1824.] On Sleam-Engines. 357 



It became important to ascertain if the safety of the French 

 engines, from their introduction to the present time, be merely- 

 owing to chance, or if it be the necessary consequence of mul- 

 tiplied precautions in their manufacture, and the previous trials 

 to which the boilers are submitted. On this point the folio wins; 

 information has been obtained respecting the cast iron boilers, 

 which are considered as the most unsafe. 



The mean-pressure, condensing engines, on Woolf's con- 

 struction, are those which are made in the principal manufactory 

 in France. With these engines the pressure may be varied 

 from that of one atmosphere to two and a half, or three atmo- 

 spheres, and is indicated by a mercurial gauge. 



The true boiler and boiling pipes in Woolf's engines (which 

 must not be confounded) are made of the purest cast iron. The 

 form of the boiler is cylindrical, its axis being horizontal. 



The thickness of the boilers and boiling pipes of large and 

 small steam-engines, varies from about an inch and a quarter 

 to an inch and three quarters. The diameter of the boiling 

 pipes is much less than that of the boiler ; for small engines it 

 is less than half, for large engines less than one-third of the 

 diameter of the boiler. 



The axes of the boiling pipes are parallel to the axis of the 

 boiler ; they are placed below it, and immediately over the fire- 

 place, in such a way that the flame is in contact with the pipes 

 only. 



As the boiler is less exposed to the fire than the pipes, it is 

 less subject to injury from its action; and if any part give way 

 from that cause, it is the lower part of the pipes and not the 

 boiler; the consequence of which is the inundation of the fire- 

 place, and extinction of the fire, as happened in one of the 

 accidents mentioned above. 



The parts of the engine are united with every possible atten- 

 tion to strength, and to closeness at the joints, so that there 

 may be no loss of power from the escape of steam. 



Before the pipes and boiler are used, they are separately 

 submitted, by a hydraulic press, to five times the pressure that 

 they will have to support when the engine is at work. 



Before any conclusions are drawn from the preceding facts 

 and observations, it may be well briefly to recapitulate them. 



High-pressure steam-engines are employed with most advan- 

 tage. 



1st. Because the greater the compression of the steam, the 

 less is the space the engine occupies. 



2d. Because it produces an equal power to that of a low- 

 pressure engine, with a smaller quantity of fuel. 



But they are considered as more dangerous than low-pressure 

 engines. Nevertheless engines may be constructed, with which 



