_Answer_. By the weight of the locomotive and to some extent by the distance between the rails. It will be found often that it is impossible to make the boiler of the size indicated by a calculation similar to the above without at the same time making the weight of the locomotive and the adhesive weight greater than was assumed. The result of such a calculation indicates, therefore, that too large a proportion of the weight of the locomotive is on the driving-wheels for the speed at which it is intended to work, and that either they should bear less weight or the speed be reduced. QUESTION 423. _In what respects is the operation of locomotive boilers different from that of nearly all other steam boilers?_ _Answer_. The amount of steam generated in proportion to the amount of heating surface is much greater in locomotive boilers than in any other kind. To produce combustion which will be sufficiently active to generate the requisite quantity of steam, the fire must be stimulated by the blast created by the exhaust steam to a degree unknown in other kinds of boilers. So rapid is the movement of the products of combustion that a smaller proportion of the heat is imparted to the water contained in the boiler, and consequently a less amount of water is evaporated in proportion to any given amount of fuel than in boilers in which combustion is less violent. The combustion is also less perfect, because the strong draft does not allow time for a perfect combination of the gases which produce combustion. The supply of steam which a locomotive boiler must furnish is also much more irregular than the demands made upon any other kind of boiler. At one time the fire must be urged to the greatest possible intensity in order to furnish steam enough to pull a train up a steep grade. When the top is reached the demand ceases, and the boiler can be cooled. The load which a locomotive can pull over a given line of road is usually limited by the utmost capacity of the boiler to supply steam at these critical periods. QUESTION 424. _What relation is there between this irregular action and the size of the boiler?_