Dr. Otto employed an open flame which was introduced into the mixture by means of a slide valve. This met with only a fair measure of success. Cerium, Lanthum and several other rare metals cause a considerable spark when brought into contact with iron or steel. The objection to this method was the expense of the Cerium plugs which required frequent renewal. The writer remembers a quaint attempt at firing the charge by means of a piece of flint and steel; the failure of this is obvious. The Diesel Engine, a great success from a thermodynamic standpoint, is fired by means of the heat produced by the compression of air, the fuel being sprayed into air which is compressed to several hundred pounds pressure. Mr. Victor Lougheed proposes ignition by means of a platinum wire rendered incandescent by a current of electricity. The plan sounds feasible, but we are still waiting to be shown. Electric ignition is applicable to all classes of engines; in fact this system made the variable speed engine as used on automobiles, etc., a possibility, as accurate timing with the electric spark covers the range from the lowest possible speed to speeds of 4,500 revolutions per minute and over. (74) Advance and Retard. While the combustion of the mixture is extremely rapid under favorable conditions, there is, nevertheless, a perceptible lapse between the instant of ignition and the final pressure established by the heat of the combustion. For this reason it is necessary that ignition should be started a certain length of time before the pressure is required if we are to expect a maximum pressure at a definite point in the stroke of the piston. The amount by which the time of ignition precedes that of combustion is called the =ADVANCE=, and is usually given in terms of angular degrees made by the crank in traveling from the time of ignition to time of maximum pressure. Since the pressure is always required at the extreme end of the compression stroke, the degree of advance is given as the angle made by the center line of the cylinder with the center line of the crank at the instant of ignition. Should the advance be given as 10°, for example, it is meant that the crank is still 10° from completing the compression when ignition occurs.