_Answer._ By attaching a sheet-iron receptacle or ash-pan (_d′ d′_, fig. 41) as it is called, under the grate, which it completely encloses from the outside air. This then serves two purposes, as it is often important when the engine is standing still to prevent any access of air to the fire-box, and therefore the ash-pan is made to fit tightly to the fire-box. Suitable doors, or _dampers_ as they are called, are placed in front and behind, and sometimes on the sides, which can be opened or closed to admit or exclude air as may be needed. QUESTION 96. _How are the tubes or flues of a locomotive arranged?_ _Answer._ They are fastened into accurately drilled holes in the tube sheet (_a_, _a_, figs. 41 and 42) which forms the front of the fire-box and in similar holes in a plate (_a′_, _a′_, fig. 41,) which forms the front end of the cylindrical part of the boiler. They thus connect the fire-box with the smoke-box. The tubes are arranged so that each tube will have a space of from ⁵⁄₈ to ⁷⁄₈ of an inch between it and those adjoining. The position of the holes for the tubes in relation to each other is determined by describing from the centre of one tube (_o_, fig. 44) a circle with a radius, _o k_, equal to the sum of the diameter of a tube and the distance which they are intended to be apart, and then subdividing this circle with the radius into six parts, _k_, _r_, _s_, _l_, _g_ and _p_. Each point of subdivision and also the centre, _o_, of the circle will be the centre of a tube. By drawing them from these centres it will be found that the distances _a b_, _c d_ between adjoining tubes will be the same between all of them. By describing circles from the centres of the outside tubes and subdividing the circles as before the position of other tubes will be determined around those first laid down. This can, of course, be carried out indefinitely. A difference in the arrangement of the tubes will be observed if, when we subdivide the first circle shown in fig. 44, instead of commencing from the intersection of a vertical line we begin from a horizontal line, _h i_, as shown in fig. 45. In the former case the tubes are said to be in _vertical rows_, and in the latter in _horizontal rows_. It is apparent from the figures and as shown by the arrows that the water can circulate in ascending currents more freely when tubes are arranged in vertical rows than when they are arranged horizontally. [Illustration: Fig. 44. Scale ¹⁄₄.] [Illustration: Fig. 45. Scale ¹⁄₄.] QUESTION 97. _How are the tubes fastened and made water-tight in the tube-sheets?_ [Illustration: Fig. 46. Scale ¹⁄₄.] [Illustration: Fig. 47.]