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. 8.) 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. 8.) be two steam vessels communicating by the tubes T T´ (marked by the same letters in fig. 7.) 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 of 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 upward, 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. 7.) by the cock M.[7] [Footnote 7: This pipe in fig. 9. is represented as proceeding from the force-pipe above the cistern C.] The communication of the pipes T T´ with the boiler can be opened and closed, alternately, by the regulator R, (fig. 7.) already described.