HEAT OR CALORIC. 95 



being SOOlbs. on the square inch, but when cooled down to the com- 

 mon working temperature, it issues with a roaring noise, so as to be 

 heard ii: U a mile, and powerfully blows a burning brand which it 

 would not do before.* 



(y.) Cause of >h.e explosion of steam boilers. According to Mr. 

 Perkins and Mr. Hazard, of Philadelphia, it is caused mainly by the 

 fact that the boiler, by want of water, becomes heated unduly, and 

 heats the steam excessively ; the water then dashing up in jets, caus- 

 ed by the ebullition, or even by the spontaneous or intentional lifting 

 of the valve, is converted into steam, in such great quantities, that it 

 cannot be retained, and therefore bursts the boiler. A boiler full of 

 steam, without access to water, it is said, may be heated even to red- 

 ness, without explosion, steam being no more expansible than an 

 equal volume of air, but if there be water present to form more 

 steam, then the pressure becomes uncontrolable. Red hot iron boil- 

 ers, by decomposing water, doubtless generate hydrogen gas, when 

 the water is suddenly let in, and this, being incapable of condensa- 

 tion, of course, greatly increases the tendency to explosion, which 

 the boiler, thus rapidly oxidized, is unable to resist. 



STEAM ARTILLERY. 



Mr. Perkins, by applying steam to the propulsion of cannon balls, 

 is able to throw sixty, four-pound balls, in a minute, " with the cor- 

 rectness of a rifled musket, and to a proportionate distance." 



A musket may be made to throw, by means of steam, from one 

 hundred to one thousand balls in a minute, and it is not doubted that 

 a constant stream of balls may be discharged during a whole day,^ 

 if required. From five hundred to one thousand bullets have actu- 

 ally been thrown per minute, the steam, all the while blowing off at 

 the escape valve. f It is said, however, that the range of shot, pro- 

 pelled by steam, is much more limited than if fired in the usual way. 

 Principle of Cupping. 



A cup partially exhausted of air, by burning paper in it,{ and sud- 

 denly applied to the soft parts of the body, allows the flesh to be forced 

 into it, by atmospheric pressure, and after scarification, the renewal 

 of the process, causes the blood to ooze out. The emission of blood, 

 at great heights, as experienced by Humboldt and his companions 

 on the Andes, was probably owing to the prevailing force of vas- 

 cular action, under a greatly diminished pressure, on the surface of 

 the body. 



* Mr. Perkins supposes that heat is matter and that its accumulation at the 

 fice imprisons the steam. 



t Am. Jour. Vol. XIII. pp. 44, 45. 



- Exhausting syringes are said to be now occasionally used. 



