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. (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 T T′ (marked by the same letters in _fig._ 11.) with the greater boiler D. [Illustration: _Fig._ 12.]