_The Effect of the Rate of Mixing the Volatile Combustible and Air on the Extent of the Combustion Space._--When studying the effects discussed in the preceding paragraphs, the rate of mixing the volatile combustible with the supply of air must be as constant as practicable. At first, tests will be made with no special mixing devices, the mixing will be accomplished entirely by the streams of air entering the furnace at the stoker, and by natural diffusion. Although there appears to be violent stirring of the gases above the fuel bed, the mixture of the gases does not become homogeneous until they are about 10 or 15 ft. from the stoker. The mixing caused by the air currents forced into the furnace at the stoker is very distinct, and can be readily observed through the peep-hole in the side wall of the Heine boiler, opposite the long combustion chamber. This mixing is shown in Fig. 20. _A_ is a current of air forced from the ash-pit directly upward through the fuel bed; _B_ and _B_ are streams of air forced above the fuel bed through numerous small openings at the furnace side of each hopper. Those currents cause the gases to flow out of the furnace in two spirals, as shown in Fig. 20. The velocity of rotation on the outside of the two spirals appears to be about 10 ft. per sec., when the rate of combustion is about 750 lb. of coal per hour. It is reasonable to expect that when the rate of mixing is increased by building piers and other mixing structures immediately back of the grate, the completeness of the combustion will be effected in less time, and a smaller combustion space will be required. Thus, the mixing structures may be an important factor in the extent of the required combustion space. To sum up, it can be said that the extent of the space required to obtain a combustion which can be considered complete for all practical purposes, depends on the following factors: (_a_).--Nature of coal, (_b_).--Rate of combustion, (_c_).--Supply of air, (_d_).--Rate of heating fuel, (_e_).--Rate of mixing volatile combustible and air. Just how much the extent of the combustion space required will be influenced by these factors is the object of the experiments under discussion. _The Scope of the Experiments._--With this object in view, as explained in the preceding paragraphs, the following series of experiments are planned: [Illustration: Fig. 20. SECTION THROUGH STOKER SHOWING MIXING OF GASES CAUSED BY CURRENTS OF AIR]