514 THE STEAM-ENGINE. 



of being opened or closed, so as to regulate the quantity of fuel supplied to the 

 fire-grate. The fuel dropping in, in small quantities, through this open valve, 

 falls on the grate, and is carried round by it, so as to leave a fresh portion of 

 the grate to receive succeeding feeds. The coals admitted through the hopper 

 are previously broken to a proper size ; and in some forms of this apparatus 

 there are two rollers, at a regulated distance asunder, the surfaces of which are 

 formed into blunt angular points, and which are kept in slow revolution by the 

 engine. Between these rollers the coals must pass before they reach the 

 valve through which the furnace is fed, and they are thus broken and reduced 

 to a regulated size. The valve which regulates the opening through which 

 the feed is admitted, is connected by chains and pulleys with the self-regula- 

 ting damper already described, so that in proportion as the damper is raised, 



the valve governing the feed may be opened. Thus, while the quantity of air 

 admitted by the damper is increased according to the demands of the engine, 

 , the quantity of fuel admitted for the feed is increased by opening the valve in 

 | the bottom of the hopper in the same proportion. Apertures are also provided 

 in the front of the grate, governed by regulators, by which the quantity of 

 air necessary and sufficient to produce the combustion of the gas evolved from 

 the fuel is admitted, these openings being also connected with the self-regula- 

 ting damper. 



A considerable portion of the heat imparted to the water in the boiler es- 

 capes 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 

 iron-conducting substances. In some cases the boiler is built round in brick- 

 work. In Cornwall, where the economy is regarded perhaps to a greater ex- 

 tent 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 consequent- 

 ly 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 exter- 

 nal air, and on entering them they have in a great degree the effect of a 

 cave. 



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 pow- 

 er expended on moving the engine itself, is called the useful effect of the 

 machine. 



The gross effect, therefore, exceeds the useful effect by the amount of power 

 spent in moving the engine, or which may be wasted or destroyed in any 

 way by the engine. 



