HOT-WATER BOILERS AND PIPES. 177 



the iron and the water, and, consequently, the latter does not 

 receive the full quantity of heat. The repulsion between heated 

 metals and water has been ascertained to exist, even at low tem- 

 peratures, being appreciably different at various temperatures 

 below the boiling point of water. But as the temperature rises 

 the repulsion increases with great rapidity ; so that iron, when 

 red-hot, completely repels water, scarcely communicating to it 

 any heat, except, perhaps, when under considerable pressure. 



It is obvious that the extent of surface exposed to the fire 

 should be in proportion to the amount of water contained in the 

 boiler and the pipes ; and it is easy to estimate these relative 

 proportions with sufficient accuracy, notwithstanding the various 

 circumstances which modify the effect. Calculating the surface 

 which a steam boiler exposes to the fire at 4 square feet for each 

 cubic foot of water evaporated per hour, and calculating the 

 latent heat of steam at 1000 degrees, we shall find that the same 

 extent of boiler surface that would evaporate a cubic foot of 

 water, of the temperature of 52, into steam, of which the tension 

 is equal to one atmosphere, would supply the requisite heat to 

 232 feet of pipe, 4 inches diameter, when its temperature is to 

 be kept at 140 degrees above that of the surrounding air. The 

 following proportions for the surface which a boiler for a hot- 

 water apparatus ought to expose to the action of the fire, will be 

 found useful. 



Surface of boiler 

 exposed to the fire. 4 inch pipe. 3 inch pipe. 2 inch pipe. 



3 square feet will heat 200 feet, or 266 feet, or 400 feet. 



5k " " " " 300 " 400 " 600 " 



7 " " " " 400 533 " 800 " 



8i " " " " 500 " 666 " 1000 " 



12 " " " 700 933 " 1400 " 



17 " " " " 1000 " 1333 " 2000 " 



A small apparatus ought, perhaps, to have rather more sur- 

 face of boiler, in proportion to the length of pipe, than a larger 

 one, as the fire is less intense, and acts with less advantage, than 

 in large furnaces. It depends, however, upon a variety of cir- 

 cumstances, whether it will be expedient to increase the quan- 

 tity of pipe, in proportion to the surface of the boiler, beyond 



