124 



OF THE GENERATION AND 



[SECT. in. 



duced, the loss of energy seems to be in the fuel, and it appears to agree very well 

 with practice to consider the loss proportional to the ratio between the surface and 

 capacity of the quantity of fuel, supposing it to be bounded by similar figures. In 

 this manner the following table is derived. 



When a boiler is made of a larger size than would supply an engine of 30 or 40 

 horse power with steam, it is much better to make two boilers, and to set them 

 side by side, and besides these there should be a reserve boiler to put in use during 

 repairs. That is, for a 40 horse engine I would recommend three 20 horse power 

 boilers ; for a 60 horse engine three 30 horse power boilers, and so on ; and for 

 smaller engines two boilers, each equivalent to the power of the engine. 



OF THE FORM OF BOILERS AS IT DEPENDS ON EFFECTS. 



222. The quantity of fire and of flue surface having been ascertained, and the 

 capacity, the next object is to consider the form of boiler best adapted for ob- 

 taining these proportions in a convenient manner. If we were to consider the 

 strength of the metal alone, they would be nearly spherical, but we well know 

 that a sphere has the least quantity of surface of any solid having the same 

 capacity. 



223. The first boilers used for engines were nearly of a spherical shape. The 

 bottom was next altered to a concave surface, the flue sides were made nearly 

 perpendicular, and the upper part still retained a hemispherical shape. This form 



