In making these experiments a record was made of the indications of a pyrometer and of the steam gauge once every minute while the engine was running. The distance run was 19 miles for the first experiment, 13 for the second and 6 for the third, with 30 loaded freight cars in the train. The last experiment was made while the engine was working on a heavy grade and very nearly up to its maximum capacity. It will thus be seen that a great deal of heat is wasted by escaping up the chimney. _Fourth_, by external radiation from the boiler. This occurs chiefly from the fact that it is not sufficiently well protected or covered with non-conducting material. The practice, or rather the neglect, of not covering the outside of the fire-box with lagging doubtless causes a very considerable loss of heat by radiation and convection from the hot boiler plates. QUESTION 400. _What is the ordinary form of fire-box employed for burning bituminous coal?_ _Answer._ It is that represented in plate II and figs. 41 and 44, and is simply a rectangular box, and for that reason it is called a _plain_ fire-box. Sometimes provision is made for admitting air into such fire-boxes through hollow or rather tubular stay-bolts, which are put into the sides and front. In most cases, too, the fire-box door has perforations for admitting air. [Illustration: Fig. 214. Scale ³⁄₈ in. = 1 foot.] QUESTION 401. _What other appliances are used for burning bituminous coal?_ _Answer._ The most common appliance which is added to the plain fire-box is what is called a _fire-brick arch_. This is shown in fig. 214. _B C_ is the arch which, as its name implies, is formed of fire-brick and extends backward and upward from a point in the tube-sheet below the tubes. In order to be self-supporting it is built in the form of an arch, the two sides of the fire-box acting as abutments for its support. The engraving represents with sufficient clearness the direction of the flames and smoke. These must take a more circuitous “_run_,” as it is called, after leaving the fire, in order to reach the tubes. Time is thus given for the gases to combine and combustion to take place. The fire-brick becomes heated, and thus to some extent prevents the gases from being cooled down below an igniting temperature by contact with the cold surface of the fire-box before combustion is complete. The fire-brick, however, soon burns out, and must be replaced, but owing to its cheapness and the ease with which it can be removed, this does not make a serious objection to its use. Air is nearly always admitted above the fire when the brick arch is used, either by tubular stay-bolts, _a_, _a_, _a_, or perforations in the door, or both. [Illustration: Fig. 215. Scale ³⁄₈ in. = 1 foot.]