224 THE POPULAR SCIENCE MONTHLY. 



*' If there are sonie who have not followed me through this simple 

 calculation, they may, perhaps, be able to reach clear views upon the 

 subject by looking at the phenomena in a somewhat different way. It 

 can readily be seen that the sudden development of this large volume 

 of gas, Avhich becomes at once a part of the atmosphere, would be 

 equivalent to a blow by the atmosphere against the rock ; or, w^hat 

 would be a more accurate representation of the phenomenon, since the 

 air is the larger mass, and acts as the anvil, a blow by the rock against 

 the air. It may seem very singular that our atmosphere can act as an 

 anvil, against which a rock can be split, and yet it is so, and, if the 

 blow has velocity enough, the atmosphere presents as effective a re- 

 sistance as would a granite ledge. The following consideration will, 

 I think, convince you that this is the case : I have here a light wooden 

 surface, say, one yard square ; the pressure of the air against the sur- 

 face is equal, as I just stated, to about nine tons ; but the air presses 

 equally on both sides, and the molecules have such great mobility 

 that, when we move the surface slowly, they readily give way, and 

 we encounter but little resistance. If, however, we push it rapidly 

 forward, the resistance greatly increases, for the air-molecules must 

 have time to change their position, and we encounter them in their 

 passage. If, now, we increase the velocity of the motion to the high- 

 est speed ever attained by a locomotive^say, one and one-fifth mile 

 per minute — we should encounter still more particles, and find a re- 

 sistance which no human muscle could overcome. Increase that ve- 

 locity ten times, to twelve miles a minute, the velocity of sound, and 

 the air would oppose such a resistance that our wooden board would 

 be shivered into splinters. Multiply again the velocity ten times, 

 and not even a plate of boiler-iron could withstand the resistance. 

 Multiply the velocity once more by ten, and we should reach the ve- 

 locity of the earth in its orbit, about twelve hundred miles a minute, 

 and, to a body moving with this velocity, the comparatively dense air 

 at the surface of the earth would present an almost impenetrable bar- 

 rier, against which the firmest rocks might be broken to fragments. 

 Indeed, this effect has been several times seen, when meteoric masses, 

 moving with these planetary velocities, penetrate our atmosphere. 

 The explosions which have been witnessed are simply the efiect of the 

 concussion against the aeriform anvil at a point where the atmosphere 

 is far less dense than it is here. So, in the case of the nitro-glycerine, 

 the rock strikes the atmosphere with such a velocity that it has the 

 effect of a solid mass, and the rock is shivered by the blow." 



Chemistry," of the molecular constitution of nitro-glycerine, and the transformations in 

 its combustion, which account for the terrific force of its explosion. — Ed. 



