ME. hood's theory. 



33 



a cubic foot of water, of the temperauu-e of fifty two degrees, into stearo, 

 of which the tension is equal to oiu' atmosphere, would supply the 

 requisite heat to two hundi'ed and thii-ty two feet of pipe, four inches in 

 diameter, when the temperatiu'e is to be kept ai one hundred and forty 

 degrees above that of the smTounding air. The following calcidations 

 showing the surface which a boiler for a hot water apparatus ought to 

 expose to the fii'e, will be found useful.'' 



Surface of Boiler 



exposed to the fire. 



4-in. Pipe. 



3-in. Pipe. 



2-in. Pipe. 



3i square 



feet wiUheat 200 feet 



or 266 feet 



or 400 feet 



H 



300 



400 



600 



7 



400 



533 



800 



Si .. 



500 



666 



1000 



12 



700 



933 



1400 



17 



1000 



1333 



2000 



" A small apparauis ought, perhaps, to have rather more surface of 

 boiler, in proportion to the length of the pipe, than a lai'ger one ; as the 

 fire is less intense, and bm-ns to less advantage in a small than in a large 

 fmnace. It depends, however, upon a variety of circumstances whether it 

 will be expedient to increase the quantity of pipe in propoiTion to the 

 surface of the boiler, beyond what is here stated ; for although many 

 causes tend to modify the efiect, the above calculation will be found a 

 good average proportion, under ordinaiy chcumstances. The effect de- 

 pends greatly upon the quality of the coals, the height of the chimney, 

 the rapidity of the ch-aught, the construction of the fiumace, and many 

 other particulars ; but it will always befoimd more economical, as regards 

 the consimiption of fuel, to work with a larger smtace of boiler at a 

 moderate heat, than to keep the boiler at its maximum temperature." 



As the data from wliich the above calculations are made assumes the 

 difference between the temperatiu'e of the pipe and the space to be 

 heated to be one hundred and forty degrees, the pipe being two himdred 

 degrees and the au* sixty degrees,) it consequently follows that if the 

 temperature be reduced either by a dimimition of heat in the apparatus, 

 or by the ak in the space to be heated being higher, then, the same surface 

 of boiler will be enough for a greater length of pipe. For example, if the 

 chfference of temperature between the water and th3 air be more than 

 one hundi'ed and twenty degrees, then the same smface will heat one 

 sixth more length of pipe, or if the difference of these temperatvires do 

 not exceed one hundred degrees, that one third more length of pipe may 

 be heated by the same means. 



