be as brief and pointed as possible.

As a foundation, we need a good supply of clean, strained gasoline in the tank. Unless the fluid is free from all impurities it is likely to clog and interfere with the working of the machinery. The tank is so placed that its elevation is sufficient to cause the gasoline to flow by gravity through the pipe, which is connected by an automatic valve with the carburettor, admitting just enough to answer the purpose desired. As the gasoline is sprayed into the carburettor a quantity of air is drawn in from the outside. The two mingle and form a highly explosive gas. To start, you give the fly-wheel a rapid swing, which causes the piston to move downward. This action sucks the gaseous mixture into the cylinder through the inlet valve. The further movement of the fly-wheel causes the piston to move upward, compressing the gas into small volume. While the gas is thus compressed it is exploded by means of an electric spark. The violent expansion of the burning gas drives the piston downward with great force. The movement opens the exhaust valve, the burnt gases escape through the exhaust pipe and the fresh mixture is drawn in again to be compressed and exploded as before. If the engine has more than one cylinder the same process is repeated in each one. This is the operation which is continued so long as the supply of gasoline holds out.

In the steam engine the vapor acts alternately on each side of the piston head, but in motor boats and automobiles it acts only on one side. The speed with which this is done is amazing and the same may be said of the steam engine.

The swift rise and fall of the piston acting through the connecting rod turns the shaft directly below, which whirls the screw around at the stern. The electric spark that explodes the vapor is generated by a dry battery or by a magneto-electric machine driven by the motor itself. There is also the "make and break" spark, to which we need give no attention. The two ends of the wires in the spark plug which is screwed into the cylinder are separated by a space barely the twentieth part of an inch, across which the spark leaps, giving out an intensely hot flash.

You understand, of course, that I have given simply the principle and method of operation of the engine belonging to a motor boat. There are many parts that must operate smoothly and with the minimum of friction. Lubricating oil is as essential as gasoline; the ignition battery must be kept dry; you must know how to operate the reversing lever, to shut off, to start and to hold the desired speed. Except when 

 Prev. P 21/140 next