168 Hisloi-y of Mechanical hiventions and 



higher temperature than the water. The steam now being in contact with 

 the heated metal, readily takes up the heat and becomes supercharged 

 with it. • Since caloric will not descend in water, it cannot be taken up 

 by the water which is below it. The steam thus supercharged 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 supercharged 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 temperature of the water, in consequence of the 

 water having been allowed to get too low, it will instantly become highly 

 elastic steam, and an explosion cannot be prevented by any safety valve 

 liitherto used. To show how the water may be suddenly brought in con- 

 tact with the overheated parts of the boiler, as well as with the super- 

 charged steam, it will be necessary 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 ; but 



• Practical engineers have frequently witnessed the destruction of the packing of 

 pistons by their becoming charred, although the steam issuing was in contact with 

 the water, the temperature of which did not exceed 230 degrees. It is very evident 

 that this steam was surcharged with heat, and was much above the temperature of 

 the water upon which it was reposing, and in a suitable state to produce explosion, 

 had the water been allowed to rise with the steam, by drawing it oft* faster than it 

 was generated. 



f Mr Moyle, a practical engineer from Cornwall, gave me the following interest- 

 ing 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 ascertained that the top of the 

 boiler, from some cause which he was then unable to determine, had become red- 

 hot ; with all possible promptitude he ordered the fire to be quenched, which proba- 

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

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



The following similar fact has been recently communicated to me by Mr Wil- 

 liams, principal manager of the Dublin and Liverpool Steam Company:— He was 

 alarmed in the night-time, during one of his passages from Ireland to Liverpool, by 

 the strong smell of burning pine, which, after a diligent search, he found to proceed 

 iroro a pine block which had been accidentally thrown on the top of the boiler, and 

 which was discovered to be on fire. 



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

 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 COO feet from it. 



