_Answer._ The _steam-whistle_, _W_, fig. 71, and shown in section on a larger scale in fig. 81, consists of an inverted metal cup or bell, _A_, made usually of brass. The lower edge of this cup is placed immediately over an annular opening, _a a_, from which the steam escapes and strikes the edge of the cup or bell, which produces a deep or shrill sound, according to the size or proportions of the whistle. The annular opening _a a_ is formed by the plate or cover, _a a_, which nearly fills the mouth of the cup _B_, which is attached to the stem _c_. The latter is screwed into the top, _D_, of the dome. Communication with the steam-space of the boiler is either opened or closed by a valve, _b_, which is attached to a sort of spindle, _d_, which extends upward inside of the stem _c_. This spindle does not entirely fill the opening in the stem _c_, so that the steam which enters when the valve _b_ is opened rises and escapes through the holes _e_, _e_, _e_, into the cup _B_ and out through the annular opening _a a_. The valve is opened by the lever _E_, whose fulcrum is at _f_. The end _g_ of this lever is connected by a rod, _h_, figs. 81 and 71, with the cab, and by a suitable handle or lever, _h′ h′_, fig. 71, it can be opened and the whistle be blown at any time by the locomotive runner or fireman to give signals to the trainmen or of the approach of a train to a station, or to warn persons to get off of the track. [Illustration: Fig. 81. Scale 1¹⁄₂ in. = 1 foot.] QUESTION 150. _How is a locomotive boiler emptied and cleaned?_ _Answer._ One or two large cocks, called _blow-off_ cocks, _X_, fig. 71, are placed near the bottom of the fire-box, either in front or behind, and sometimes on the side. By opening either of these the water in the boiler is blown out, and much of the loose mud and dirt is carried out with the water. The cock _X_, fig. 71, is opened by a handle, _w_, which is connected with the cock by a rod. In order to clean out the mud and scale which are not entirely loose, what are called _mud-holes_ or _hand-holes_ are placed in the corners of the fire-box near the bottom. These are oval-shaped holes, about 4¹⁄₂ inches long and 2¹⁄₂ inches wide, and covered with two metal plates, one of which is put inside the boiler and the other outside, and fastened with a bolt through both. Another hand-hole is sometimes placed in the bottom of the front tube-sheet. When the boiler is emptied of water these hand-holes are uncovered, and as much dirt is removed as can be scraped out of these holes. A hose pipe is then inserted and a strong stream of water is forced in, which washes out nearly all the loose dirt, so as to leave the boiler comparatively clean.