_Answer._ Because when the engine is working hard, that is, pulling a heavy load up a grade, more steam and consequently more water are consumed than when it is not working so hard, and therefore more water must be forced in to supply the place of that which is used in the form of steam. If more water is forced in than is consumed, the water will rise and fill the steam-space and a part of it will then be carried into the cylinders without being evaporated. If too little water is forced into the boiler, the heating surface will not be covered, and there will consequently be danger that those portions which are exposed to the fire will be overheated and injured. QUESTION 128. _How is the supply of water which is fed into the boiler by the pump regulated?_ _Answer._ By a cock in the suction-pipe called a _feed-cock_, which can be regulated by the locomotive runner, so that more or less water is supplied to the pump. There is also a valve in the water tank by which the supply of water can be regulated. QUESTION 129. _On what part of the locomotive are the pumps usually placed?_ _Answer._ They are usually attached to the frames behind the cylinders, and are worked by the piston-rod, as will be more fully explained hereafter; but they are sometimes placed inside of the frames, that is, between the wheels, and worked from an eccentric on one of the axles, and sometimes they are placed outside of the wheels near the back part of the locomotive, and worked from short cranks attached to the crank-pins. QUESTION 130. _What provision is made for preventing the water in the pumps from freezing in cold weather?_ _Answer._ Pipes which communicate with the steam-space of the boiler are attached to each of the suction-pipes, so that, by opening valves in the former, steam is admitted into the suction-pipes to heat the water in them. By admitting this hot water into the pump, it is kept warm, and the water is thus prevented from freezing. [Illustration: Fig. 69.] QUESTION 131. _What is an “injector”?_ _Answer._ It is an instrument in which a jet of steam from the boiler mingles with and forces a continuous jet of water into the same boiler against its own pressure. QUESTION 132. _What is the action of the injector and what are the names of its essential parts?_ _Answer._ All injectors have certain parts in common. These may be shown in the simplest form of instrument, as in the fixed-nozzle injector, a section of which (omitting all detail of construction) is shown in fig. 69.