traffic occurs, and on the street being freed from the block all the cars in that street start at once. In electric railways the fluctuations are even greater than on tramways, because there are not so many motors simultaneously in use whose demand for current may, as in very large tramway systems, more or less average out to a fairly steady load. The generators in a traction station are thus subjected to great and quick changes of load. To mitigate the excessive stresses brought on to the generating plant by these violent fluctuations of load, storage batteries may be used which act as a kind of reservoir of energy, taking in a charging current at the times that the line requires less than a certain amount of current, and giving a discharge current and thus helping the engines at the time that the demand for current on the line becomes excessive. The storage battery acts as a sort of elastic buffer between the power-house and the trains, and it is therefore called a "buffer battery." The buffer battery may be used either in the main power-house or in the sub-station; in either case its advantages are that the generating machinery may be reduced in size. It has not to be large enough for the occasional and excessive demand, but only
