Let S be a pipe, called the _suction pipe_, descending into [Pg052] 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.[9] 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 T. 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 [Pg053] 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.