In the "two-cycle" engine, the piston first moves to the left, compressing a charge already present in the cylinder at _F_, and meanwhile drawing a fresh supply through the valve _A_ and passages _C_ to the space _D_. On the return stroke, the exploded gas in _F_ expands, doing its work, while that in _D_ is slightly compressed, the valve _A_ being now closed. When the piston, moving toward the right, opens the passage _E_, the burnt gas rushes out. A little later, when the passage _I_ is exposed, the fresh compressed gas in _D_ rushes through _C_, _B_, and _I_ to _F_. The operation may now be repeated. Only two strokes have been necessary. The cylinder develops power twice as rapidly as before: but at the cost of some waste of gas, since the inlet (_I_) and outlet (_E_) passages are for a brief interval _both open at once_: a condition not altogether remedied by the use of a deflector at _G_. A two-cycle cylinder should give nearly twice the power of a four-cycle cylinder of the same size, and the two-cycle engine should weigh less, per horse-power; but it requires from 10 to 30% more fuel, and fuel also counts in the total weight. [Illustration: ACTION OF TWO-CYCLE ENGINE] The high temperatures in the cylinder would soon make the cast-iron walls red-hot, unless the latter where artificially cooled. The usual method of cooling is to make the walls hollow and circulate water through them. This involves a pump, a quantity of water, and a "radiator" (cooling machine) so that the water can be used over and over again. To cool by air blowing over the surface of the cylinder is relatively ineffective: but has been made possible in automobiles by building fins on the cylinders so as to increase the amount of cooling surface. When the motors are worked at high capacity, or when two-cycle motors are used, the heat is generated so rapidly that this method of cooling is regarded as inapplicable. By rapidly rotating the cylinders themselves through the air, as in motors like the Gnome, air cooling is made sufficiently adequate, but the expenditure of power in producing this rotation has perhaps not been sufficiently regarded. [Illustration: MOTOR AND PROPELLER (Detroit Aeronautic Construction Co.)] Possible progress in weight economy is destined to be limited by the necessity for reserve motor equipment. The engine used is usually the four-cycle, single-acting, four-cylinder gasoline motor of the automobile, designed for great lightness. The power from each cylinder of such a motor is approximately that obtained by dividing the square of the diameter in inches by the figure 2-1/2. Thus a five-inch cylinder should give ten horse-power--at normal piston speed. On account of friction losses and the wastefulness of a screw propeller, not more than half this power is actually available for propulsion.