_Answer._ It should be started very slowly, so as not to heat any one part suddenly. Probably the greatest strains which a locomotive boiler has to bear are those due to the unequal expansion and contraction of its different parts. When the fire is started, of course the parts exposed to it are heated first, and consequently expand before the others. Now, if the fire is kindled rapidly, the heating surfaces will become very hot before the heat is communicated to the parts not exposed to the fire. Thus the tubes, for example, will be expanded so as to be considerably longer than the outside shell of the boiler, and therefore there will be a severe strain on the tube-plates, which will be communicated to the fire-box, stay-bolts, braces, etc. The inside plates of the fire-box will also become much hotter than those on the outside, and as it is rigidly fastened to the bar to which both the inside and the outside shells are fastened at the bottom, its expansion will all be upward, which thus strains the stay-bolts in that direction. As the motion due to this expansion is greatest near the top of the fire-box, the top stay-bolts are of course strained the most, and it is those in that position, as has already been pointed out, which are the most liable to break. When steel plates are used the expansion or contraction often cracks them, and sometimes, hours after the fire is withdrawn from the fire-box, the inside plates will crack with a report like that of a pistol. It is therefore very important both to heat and cool a locomotive boiler very slowly, and the fire should always be kindled several hours before the engine starts on its run. QUESTION 475. _What should be done when the locomotive leaves the engine-house and before the train is started?_