SECT. m .] CONDENSATION OF STEAM. 117 



experiment, made by Professor Leslie, 1 '828 s (T t) = 948 + t, when one cubic 

 foot of water is to be converted into steam in an hour ; or 



948 + t 



828 (T - 



201. When a mass of fuel is in combustion in a close fire-place, we have 

 shown that it is not desirable for its temperature to exceed 1200, (art. 192.) 

 Now the surface of the boiler must be at some distance from the fuel, to allow it to 

 develope flame, and therefore the heat having a larger surface to act upon its 

 intensity is less, but at a mean ought not to be less than about 800, for coal ; 

 consequently we may insert 800 for T. For low pressure steam t = 225, hence, 



For steam of 300, the force of which is about 40 Ibs. per circular inch above 

 the pressure of the atmosphere, we have 



948 + 300 



= = ' 



These examples will be sufficient to show the increased quantity of surface 

 required for high pressure steam, and we may now proceed to estimate the quantity 

 of side flue. 



202. At an average for coal, 2000 cubic feet of gaseous matter, heated to 

 800, is generated, and required for combustion to produce the above effect ; and 

 the specific heat of air being '00032, its effect will be equivalent to heating a cubic 

 foot of water -00032 x 2000 x (800 - f) = -64 (800 - t). Now it will be suf- 

 ficiently accurate for our purpose to consider the effective excess of temperature to 

 be a little less than the mean between 800 and t ; consequently, 



<) = -64 (800 - ; or S ' = 1-94 feet. 



203. Comparing -64 (800 - f) with 948 + t, we find that the whole energy of 

 the side of flues will amount only to about one-fourth of the effect of the bottom 

 ones ; we may therefore reduce the fire surface found by the rule one-fourth for 

 each cubic foot of water evaporated per hour. The rule will then become 



3 (948 + t ) 948 + t 



4 x -828 (800 - o 1-1 (800 - *)' 



204. But in a steam engine boiler this would barely keep the boiler supplied, 

 whereas it is shown that there should be a capability of supplying steam with 

 double the rapidity actually required, otherwise the pressure on the piston will be 



1 Inquiry into the Nature of Heat. Experiment 51 and 52. 



