(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 [Pg050] generate the steam, which is called the boiler; and the second, that in which the steam is applied as a moving power.

Fig. 11.

The former apparatus in Savery's engine consists of two strong boilers, sections of which are represented at D and E in fig. 11.; D the greater boiler, and E the less. The tubes T and T′ communicate with the working apparatus, which we shall presently describe. A thin plate of metal R, is applied closely to the top of the great boiler D, 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 G should be at a level of a little less [Pg051] than one third of the whole depth, and the mouth of G′ at 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 M, 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 I. 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.