It will be obvious, therefore, that during the normal action of the engine suction, no air will enter the duct T; but when an undue vacuum exists in the chamber O, the ball valve U is raised, and additional air is supplied to the carbureted air within the chamber. [Illustration: Fig. 82. Complete Carbureter.] CARBURETER ADJUSTMENT.--Each of these four elements has some particular method of adjustment, as will be more particularly noticed in the completely assembled carbureter, made up of the foregoing illustrations, in which the details are refined and shown as actually made in one of the well known types of carbureters. Fig. 82 shows the different parts arranged in a practical manner, in which the regulating arm for controlling the throttle, as well as the secondary air supply and the gasoline inlets are capable of being adjusted by special means. SPECIAL POINTS CONCERNING CARBURETERS.--A rich mixture is undesirable, except in the case of heavy loads and at slow speed, for various reasons. It does not burn quickly, or explode as readily as a lean one, and owing to the slow combustion the temperature in the engine cylinder remains high to the end of the stroke. THIN MIXTURES.--On the other hand, a thin mixture will compress better and burn with greater facility, and at the same time heat the cylinder less than the rich mixture, to say nothing of the saving in fuel. It has long been recognized that a carbureter will not act uniformly with all engines. Some have better compression than others, and some have more efficient sparking means. This has a bearing on the character of the fuel delivered to the cylinders. SPEEDS AND MIXTURES.--There is also a wide difference in the performances of engines at high and at low speeds, as to the quality of the mixtures required, so it will be seen that a carbureter which is capable of being controlled for all emergencies, is the one to select. Above all, the structure should be such that the valves can be easily taken out for inspection and repairs. It is impossible to prevent grit from finding its way into the gasoline, and it is astonishing how the smallest piece of fiber, finding a lodgment in a valve, will disarrange the entire power system. SURFACE CARBURETER.--These devices depend on presenting as large an area of gasoline as possible, and then conducting the air flow over the surface so as to take up the volatile hydro-carbon. THE FLOAT.--Such devices also require a float to regulate the inflow of fuel, and the distinctive feature of construction depends on increasing or decreasing the area so exposed to the moving air column.