Still, the certainty and simplicity of this method have distinct advantages.
In forced circulation a pump, either rotary or semi-rotary, is used, the direction of the flow being such that the water passes from pump to cylinder, thence to radiator, on to tank, and then through pump again, thus completing its circuit. The water in this way gets the maximum cooling effect from the radiator, and the body of water in the tank is kept cool. On account of the high speed of an oil engine and the comparatively small amount of power required to circulate the water, centrifugal pumps are becoming almost universal. As there are no valves to get out of order, and high speed is obtainable without extra gearing, this type of pump is very suitable. Semi-rotary pumps are also used, but necessitate lower speeds, and consequently extra parts to effect this. In the centrifugal pump the water is kept in motion by a fan-wheel working in an enclosed space; there being only just clearance for the fan, the centrifugal force thus obtained is utilised to project the water into the outlet pipe and up to the highest level of the system. In the semi-rotary pump a lifting force is obtained by means of the see-saw motion of a plunger with two valves working alternately. Another type of high-speed pump consists of two small gear wheels in mesh with one another in an enclosed space, with just sufficient space for their free revolution. The water is lifted and forced upwards by the intermeshing teeth acting in the enclosed as pistons.
RADIATORS
To enable the same water to be used continuously with little loss from evaporation, it must be cooled by some means; or in other words, the heat from the explosion in the cylinder absorbed by the water must be dissipated before that water can be used again, and the more perfect this dissipation is, the less body of water is required to carry on the car. To dissipate heat quickly, it is necessary to provide a large surface for the cooling medium to act on. As the air is the most con-
