incident to Steam Boilers. 127 



use of high-pressure steam has become general : in the former 

 district it is, I believe, universal, and is applied to condensing 

 engines not differing very much from Boulton and Watt's 

 construction, among vi^hich engines are many of enormous 

 power, and the largest in the world. The steam is commonly 

 so as to balance from 15 to 40 pounds on each inch of the 

 safety-valve ; and some difference of opinion exists among the 

 engineers as to the importance of using it at a higher or lower 

 degree of pressure. 



It will be necessary to describe the boilers which have been 

 employed, in order to understand the subject, and to notice those 

 which have been subject to accidents ; which indeed, as far as 

 I know, have been confined to one sort of boiler, — or at least 

 such accidents as have been attended with any fatal or dis- 

 tressing consequences. 



This description of boiler, though appearing therefore to be 

 the most hazardous, is yet most generally adopted ; and as 

 it is believed to have some advantages over others in other 

 respects and under certain circumstances, it will probably 

 continue to be generally preferred, or at least until some con- 

 struction that shall unite these advantages with more perfect 

 security may be brought into use : this, indeed, it will not be 

 very easy to do, as the experiments on boilers have been mul- 

 tiplied to a great extent in Cornwall, and the expense in- 

 curred by many of the mines in this way has been so great, that 

 but few of the managers will probably be inclined to enter 

 upon them again without some very clear prospect of success. 



The steam boilers which I mean to describe as the most 

 common, are those which are constructed by fixing one tube 

 within another: the interior one containing the fireplace, 

 and the space between it and the exterior containing water, 

 and in the upper part steam. This kind of boiler was, I be- 

 lieve, first introduced by Trevithic for his simple high-pres- 

 sure engines : he made the outer tube of cast iron, and the 

 inner one, which was often recurved so as to make a double 

 circuit within, of wrought iron. At present both the tubes are 

 made of wrought iron or rolled plates : the form is simply that 

 of one straight tube passing through the other ; the ends of 

 the boiler fix the tubes together, so that the interior tube is 

 open at both ends ; at one of which is placed the fire-grate, 

 and at the other the smoke and flame pass out, and are con- 

 veyed to the stack or chimney most commonly by flues passing 

 under and along the sides of the outer case. 



The following sketch will show the cross sections : they are 

 commonly froni 20 to 35 feet in length, the diameter of the 

 iimer tube from 3 to 4 feet, and of the outer one from 5^ 



to 



