by means of electricity, for doing a great variety of work and for furnishing electric light and heat, all of which we shall presently study. "Just before we go into this central station, can you tell me how the elevator is run in our apartment house?" "It is an electric elevator," said Harold. "And where does the electricity come from?" I inquired. "Well, I know that it comes from the street mains, but do they come from this power station?" "Yes," said I, "and we will now go in and see the steam-engines which lift you up stairs many times each day by sending electricity to run that elevator. If you choose to do so, you may claim for purposes of discussion that your elevator is run by steam." As we entered the building we came first to the dynamo room and both boys noticed that the tone which met their ears was that which I had produced for them in the telephone the night before. "I shall try to show you before we get through," I said, "that these dynamos are doing something which makes iron pulsate sixty times a second and that that is the cause of the pitch of this tone. But let us begin with the coal which is the source of all this power. "This particular station at the present time is burning forty tons of coal an hour. That is as much as Mr. ---- uses to heat his twelve-room house for a whole year. One pound of coal is capable of liberating enough energy to supply 5-3/4 horse-power for an hour. (Written for short 5-3/4 H.P.H.) One ton of coal is capable of furnishing (2,000 × 5-3/4) 11,500 H.P.H. Forty tons would yield 460,000 H.P.H. But the best furnaces, boilers, and steam-engines are terribly wasteful of energy. About nine tenths of all this energy is wasted and only one tenth, or about 46,000 horse-power per hour, is delivered by the steam-engines to the dynamos. "Coal is already scarce in the world and the supply is rapidly being exhausted. Meanwhile we are growing more dependent upon coal. A century ago we used scarcely any power except that of men, horses, and oxen, and what little heat men then used came chiefly from wood. They lived in cold houses, attended cold churches and schools, did not ride in steam or electric cars, and did not have power plants. Our wood is nearly all gone, our coal is going, and we are very rapidly growing more dependent upon heat and power, our chief source of which is coal. Wind power is too uncertain to depend upon, and we turned our backs upon water-power when we began to crowd into cities. What little water-power there is, however, is nearly all in use. "There is great need both that we learn how to save the major part of the energy of the coal which we now waste, and that we find a substitute for the coal to use when that is gone.