and the atmospheric pressure will force the water through the valve A into the vessel v, which it will nearly fill, a small quantity of air, however, remaining above it.

Thus far the mechanical agency employed in elevating the water is the atmospheric pressure; and the power of steam is no further employed than in the production of a vacuum.