Piston friction is much the largest item of mechanical loss in an engine, amounting to fully one-half the indicated horsepower at light loads. Although opinions differ as to the most desirable temperature of the jacket-water for full-load operation, no question has arisen as to that for part load. It should be as high as possible, in order that piston friction can be reduced by keeping down the viscosity of the oil on the bearing surfaces, and that complete vaporization of the fuel may be secured. By reducing the friction of the piston and improving the vaporization, steam-cooling increases economy, which, on a number of cars of different makes, has been found to average 20 per cent more miles per gallon. Water is practically a non-conductor of heat. Boiling water, or a mixture of water and steam, is far more effective for cooling than is water that is not boiling.