Gas engines are in most cases mounted on a metal bed-plate, and for small engines the weight of the machine is very often sufficient to keep it in place; it is more satisfactory, however, to bolt it firmly down to a bed of concrete or stone. Plenty of room must be allowed round about to enable cleaning and repairs to be executed; space is also necessary for turning the fly-wheel round if no self-starting mechanism is provided. Above two or three horse-power water must be used for cooling the cylinder, mere cooling by air circulation being insufficient for carrying off the waste heat except in very small motors. It occasionally happens that water is laid on, which can be used for the water jacket, the heated water being simply allowed to run to waste. Unless the water is exceedingly cheap this arrangement is far too expensive, and it becomes necessary to erect a separate circulating system, in which the water is used over and over again. Makers provide for this purpose galvanized iron tanks, which are connected by two pipes to the water jacket. The tank being filled with water, and the engines started, circulation takes place in a manner similar to the ordinary household hot-water systems. The lower opening of the water jacket is connected to the lower pipe from the tank, and the second pipe, starting from a point near the top of the tank, is connected to the upper opening of the cooling jacket. The difference of density between hot and cold water causes the former to rise and flow back to the reservoir along the upper pipe, and cold water rushes in to take its place. About 400 to 500 litres of cold water are required per horse-power, but it largely depends on the rate at which the tank cools; in some cases only 200 litres being found quite sufficient. The temperature of the water leaving the jacket should be about 60° C. It is more economical to let it rise to 80° C., but in this case very good lubricating oil is required for the cylinder. A tap in the lower pipe, and a thermometer placed against the upper one, will enable the attendant to adjust this temperature.