tricity, silently if possible, but so completely as to allow no escape in case of a flash. There are no complicating conditions, such as petroleum-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 directly 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 destroyed 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 directions. 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. He was not quite positive which preceded, the motion upward or downward, but he thought that downward. That would indicate that the upward motion 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 injured 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
