At this point I was reminded by the boy who had received a shock from the engine that morning that he had touched only one binding post. How then had he closed a circuit through his body, and how could he receive such a terrible shock when there were only a few battery cells to produce the electric current. I replied that he had the distinction of having encountered about a 5000-volt current. In the language of the newspapers he might say, _Took 5000 volts and still live._ We must next proceed to show how he really did close the circuit and how the spark coil enables a battery of a few dry cells to produce exceedingly high voltages. XI THE ELECTRICAL SPARKING EQUIPMENT FOR A GASOLENE ENGINE Under the shade of a great sugar maple, with Millville Lake spread before us, we took apart and examined the entire equipment for producing the electric sparks to explode the mixture of gasolene and air in the cylinders of our motor boat. The engine has two cylinders. For each cylinder there is a separate battery and spark coil. Inasmuch as the electrical outfit is duplicated for each cylinder it will be necessary for us to consider the case of one cylinder only. When this engine is running, 700 explosions per minute are produced in each cylinder. In one-twelfth of a second the following four events take place: 1. The cylinder is swept clear of the products of combustion formed by the last explosion. 2. Four drops of gasolene are vaporized and mixed with one quart of air and pushed into the cylinder by the pressure of the atmosphere. 3. This mixture is compressed by the piston in the cylinder to about one-fifth its original volume. 4. The mixture is heated to its kindling temperature, which is above 2000 degrees. It then burns with a sudden expansion, which drives the piston before it and pushes the crank which is concealed in the lower end of the cylinder half-way around. The crank is attached to the shaft, which carries the fly-wheel upon one end and the propeller wheel upon the other end. The momentum of the moving parts--chiefly that of the fly-wheel--suffices to accomplish the remaining half of the revolution. That any machine could be devised which could repeat these four events 700 times a minute was unthinkable a few years ago. The first men who thought that a gasolene engine could be a practical thing were considered visionaries, but now they are found to be more practicable than steam engines. They are so efficient that they compete with the steam engine upon its own ground, and, in addition, they have opened up regions of usefulness which the steam engine can never exploit. So far as we can see, they have a permanent monopoly of the navigation of the air. It is with the fourth event mentioned above, viz., kindling the explosive mixture, that we are now concerned. The high temperature required for this is obtained by forcing an electrical current against resistance.