Polytechnic Association Proceedings. 713 



has been in a measure superseded by the flue boiler; and this, still 

 uneconomical, by the tubular, in its many varieties. The use of 

 tubes greatly reduced the space occupied; and by subdividing the 

 water and introducing a large amount of heating surface composed 

 of thin iron (tubes), accomplished another very valuable result — 

 economy of fuel. 



The tubular, however, still retains the dangerous shell, intrin- 

 sically weak, owing to its large diameter. It is this shell, contain- 

 ing the whole volume of water and steam, that always, or nearly 

 always, explodes, and usually resulting in fearful havoc. 



After years of use, the diiference in expansion between the shell, 

 fire box, tube sheets, and tubes, works the ends of the tubes loose 

 in the tube sheets, and while they are nearly as good as new, except 

 half an inch at the ends, they have to be thrown aside and replaced 

 b}-^ new ones, because re-expanding them to make them tight has 

 injured the ends. 



It is the shell and fire box that require frequent patching: these 

 repairs constantly weakening the remaining portions of the sheets, 

 as it is well established that a single riveted joint has but fifty-six 

 per cent of the strength of the plate before punching and riveting, 

 and this with the best workmanship. This shows that the tubes 

 are the most durable part of the boiler, and the only damage they 

 suffer is due to irregularity of expansion and contraction, which 

 fault is avoided in the boiler under consideration. Boilers made 

 with shells may bear a high degree of cold water pressure when 

 all parts remain at one temperature; but the application of heat on 

 one side may produce such immense strain by inequality of expan- 

 sion, as to nearly break up the boiler without steam pressure. This 

 explains why some new boilers burst at the first firing, after having 

 borne the cold-water test. The parts of the safety boiler being 

 uniform in length and size, will expand and contract equall}-; but 

 any inequality would have no bad effect, owing to the manner of 

 attaching the parts together. 



This uniformity of parts in size renders the largest boilers as 

 strong as the smallest, which is not the case in the ordinaiy forms; 

 and hence large marine boilers are usually worked at very low 

 pressures, it being regarded as dangerous to cany high pressures, 

 on account of the weakness inseparable from the common forms of 

 construction. 



The principal causes that operate to produce explosions in the 

 old style of boiler, are inequality of expansion, caused by one part 



