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 164- 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. 



f) 



