As will be seen, the piston must move down to draw in a supply of the explosive mixture; it then moves upward to compress it, and on the second down stroke it is pushed by the force of the explosion. From this action it can be clearly realized that the power developed by the motor comes from the force exerted by explosions at every alternate revolution of the shaft. On that account the cams that move the valve _h_ and the lever _l_ are placed on a separate shaft, which is geared to the main shaft in the ratio of two to one; that is, the wheel _K_ is twice the diameter of the wheel _J_. As the force of the piston acts on the shaft only once in every two revolutions it is necessary to provide a heavy fly wheel _O_, which will store up enough momentum to continue the rotation of the motor through the ineffective revolution. Before the motor can put forth an effort it is necessary for the piston to move downward so as to draw in a supply of explosive gases and then to move up so as to compress them and produce an explosion; therefore, the motor will not start of its own accord, but must be set in motion. In the act of starting the wheel _O_ is turned by hand. The combustion of the gasoline vapor within the chamber _Q_ and the upper end of the cylinder develops a large amount of heat, and unless means are provided for dissipating it the temperature will soon rise to a point that will interfere with the proper action of the motor. Two ways are employed to carry off the heat. One is by surrounding the cylinder with a water jacket, as shown in the diagram at _NN_; and the other is to provide the exterior of the cylinder with numerous thin ribs so as to increase the surface exposed to the air and thus increase the radiation. [Illustration: FIG. 2. PETROLEUM SPIRIT MOTOR.] The electric spark is a very effective igniter for the explosive mixture, and, by properly setting cam _n_ the explosion can be made to take place just at the position of the piston that may be found the most desirable; but the points at _i_ are liable to get out of order, and the battery that actuates the induction coil _M_ and the coil itself can become a source of more or less trouble, and on that account the igniting is effected in some motors by means of a hot tube. When this is used the cam _n_, the lever _l_ and the electrical parts of the apparatus are not required. In their stead a tube is placed on the upper side of the chamber _Q_ and this tube is maintained at a red heat by means of a flame impinging against its outer surface. When the explosive mixture is compressed it rises in the interior of the hot tube, and when it reaches the portion that is hot enough to produce combustion an explosion takes place. By many engineers this arrangement is regarded as superior to the electric spark on account of its simplicity.