ON CONSUMPTION OF FUEL AND PREVENTION OF SMOKE. 105 



turns, unite in proving that part of the ceconomy is due to the boiler ; and 

 the proportion of flue surface on the Cornish construction being so much 

 greater, we reasonably infer that the recipient surface of the hemispherical 

 and waggon-boilers is insufficient for the amount of fire-bar surface acting 

 upon it. 



These observations have in a great measure been corroborated by the in- 

 troduction into the Lancashire districts of the cylindrical form with a large 

 circular flue, extending the whole length of the boiler. In this flue the fur- 

 nace is placed, and being confined within certain limits it no longer admits 

 of disproportionate enlargement, but from the very nature of its construction 

 forces old plans and old prejudices to yield to positive improvement. 



The effect of the change is a progressive and improved ceconomy in the 

 consumption of coal, with a larger extent of flue surface, and, what is pro- 

 bably of equal value, a stronger and much more perfect boiler. 



Irrespective of the changes of form and management of boilers which are 

 in progress, it may be proper to notice a still further improvement in con- 

 struction which has recently taken place, and where a still greater ceconomy 

 is effected. This is a mean between the Cornish single flue boiler and the 

 tubular boiler; it is perfectly cylindrical, and contains two circular flues, 

 varying from 2 feet 6 inches to 2 feet 9 inches diameter, extending through- 

 out its whole length, as represented and explained in another place in draw- 

 ings which are annexed. Towards the front end the flues are made slightly 

 elliptical, in order to receive the furnace grate-bars, hearth-plates, &c., to 

 give sufficient space over the fire, and to admit a free current of air under 

 the ash-pit. On this plan it will be observed that each furnace is surrounded 

 by water in every direction, with large intermediate spaces to allow a free 

 circulation of the water, as the globules of heat rise from the radiant surface 

 over the fires and the other intensely heated parts of the flues. Another ad- 

 vantage is the position of the i-eceptacle for the sedimentary deposits, which 

 do not take place over the furnace, as in the old construction, but in the lower 

 region of the boiler, where the temperature is lowest, thus affording greater 

 security from incrustation and other causes of an injurious tendency. 



On the evaporative powers of boilers, it has already been shown, that the 

 process to be conducted with ceconomy depends upon one of two causes, or 

 both J first, on the due and perfect proportions of the furnace; secondly, 

 which is more probable, on the quantity of flue surface exposed to the action 

 of heat : no doubt they are both important agents in the procuration and 

 generating of steam, but the recipient surface is so important, that the mea- 

 sure of all boilers as to their ceconomy and efficiency in a great degree de- 

 pends upon the enlargement of those important parts. Taking, therefore, 

 the amount of the flue surface in a boiler exposed to the passing currents of 

 heat as a criterion of its ceconomic value, we shall then have according to 

 computation a summary of comparison as follows: — 



