HOT-WATER BOILERS AND PIPES. 



179 



Fig. 35. 



long, there will be about 8- square feet of surface exposed to the 

 fire, that is, to its direct action underneath ; and suppose, also, 

 that there are four external flues, one on each side, or sup- 

 posing that the flue went all round the boiler, top and all, we 

 may calculate that nearly one half of the effect is produced by 

 these flues which would have been obtained had the direct action 

 of the fire been employed on a like extent of surface ; therefore 

 the flues will be equal to 5 square feet, making altogether 13| 

 square feet as the available heating surface of a boiler of this shape 

 and size, which we consider far superior to the old form of boiler, 

 as shown in the following cut, (Fig. 36.) A boiler of the size 



Fig. 36. 



here described (Fig. 35) would be sufficient to heat about 800 

 feet of pipe, 4 inches diameter, when the excess of its tempera- 

 ture above that of the surrounding air is 140, as before stated : 

 a boiler of the same shape, 24 inches, has about 11 square feet 

 of surface directly acted upon by the fire ; one 36 inches long 

 has 16 square feet of surface ; and one 42 inches long has 19 

 square feet of surface ; the increase being directly proportioned 



in the simple ratio to the length. 

 IB 



