In Fig. 183, the entire plant is shown in place. It occupies a place inside the building, usually in the basement. In the figure the carburetor is marked 1; the mixer 2 stands at the end of the blower, which is numbered 3. The motive power of the blower is furnished by a heavy weight, which is raised by a block and tackle, the cord of which is attached to the drum and fastened to the shaft of the blower. The force furnished by the weight 4 drives the blower and maintains a constant pressure on the gas in the system. The pipe 8 conducts the air from the blower to the carburetor, which is located underground, below the frost line and 25 or 30 feet away from the building. [Illustration: FIG. 184.--Carburetor for cold-process gasoline lighting plant.] The carburetor in this case is also the storage tank, as shown in detail in Fig. 184. The carburetor is divided laterally into two or more compartments, depending on the size of the plant to be accommodated. That shown in Fig. 184 contains four compartments and is intended for a large plant. The construction is such that the compartments are only partly filled with gasoline, and arranged to permit the air from the blower, which enters at the pipe marked air, to pass through each compartment in succession, beginning at the bottom, in order that it may become completely saturated with gasoline vapor. As an additional means of aiding the saturation of the passing air, the compartments in this carburetor are provided with spiral passages through which the air must pass, so that when it reaches the outlet pipe, marked gas, the air is completely filled with gasoline vapor. The vapor-saturated air now leaves the carburetor by pipe 9, in Fig. 183, and enters the mixing chamber 2, where it is mixed with the required amount of atmospheric air, to make it completely combustible when burned at the burner. The mixing chamber is shown in detail in Fig. 185. The mixing is done automatically and the quality of the gas is uniform, regardless of the varying conditions of the attending temperature and the quality of the gasoline in the carburetor. The vitally important feature of any gas machine is, that a constant amount of gasoline vapor be carried to the burners. If the gas contains too great an amount of gasoline vapor, a smoky flame will be the result; if an insufficient amount of gasoline is present, the flame will be pale and give out little light. When freshly charged, the gasoline in the carburetor will vaporize very readily, and a large amount of air must be added to the gas to reduce it to the proper consistency; but from old gasoline, which has lost most of the highly volatile matter, a smaller proportion of atmospheric air will be demanded. For this reason, a mixing regulator that will always deliver gas containing the same amount of gasoline vapor is necessary to give satisfactory service. The mixer shown in Fig. 185 accomplishes this office by reason of the specific gravity of the gas.