"My garden hose will usually throw a stream across the street, which is very desirable when one wishes to sprinkle the street, but this pressure is disastrous when I wish to sprinkle the flowers. Turning down the stop-cock at the nozzle makes it shoot a smaller stream but more spiteful in pressure, knocking the flowers to pieces and washing the soil away from their roots. But if I partially close the stop-cock at the side of the house where the hose is attached I may have the stream of water flow as gently as I choose. "I should meet precisely the same situation if I tried to ring an ordinary electric bell by a 110-volt current, and I should use the same method of overcoming the difficulty. "The great virtue of the dynamo is that it can furnish a large supply so that the voltage is kept constant on a great flow of current. [Illustration: Fig. 126] "I have not forgotten the question, but have tried to work toward its answer all this time. The question is, why did Ernest get a shock this morning when he touched only one binding post, and when the battery of five cells is not capable of giving shocks to any one who touches its binding posts directly? We need one more diagram to give the final answer. In Fig. 126 _e_ represents the binding post from which the shock was received. _B_ is the battery of five cells, _C_ is the spark coil, _G_ is the engine cylinder, _f_ is the spark plug. When one wishes to start the engine he closes the switch _S_. This makes a continuous conductor from the battery to the metal cylinder itself. As the engine rolls over it closes the gap in the conductor at _d_ for an instant. The primary circuit is then completed and the current passes from _B_ to the cylinder, through the metal of the cylinder to _d_, then to the coil _C_, where it passes through a portion of the coil and then back to the battery. The vibrator on the coil causes the magnetic field to rapidly vary in strength. This induces a secondary current in the whole coil which, because it passes through a very great number of turns, has a high voltage. This passes from _C_ through _B_ to the base of the engine, then up the walls of the cylinder to the plug _f_, then jumps across the gap at _a_, causing the spark which explodes the mixture of gasolene and air in the cylinder. The spark plug _f_ is porcelain--an exceedingly good insulator. Through the centre of this passes a wire from _a_ to _e_. The current passes up this and back to _C_. Now the engine rests upon the floor of the boat, and Ernest stood upon the same floor. The wood of this floor when dry and clean is a very good insulator, but when wet, and particularly when wet with water that has ever so slight an amount of any salt in solution, it becomes a conductor for such high tension currents. When therefore Ernest, standing upon the floor of the boat, touched the binding post, _e_, this induced current of high voltage found it about as easy to pass