404 The Economic Production of Artificial Heat. 



of a body, the more capable it is of heating another; the 

 capacity, therefore, of that other for heat is practically increased. 

 When the capacity of a boiler for absorbing heat is augmented, 

 its size may be diminished ; and hence, a proper draught will 

 render a smaller boiler sufficient, or will leave more of a larger 

 to take up the residue of the heat after its first violence is 

 expended. For, one square foot of boiling. heating surface, 

 with a suitable draught, will produce as much effect as five 

 square feet of equally efficient surface, when the draught is four 

 times what it should be. The difference between the heating 

 effect of a well-regulated and an excessive draught is very great ; 

 for, while the one will produce a temperature of 3000° and up- 

 wards, the other may not afford one of 2000° : a little seeming- 

 waste, in producing the former is, therefore, real economy. 

 The draught may, however, be so violent, as not to allow time 

 for the heat to be imparted to the boiler ; or it may be so lan- 

 guid as, in certain cases, to be attended with inconvenience, 

 and even danger. Thus, if the temperature of a chimney or 

 flue is not sufficiently high, the carbonic acid which is formed 

 by combustion, and which is one and a-half times as heavy as 

 common air, will flow backwards. By a certain arrangement of 

 an American stove, a small quantity of wood may be kept 

 smouldering for the whole night : this is a convenient way of 

 maintaining a moderate temperature without any trouble or 

 attendance ; but in sleeping apartments it is not unaccompanied 

 by danger. 



Conduction of heat, from the boiler to the surrounding solids, 

 and radiation from its surface, are also causes of heat being 

 wasted. Locomotive boilers are particularly subject to these 

 inconveniences ; and the evil is much greater when the cylinders 

 .are outside. The excellence of what is termed a " Cornish" 

 boiler, consists, almost exclusively, in the care with which it is 

 insulated by felt, brick, and other non-conductors of heat. 



Finally, heat is lost by the passing off of uncombined fuel. 

 This may occur mechanically, from fuel being dropped among 

 the ashes, which must happen if the coal is small, or if the 

 distance between the bars is too great : they must not, how- 

 ever, be so close as to prevent a proper supply of air. Some 

 kinds of coal break down with great rapidity during transmis- 

 sion, and even in the very steamers in which they are used ; 

 and some, from the water which is chemically combined with 

 them, split up and fall to powder when heated. Waste from 

 uncombined fuel may occur also from the production of smoke ; 

 that is, from incomplete combustion. The prevention of visi- 

 ble smoke has for a long time occupied the attention of men 

 of science. It is indispensable to the economical application 

 of fuel ; because smoke consists of carbon and other matters, 



