STEAM. 



In general, it may be stated that the strongest coals, meaning by 

 that term those which have the greatest evaporating power, are 

 those which are richest in carbon. 



The practical evaporating power of a given species of coal varies 

 with the form, construction, and magnitude of the furnace and 

 boiler. That portion of the heat which does not reach the water 

 is dissipated in various ways. A part of it is lost by radiation 

 from the grate ; a part by radiation from the boiler ; a part is 

 carried by the heated gases of combustion into the chimney. The 

 first two sources of waste of heat are reduced to a very small 

 amount by a variety of ingenious contrivances. But the last is 

 indispensable to the maintenance of the combustion, and ought to 

 be considered as the power by which the furnace is worked, rather 

 than a waste of heat. 



20. The grate upon which the fuel is placed is surrounded on 

 every side by parts of the boiler within which water is contained. 



In some boilers, even the ash-pit is a part of the surface of the 

 boiler under which there is water. In this case, all the heat 

 radiated from the grate, and the fuel upon it, is transmitted to 

 the boiler ; and in all cases the furnace is surrounded on every 

 side, except the bottom of the grate or ash-pit, with surfaces 

 having water within them. 



21. The waste of heat by radiation from the surfaces of the 

 boiler, steam-pipes, cylinder, and other parts of the machinery in 

 which steam is contained, or through which it passes, is dimi- 

 nished by various expedients, which in general consist in 

 surrounding such surfaces with packing, casing, or coating, 

 composed of materials which are non-conductors, or at least very 

 imperfect conductors of heat. 



In some cases the boiler is built round in brick work. In 

 Cornwall, where economy is carried 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 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. 

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