54 Perkins' 1 Steam Engine. 



caloric will not descend in water, it cannot be taken up by 

 the water which is below it. The steam thus surcharged, 

 will heat the upper surface of the boiler, in some cases red 

 hot,* and will ignite coals, or any other combustible matter 

 which may be in contact with it. If the water which is 

 kept below the surcharged steam, by the pressure of it, 

 should, by any circumstance, be made to take up the excess 

 of caloric in the steam, as well as that from the upper part 

 of the boiler, which has become heated above the tempe- 

 rature of the water, in consequence of the water having been 

 allowed to get too low, it will instantly become highly elas- 

 tic steam, and an explosion cannot be prevented by any 

 safety valve hitherto used. To show how the water may be 

 suddenly brought in contact with the over-heated parts of the 

 boiler, as well as the surcharged steam, it will be neces- 

 sary to state the following facts. 



As long as water is not heated above 212 degrees, it 

 will simply boil, and give off atmospheric steam, without the 

 water having any tendency to rise with it ; but as it becomes 

 more and more elevated in temperature, its disposition to rise 

 with the steam becomes more and more apparent. As the 

 steam presses on the surface of the water, in the same ra- 

 tio as the water increases in temperature, it only boils with- 

 out rising, as when at atmospheric pressure; but if the 

 steam should be drawn off faster than it is generated, this 



* Mr. Moyle, a practical engineer from Cornwall, gave me the fol- 

 lowing interesting fact : 



On going into his boiler room, he observed a ladder, the foot of 

 which rested on the top of his boiler, to be in flames : he instantly as- 

 certained that the top of the boiler, from some cause which he was 

 then unable to determine, had become red hot ; with all possible prompt- 

 itude he ordered the fire to be quenched, which probably saved his 

 premises and perhaps his life. Mr. Moyle found, upon examining the 

 boiler when cold, that very little water remained in it. 



A stronger case still, was that of an explosion at the iron foundry 

 at Pittsburg, North America. As is the practice in North America, a 

 high pressure engine, of sixty or eighty horse power, was supplied 

 with steam from three separate cylindrical boilers, each being thirty 

 inches diameter, and eighteen feet long. One of these boilers had 

 for some time been observed to be getting red hot ; but, as the other 

 two supplied a sufficiency of steam for the work then doing, it was 

 disregarded, until it exploded. The main body of the boiler separated 

 from one of its ends, at an angle of 45 degrees, and passed off like 

 a rocket through the roof of the building, and landed about 600 feet 

 from it. 



