In using water this artifice is not necessary, because it will absorb heat instantly along the surface in contact with the metal, and quickly change the heated particles in favor of the cooler portions. WATER COOLING.--While heat will cause a circulation of water in a definite direction, for the foregoing reason, it has been found that, in practice, it is more practical to keep up the movement by mechanical means. This is done by a pump placed in the line of the circulating pipe, and usually so arranged that the cold, or coldest, water is forced into the circulating area around the cylinders. [Illustration: Fig. 73. Movement of Heated Water.] GRAVITY SYSTEM.--The natural circulation is founded on the principle of the well known law, that heated water will flow upwardly, hence, if a cylinder, such as A, Fig. 73, which has a water jacket around it, has its lower end connected by a pipe B, from the bottom of a water reservoir C, and the upper end of the jacket is provided with a pipe connection D, with the upper part of the reservoir, the water will flow from the bottom of the reservoir to the jacket, and from the top of the jacket to the reservoir, in the direction of the arrows. LOCATING THE RESERVOIR.--This flow would be materially increased if the reservoir should be located a considerable distance above the jacket. But in an automobile it would be difficult to use an elevated reservoir, and, furthermore, as means must be provided to cool the water, such disposition of the reservoir would be still more impracticable. [Illustration: Fig. 74. Cooling System.] The area forward of the engine is the most available space for placing the water tank, and, especially for the reasons that the radiator itself may be utilized for inclosing the engine hood, and because the air, which is only partially heated in passing through the radiator, serves to keep the space within the hood reasonably cool. FORCE SYSTEM OF COOLING.--Under the circumstances the water should be caused to circulate by mechanical means, which, while it adds another operative element to the machinery, is nevertheless so much more effective that it is worth the care, attention and expense which are involved. THE RADIATOR CONNECTION.--In Fig. 74 a radiator, engine and circulating system are connected together to show the relative arrangement of the various elements, in which the pump A is placed in the pipe line B running from the lower end of the radiator C to the manifold D at the lower end of the water jacket of the engine. The upper end of the radiator is connected by a pipe E with the top of the jacket, and the pipes are thus so disposed as to be free of the other mechanism, and are all contained within the hood of the engine. A fan F, suitably geared to the crane shaft of the engine, provides a means for inducing an air current through the radiator whenever the engine is running.