116 OF THE GENERATION AND [SECT. in. 



the fire, the object being to increase the action of the fire without hastening the 

 smoke too rapidly along the flues. By means of the formula in art. 172. we 

 easily compute the area of the spaces between the grating under these circum- 

 stances. For coals, the quantity for generating the steam of a cubic foot of water 

 is 2000 cubic feet, the temperature not less than 800, and the height producing 

 the motion being A feet, we have 



70 



-= = the area of the spaces ; 



V " 



and, the bars being usually equal in thickness to about three times the space 

 between them, 



'280 



= = the area of the grating for coals in inches ; 



2 



or = = the area in feet for one horse power. 

 \/ h 



When the height from the ash-pit, to where the smoke enters the chimney, is 

 four feet, then the area is one foot ; and one foot of area of grate for each horse 

 power is the common rule of practical engineers. 



The proportion of aperture to the solid part of the bar is not always the same, 

 but it ought to be about in the proportion above stated, as air expands to nearly 2^ 

 times in bulk while in the fire. 



199. For burning wood and peat the area of the grate must be 



for each cubic foot of water converted into steam ; where h is the depth of the ash- 

 pit in feet ; the increased area being gained by increasing the size of the bars. 



OF THE SURFACE OF BOILER TO RECEIVE THE EFFECT OF THE FIRE. 



200. The surface of boiler to produce a given effect must be sufficient to 

 receive the heat which will produce the supply of steam ; and as fire or bottom 

 surface is the most effectual, that kind of surface should be of sufficient area to 

 receive the whole effect of the fire ; while the flue surface, or sides, may receive the 

 effect of the smoke. Hence we have an easy mode of determining the proportions. 

 The mean heat of a close fire-place may be considered T ; and if t be the tem- 

 perature of the steam, and s the bottom surface, then the heat of conversion of 

 water to steam being 1000, added to its temperature less 52, we have, from an 



