A considerable portion of the heat imparted to the water [Pg285] in the boiler escapes by radiation from the surface of the boiler, steam-pipes, and other parts of the machinery in contact with the steam and hot water. The effects of this are rendered very apparent in marine engines, where a large quantity of water is found to be condensed in the great steam-pipes leading from the boiler to the cylinder. In stationary land boilers this loss of heat is usually diminished, and in some cases in a great degree removed, by surrounding the boiler with non-conducting substances. In some cases the boiler is built round in brick work. In Cornwall, where the economy is regarded perhaps to a greater extent than elsewhere, the boiler and steam-pipes are surrounded with a packing of sawdust, which being almost a non-conductor of heat, is impervious to the heat proceeding from the surfaces with which it is in contact, and consequently confines all the heat within the boiler. In marine boilers it has been the practice recently to clothe the boiler and steam-pipes with a coating of felt, which is attended with a similar effect. When these remedies are properly applied, the loss of heat proceeding from the radiation of the boiler is reduced to an extremely small amount. The engine-houses of some of the Cornish engines, where the boiler generates steam at a very high temperature, are nevertheless frequently maintained at a lower temperature than the external air, and on entering them they have in a great degree the effect of a cave. (167.) All mechanical action is measured by the amount of force exercised, or resistance overcome, and the space through which that force has acted, or through which the resistance has been moved. The gross amount of mechanical action developed by the moving power of an engine, is expended partly on moving the engine itself, and partly on overcoming the resistance on which the engine is intended to act. That part of the mechanical energy of the moving power which is expended on the resistance or load which the engine moves exclusively, and of the power expended on moving the engine itself, is called _the useful effect_ of the machine. The _gross effect_, therefore, exceeds the _useful effect_ by the [Pg286] amount of power spent in moving the engine, or which may be wasted or destroyed in any way by the engine.