Throughout this process the steam, which mixed with the air fills the cylinder, condenser and air-pumps will be only partially condensed in the last two, and it will escape mixed with air through the valve K, and this process will continue until all the atmospheric air which at first filled the cylinder, tubes, condenser and air-pump barrel shall be expelled through the valve K, and these various spaces shall be filled with pure steam. When that has happened let us suppose all the valves closed. In closing the valve I the flow of steam to the condenser will be stopped, and the steam contained in it will speedily be condensed by the cold surface of the condenser, so that a vacuum will be produced in the condenser, the condensed steam falling in the form of water to the bottom. In like manner, and for like reasons, a vacuum will be produced in the air-pump. The valve M, and the valves in the air-pump piston will be closed by their own weight. By this process, which is called _blowing through_, the atmospheric air, and other permanent gases, which filled the cylinder, tubes, condenser and air-pump are expelled, and these spaces will be a vacuum. The engine is then prepared to be started, which is effected in the following manner:—The upper steam valve G is opened, and steam allowed to flow from the boiler through the passage leading to the top of the cylinder. This steam cannot pass to the bottom of the cylinder, since the lower steam valve H is closed. The space in the cylinder below the piston being therefore a vacuum, and the steam pressing above it the piston will be pressed downwards with a corresponding force. When it has arrived at the bottom of the cylinder the steam valve G must be closed, and at the same time the valve H opened. The valve I leading to the condenser being also closed, the steam [Pg138] which fills the cylinder above the piston is now admitted to circulate through the open valve H below the piston, so that the piston is pressed equally upwards and downwards by steam, and there is no force to resist its movement save its friction with the cylinder. The weight of the pump rods on the opposite end of the beam being more than equivalent to overcome this the piston is drawn to the top of the cylinder, and pushes before it the steam which is drawn through the tube T, and the open valve H, and passes into the cylinder C below the piston. [Illustration: _Fig._ 22.]