180 HOT-WATER BOILERS AND PIPES. 



A circular boiler, 30 inches diameter, with a 9 inch circular 

 flue running round the outside, will expose nearly the same 

 extent of surface to the fire as the one just described, (Fig. 35,) 

 both being the same length, and therefore the one will be as 

 effective as the other ; a slight diminution on the perpendicular 

 length of the curve makes but little difference to its capacity for 

 radiating caloric. 



The surfaces of any size of this shaped boiler can easily be 

 calculated by the same rule ; but, instead of varying in the sim- 

 ple ratio of the length or diameter, it will be found to be propor- 

 tional to the square of the diameter, so that the proportion of 

 surface increases more rapidly than in the arched boiler. Thus, 

 a circular boiler, 24 inches diameter, has 8J square feet of sur- 

 face exposed to the fire ; a 30 inch has 13f square feet ; a 36 

 inch has 19f square feet ; and a 42 inch has 26f square feet 

 exposed to the fire ; the small sizes having proportionally less 

 surface, and the large sizes more than the high-arched boilers. 



The rules which are here given regarding boilers, are framed 

 to suit common occurrence, arid intended to guide practical men 

 who have the management and working of common hot-water 

 apparatus. There are some cases, however, where apparatus 

 of great magnitude is necessary, in which these rules will not 

 apply without modification. But as such instances are com- 

 paratively rare, and, moreover, as no person that is a novice in 

 the practical application of this principle of warming, will be 

 likely to undertake, for his first essay, the responsible erection 

 of an apparatus of great dimensions, it is the less necessary to 

 enter at length into such cases as may be supposed to render 

 any alterations of these principles necessary. 



It may, however, be observed, that cases may occur where a 

 peculiar construction of apparatus may be desirable ; for instance, 

 where, from a large quantity of required surface a furnace of 

 very great power would be necessary ; and, in that case, a boiler 

 which exposes a large surface, while it possesses but a small 

 capacity, would obviously be injudicious, because the intense 

 heat acting on a small body of water would probably generate 

 steam to a high degree of elasticity in the boiler, and not only 



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