130 OF THE GENERATION AND [SECT. HI. 



common boiler, with scarcely any sensible advantage, or at most not more than is 

 gained by making a flue through the boiler. 



235. The reasons for avoiding the complicated forms of the tubular boilers of 

 Rumford and Woolf require very little illustration. We are certain that if a boiler 

 has the proper quantity of surface and capacity, it will be effective ; and that all that 

 can be done in this respect by a tubular boiler is to obtain these proportions 

 perhaps in a less space : but if a more simple form be afforded, it certainly 

 claims our preference. As to safety there can be no difference, unless the capacity 

 of the cylinder be reduced to less than would contain the proper store of steam : 

 for it is to be recollected that the stress on the larger cylinder is unalterable by 

 either the disposition or the size of the small tubes ; and half the capacity of this 

 cylinder must be capable of holding the store of steam. 



Another objection to these boilers is the necessity of using cast iron ; but of the 

 defect of this material for boilers it will be necessary to treat further in giving 

 the rules for the strength of them. 



236. We have now to consider boilers which have the fire within them. 

 They have been long a favourite species with speculative mechanics, and par- 

 ticularly since the high pressure steam engine was brought into use by Trevithick. 

 It seems a most compact and convenient mode of applying heat, and if we could 

 for a moment forget the current of heat blown up the chimney, one might with 

 some people imagine that the whole of the fire-place being within the boiler it 

 must give out its heat to it alone : such an opinion is however absurd. 



It is also urged that it is safe, because the part exposed to the heat of the fire 

 being within the boiler, when it is destroyed the steam will burst inwardly ; and 

 this is freely admitted to be true, only it imposes the necessity of having a larger 

 boiler, which of course is more dangerous. 



237. The proportions of these boilers will be found to depend on the following 

 circumstances. That part of the area of the tube appropriated to supply air to the 

 ash-pit must be of sufficient size for the purpose ; which determines the diameter 

 of the tube. The area of the grating must be considered, (see art. 198.) and then 

 the length of the tube must be at least sufficient to make its superficial contents 

 equal to the surface required for the fire. (See art. 204.) The capacity of the 

 boiler must next be adjusted, so that deducting the space occupied by the tube 

 containing the fire, the quantity remaining will contain the necessary store of water 

 and steam. (See art. 215.) 



238. If the nature of the application admit of a supply boiler being added, to 

 receive and heat the water required to replace that boiled off, then the internal flue 

 should have only the quantity of fire surface, and the smoke should be returned 



