In operation, the air in the pressure tank furnishes the force which sends the water through the pipes to the various points, and forces it through the taps at the desired rate. If for any reason the air in the tank escapes, the propelling force is destroyed. This may occur by reason of absorption of the air by the water, due to the pressure to which it is subjected; or to small air leaks that may develop in the joints, which allow the air to escape. To overcome the possibility of these occurrences, arrangement is made whereby air may be pumped into the tank by the same pump as that which supplies the water. In this way, the air is introduced with the water, which bubbles up through it to the surface. If at any time the pressure in the tank is lost, it may be replaced by pumping air alone into the tank. =Power Water-supply Plants.=--Where the pump is expected to furnish water to any considerable amount beyond that for household use, it is desirable that the plant be power-driven. If the work of watering stock, lawn sprinkling, etc., is intended, the tank and pump must be enlarged to suit the desired amount of water, and a gasoline engine, windmill or electric motor will be used for power. Where local conditions will permit, a hydraulic ram may be substituted for the pump and the pressure tank used for additional pressure and storage. [Illustration: FIG. 146.--Tank pump operated by a small gasoline engine.] Fig. 146 shows a plant in which the pump is driven by a gasoline engine. In the figure, the engine _E_ is shown connected by a belt to a speed-reducing device or “jack,” marked _J_. The object of this machine is to reduce the speed of rotation and charge it to the required motion for operating the pump. The jack is connected to the pump by a rod attached to a large gear, so as to produce the desired crank motion; and the opposite end of the rod is attached to the pump handle. The rod may be detached at any time and the pump worked by hand. =Electric Power Water Supply.=--Fig. 147 shows another type of power plant in which an electric motor operates the pump. In this style of plant, the pulley on the electric motor _M_ is connected by a belt to the large wheel _W_, from which the crank motion is secured for driving the pump _P_. This machine is provided with an automatic starting and stopping device, which automatically controls the supply of water in the system. Whenever the pressure in the tank falls to a certain point, the change of pressure produced on the diaphram valve _A_ starts the motor, and the pump sends water into the tank until the pressure in the tank again reaches the amount for which the valve is set, at which time the valve disconnects the electric contact to the motor and the pump stops working. [Illustration: FIG. 147.--Pressure tank supplied by an electrically driven pump.]