354 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



and explosions that have arisen from this cause have caused the aban- 

 donment of this method of heating, which is only employed in factories 

 where there is waste steam to utilize. 



A mixed system, which appears to be very advantageous, has been 

 invented by M. Gronvelle, an. engineer. This consists in combining 

 hot water heating with heating by steam. The latter is employed to 

 heat the water contained in stoves fixed on the various flats or rooms 

 of a building, like those described under the head of hot water appar- 

 atus open to the air. Hospitals, prisons, and large public offices can 

 be healthily and economically warmed in this way. 



There remains to be mentioned a method of heating which is useful 

 in certain respects, and for special purposes, if not from an economical 

 point of view, certainly for its cleanliness and handiness, but which 

 can only be made use of in towns. We refer to heating by gas. 

 But here we must not be misled ; there is nothing but a substitution 

 of one fuel for another, and the apparatus invented for the applica- 

 tion of this system have only one object, namely, to make use of a 

 certain number of gas jets to raise the temperature of stoves, cooking 

 ovens, manufacturing apparatus, or the like. We merely mention 

 these different utilizations, without speaking further of a method of 

 heating which is based on the same principles as those in which coal, 

 wood, coke, or turf, are employed as fuel. 



in. Ox FUELS, 



THIS leads us naturally to say a few words on the fuels themselves 

 and their relative value, with regard to the heat they develop; 

 a question by no means unimportant in relation to heating 

 apparatus. 



Wood was the first fuel employed, and even now whole countries 

 use no other. It is certainly one of the most agreeable, but.it is also 

 one of the most, costly, except in countries covered with forests and 

 remote from coal mines. Coal, anthracite, lignite and other mineral 

 fuels, and coke, which is the residue left after the distillation of coal, are 

 more and more frequently employed, as they afford a more economical 

 method of heating. In some countries peat is used ; in fact it is 



