_Answer._ The advantage of using steam expansively has already been explained in Part V.; it is more economical to use steam of a high pressure which is done by keeping the throttle-valve wide open, and then regulating the speed by cutting off shorter--that is, expanding it more. If the speed is reduced by partly closing the throttle-valve, the steam is wire-drawn and, as was shown in answer to Question 59, it then produces much less useful effect than it would if it was admitted into the cylinder at full boiler pressure. It is found, however, that in many cases if the steam is cut off very short the final pressure when it escapes is so low that it does not produce blast enough to stimulate the fire, and therefore the boiler will not make enough steam. This is more liable to occur with engines which have small than with those which have large boilers, or when the boilers are in bad condition or the fuel is of poor quality. When it does occur it is necessary to work steam during a longer portion of the stroke, so as to increase the final pressure when it is exhausted and regulate the speed with the throttle-valve. Of course this is very wasteful, but it is often the best which can be done and pull the train. There is also another practical difficulty in using steam of a high pressure and running with the throttle wide open and regulating the speed with the reverse lever alone. The link motion, as has already been explained, will not be effective in cutting off at a point below about one-quarter of the stroke. Now it often happens, even when cutting off at that short point, with light trains on a level or slightly descending grade, that the speed will be too great if the throttle is wide open and with full steam pressure in the boiler. When this is the case, it is absolutely necessary to reduce the speed either by partly closing the throttle, or reducing the pressure in the boiler. Undoubtedly if valve-gear for locomotives was so constructed that steam could be cut off effectively at a shorter point of the stroke, it would result in increased economy in the use of steam. The runner should aim to run at as nearly uniform speed as possible, and in order to do so should divide the distance between stopping points and the time given for running it into as small divisions as he conveniently can, so as to be able to tell as often as possible whether he is running too fast or too slow, and thus travel over the shorter spaces in corresponding periods of time. QUESTION 478. _How should the boiler be fed?_