from the metal of the engine cylinder through the wood to his body and through his body to _e_ as to jump across the short air gap at _a_. There are two things upon which he may congratulate himself. "1. While the coil stepped up the voltage so high it reduced the available quantity of the current, so that the shock was a safe one. "2. He received only a portion of the current which passed. The major part of it passed across the gap at _a_, otherwise we should have noticed that the engine missed an explosion when he touched the binding post." The only part of this electrical outfit from which one may receive a shock is that line from _e_ to _C_. The greatest difference in electric pressure is always to be found between the two extremities of the electric generator; as, for example, between the carbon end and the zinc end of the battery, the positive and negative poles of the dynamos; the right-hand and left-hand end of this coil. Since the right-hand end is connected by good conductors with the metal of the engine and with the floor of the boat and through it with our bodies, we are in the same electrical condition as the right end of the coil; but the left-hand end and the wire connecting it with _e_ are forced by the varying magnetic field into a very different state of electric tension, and it is insulated from the engine and from us by the porcelain spark plug. We say that the "difference in potential" between the two sides of this system is 5000 to 10,000 volts. The water in this lake flows through the stream at the other end of the lake to the ocean. The water of the ocean evaporates to form clouds. Clouds drift over the land and drop their rain to replenish the lake. The difference in water level between this lake and the ocean is twenty feet. A difference in water level is what makes it a water power and it is what occasioned the building of our mill. This difference of water level corresponds in our electric generators to the difference in potential. The difference in potential maintained by our battery of five cells when not producing current is 7.5 volts. The difference in potential between the two ends of our coil, when the battery is agitating its magnetic field, is perhaps a thousand times as much, or 7500 volts. The boys took their swim in the lake and afterward, while we were all on shore lying on the grass, they brought up again the question of the machine-shop. They were anxious to know if I had any plans in regard to it. I said I had been thinking about it a good deal over night but had been waiting to hear their plans. Well, they thought it would be good to have a turning lathe, but could not think of anything else unless it might be a grindstone run by power. I said I had thought of a Central Station Electric Plant. At this they all sat up.