The tank in Fig. 188 is made of sheet metal of such weight as will safely withstand the pressure necessary in its use. It is arranged with an opening _E_, for filling with gasoline, a pressure gage for indicating the air pressure to which the gasoline is subjected, and two needle valves; _C_, for attaching an air pump and _D_, to which the hollow wire is attached for distributing the gasoline to the places of use. The tank is filled with gasoline to about the line _A_, and then air pressure is applied with an ordinary air pump to say 20 pounds to the square inch. This pressure will be much more than will be necessary to force the gasoline through the tubes but it is intended to last for a considerable length of time. [Illustration: FIG. 188.--Simple gasoline pressure-tank. FIG. 189.--Double-pressure tank for constant pressure service in gasoline lighting systems.] The principle of operation is that known in physics as Boyle’s law, that “the temperature being constant, the pressure of a confined gas will be inversely as its volume.” That is, if the tank is perfectly tight, the pressure above the line _A_, in the tank, will gradually become less as the gasoline is used and when its level is at the line _B_, where the volume is twice the original amount, the pressure will be one-half what it was originally, and will still be sufficient to force the gasoline through the tubes to the lamps. It is evident that once the tank is charged and the air pressure applied it will require no further attention until a considerable part of the gasoline is consumed. If at any time the pressure in the tank becomes too low to feed the lamps, a few strokes of the pump will raise it to the required amount. While the single tank does the required work, its use is not perfect because the pressure is constantly varying. If a lamp is set to burn at a definite pressure, any decrease in the gasoline supply due to falling pressure will change the amount of light given by the lamp; while the variation in the pressure of the single supply tank is not great, a more perfect effect is attained in the double type of tank as that of Fig. 189. The object attained in the use of two tanks differs with different manufacturers. The tank shown in Fig. 183, being intended to maintain a constant pressure on the gasoline, is quite different from those described in Fig. 197 in use with the central-generator system of lighting, to be described later. In Fig. 189 tank No. 1 is for air supply alone and tank No. 2 is the storage tank for gasoline. Between the two tanks is a pressure-regulating valve 6-7, which keeps a constant pressure on tank No. 2 so long as the air pressure of the tank No. 1 is equal or greater than the other. The gasoline in tank No. 2 will therefore be always under the same pressure and when the lamps are once burning the gasoline supply to each lamp will be a constant amount.