All heat engines based on the principle of the explosion of a volume of gas require an arrangement for drawing superfluous heat from the cylinder, otherwise it would soon become red-hot. This cooling is obtained in small engines by the circulation of air about a large surface especially attached to the cylinder walls, but in larger engines this means is insufficient, and it becomes necessary to use water as a cooling agent. If water is laid on there is no difficulty about this, but it occasionally happens that water is too expensive, or that it is unobtainable; in such cases it is usual to erect a large cylindrical reservoir of galvanized iron holding about ten litres of water per horse-power of the engine. This reservoir is connected by two pipes from the top and bottom to the water jacket of the engine cylinder, and circulation takes place by the difference of density of hot and cold water. In the Grob motor a somewhat different device is resorted to. The water arriving from the jacket at a temperature of about 70° C. is divided into a number of fine streams, which pass up through a network inside the reservoir. At the same time a small centrifugal pump driven by the engine forces in addition a stream of air through the water; the effect is to cool the water before it returns to the jacket to a temperature of from 80° to 90° Fahrenheit. The Grob motor is built in the same manner as the Capitaine engine, which we shall describe next, but it does not work either as smoothly or efficiently as the latter engine. It possesses, in addition, other faults which are objectionable. The vaporizing apparatus is particularly troublesome, for the result of gasifying petroleum at such a high temperature is very often to decompose it, and thereby foul the cylinder and valve mechanism. [Illustration: FIG. 31.—Exterior View of Grob Oil Motor.] _Capitaine petroleum motor_ (Figs. 32, 33, 34).—More than seventeen years have elapsed since Emile Capitaine, already well known for his experiments on gas engines, first tried his hand at oil motors. His first patent dates from the year 1879, and since that time he has continuously worked at the subject, expending much patience and money in order to produce a high-speed motor capable of using ordinary petroleum of ·88 specific gravity. The result of his labours is a machine which is excellent in all respects, requiring no igniting or heating apparatus. In the earlier type it was found that the motor would not work at less than three-quarters of its full load, because when it was doing less work the vaporizer became cooled in the intervals which occurred between the rarer explosions. This motor could only work under certain conditions, and after further trials and experiments Capitaine brought out a motor which worked at small loads as well as full loads, and which only consumed fuel in proportion to the power developed. This machine was considered too complicated, and a fresh type was brought out.