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
Fig. 2. Petroleum spirit Motor.
ribs so as to increase the surface exposed to the air and thus increase the radiation.
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 I 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 ex-
