i62 Theory tf Comhujiion^ applied to the Improvement of Fire-places. 



ilecompofition of the fuel is more complete. This greatly depends on the ftream of air, which 

 pafles through the burning fuel ; that is to fay, upon the velocity of this ftream, which muft 

 be definite, in order to produce the greatcft poflible effect with fuel of any particular kind. 

 Without air there is no combuftion ; a fmall portion of air will maintain the combuftion but 

 feebly ; a greater quantity will give intenfity to the heat, and rapidly decompofe the air and the 

 fuel ; a ftill greater quantity will carry ofF more heat than it can generate, that is to fay, it 

 will blow the fire out *. The current of air, by which a fire is excited, may be made to flow 

 by mechanical means, fuch as bellows ; or by the ftatical efFedl of its own expanfion. This 

 laft is the common proccfs operated by chimnies ; and it is of great pra6lical advantao-e to be able 

 to regulate this current. If we fuppofe the dimenfions of the fire-place, and the heio-ht of the 

 chimney, to be fuch as are beft calculated to produce a rapid combuftion of the fuel, it may, in 

 many inftances, be defirable to moderate that combuftion, by diminifhing the quantity of air; 

 this is done by perfe£lly clofing the door or aperture, through which fuel is conveyed to the 

 fire-place, and by fecuring a command of the other two pafTages, namely, that of the afli-holc 

 under the grate and that of the chimney, by proper regifters, which may be opened to any 

 required extent, or clofed at pleafure. By thefe arrangements, the combuftion is governed by 

 the operator, and may be entirely ftopped by clofing both regifters. The Author obferves, 

 that nearly the fame efiefts as thofe of the chimney regifter may be produced, by caufing the 

 fmolce to defcend feveral feet after it has quitted the fire-place before it afcends again ; and from 

 a late converfation with him, I underftand that this defcent is of the utmoft importance and value in 

 its efFeft. As the cooled air or vapour always occupies the loweft place, it muft follow, that no 

 part of the ftream can pafs up the chimney, till it has imparted fo much of its heat, as to caufe 

 it to defcend to the bottom ; whereas, in the ordinary form of chimnies, it is the moft heated 

 part which flies immediately and rapidly up the chimney. Many fire-places have been greatly 

 improved by this fimple addition. The fize of the fire-place, as well as of the fuel, are both of 

 confiderable importance. The air, which in common conftrudions is fufFered to pafs over 

 the fire, is a thief which carries ofF heat without aflifting the combuftion : a large grate, 

 partly covered with fuel, produces a fimilar efFedt by admitting air through the vicinities, 

 which fimply becomes heated by robbing the boiler, and the reft of the apparatus with which 

 it may come in contaft. The Count recommends a grate in the form of a fegment of a fphere, 

 which affords the advantage of the fuel rolling to the loweft part, as it becomes fmalkr; and 

 he p-ivcs the form of a cone to the pafiage beneath the grate, converging downwards, till the 

 diameter becomes only one third of that of the grate. Inftead of the grate itfelf, he has intro- 

 duced earthen pans perforated with holes to admit the air, and thinks they anfwer even better 

 than the grates themfelves. 



The heat generated in the combuftion of fuel, manifefts itfelf in two ways ; namely, in the 

 hot vapour which rifes from the fire, and in the rays which pafs off in ftraight lines in all di- 

 reflions. It is not known what proportion this radiant heat, as it has been called, bears to that 



* Philofopliical Journal, I. 515 



which 



