A REMARKABLE EXPLOSIOX. 813 



tricity, silently if possible, but so completely as to allow no escape in 

 case of a flash. There are no complicating conditions, such as petro- 

 leum-tanks present. Nothing, either in the material itself or in the 

 air around, makes that a better conductor than neighboring objects. 

 But, in the case of petroleum-tanks, gases are constantly rising from 

 the petroleum and escaping into the air around, and particularly di- 

 rectly above. They frequently rise many feet above the tank, and 

 experience proves that the gas, or the mixture of the air and gas, is a 

 much better conductor than the air itself. So the tank is likely to 

 become the path chosen by any descending flash, and the problem of 

 protection is not simply to furnish a conductor from the top of the 

 tank, but one that shall conduct the electricity from the top of the 

 ascending gas, always an uncertain height. So far, no plan has proved 

 completely successful. 



The phenomena show clearly that two sources of danger arising 

 from such terrific explosions must be guarded against. The glass 

 broken within the first two miles proved a rush of air toward the de- 

 stroyed magazine. The sudden up-rush of gas, the mass very highly 

 heated, caused a vacuum, and the subsequent cooling added to the 

 effect. The air rushed toward that vacuum from all directions, and 

 when it was contained in a confined space, as a closed room, it quickly 

 broke the glass, shattering it into small fragments, which fell outward. 

 But the force which did this work was spent within a comparatively 

 narrow area. Beyond that it only appeared as the back-and-forward 

 movement of an ordinary sound-wave. The distance to which this 

 was carried could not be determined, because beyond some seven or 

 eight miles the report was not distinguished from the ordinary roll of 

 the thunder. 



This explosion produced an earth-wave as well as an air- wave. The 

 force of the dynamite, exerted largely downward, not only tore the 

 ground out to make the hole, but forced it away sidewise in all direc- 

 tions. This formed a ridge around the hole, and at the same time it 

 produced a wave, that is, an up-and-down movement in the earth. One 

 observer, who was sitting quietly in a chair about six miles from the 

 magazine at the time of the explosion, described the sensation which 

 he felt as a quick movement down and up again. lie was not quite 

 positive which preceded, the motion upward or downward, but he 

 thought that downward. That would indicate that the upward mo- 

 tion of the earth was first, since the human body has the sensation of 

 moving in the opposite direction to the motion of the wave, and that 

 agrees with the appearance of the hole. This earth-wave made dishes 

 rattle in all places where it was felt perceptibly. In the central part 

 of Chicago many plate-glass windows were cracked. These were in- 

 jured by the earth-wave, not by the air-wave. They were simply 

 shattered from the motion of the surrounding walls, but were not 

 forced either inward or outward. One observer stated that a pane of 



