Popular Science Monthly/Volume 30/April 1887/A Remarkable Explosion
By Professor L. R. F. GRIFFIN.
MODERN industrial operations necessarily employ great quantities of powerful explosives, of which gunpowder and some of the forms of nitroglycerin are the most important. Nitroglycerin, for convenience in handling, is now commonly absorbed, by Richmond infusorial earth, and is then known as dynamite. The use of these substances is not confined to the country, where they can be stored with comparative safety, but many engineering operations in cities require their aid to secure economical construction. This often necessitates their storage in considerable quantities, so that it becomes a source of danger. Special precautions are necessary to reduce the danger as much as possible, and to confine the effects of any accidental explosion within the narrowest limits. Usually, making the buildings used as magazines low, with strong walls and very light roofs, has been considered sufficient. Then, if explosion comes through accident, the explosive material spends its force upward, and the only damage to neighboring property arises from the shock given to the air. This plan may have been ample protection when gunpowder alone was stored, but the large substitution of dynamite in blasting has led to storing that explosive in the magazines, and a recent occurrence dangerously near Chicago has shown that it is by no means sufficient.
On Sunday morning, August 29th, Chicago and places in its immediate vicinity were startled by a sudden jar, followed by a dull thud, as of a distant gun of large size. It was sufficiently violent to shake buildings six miles distant, so that, although a very severe thunder-storm was occurring at the time, guests in some of the hotels rushed frantically down-stairs, suspecting an earthquake. Plaster fell in the Immanuel Church, more than five miles away, so that it was at first supposed to have been struck by lightning, and a large plate-glass window in the Board of Trade building, about seven miles distant, was cracked, and the clock on its tower was put back three seconds. An examination showed that Laflin & Rand's powder-magazine, one of a group of eleven, standing on a comparatively open area of some forty acres, about a mile and a half west of the village of Brighton, seven miles distant from the center of the city, had exploded, being struck by lightning. It contained at the time some fifty tons of ordinary gunpowder and fifteen tons of dynamite. The brick walls of the building were pulverized and scattered over a wide area; the limestone foundation was torn up, and a large part of the material broken into small pieces, the most of it carried through the air from forty to eighty rods; and a hole was torn in the ground, there mainly tough clay, about one hundred and fifty feet long, forty feet wide, and from ten to twenty feet deep. All the buildings in the immediate vicinity were demolished, while those which stood within reach of the flying stones were more or less riddled. The loss of life was very small, considering the extent of the damage, only one person having been killed outright, although several others were severely injured, of whom some have since died.
Such are the primary facts connected with the explosion itself. An examination of the ground in the vicinity, and of many of the buildings ruined near by, together with others at considerable distance more or less injured, has developed certain minor facts that bear upon the general subject of explosions. Especially do they seem to show that such explosions may produce an earth-wave which may do damage at great distances, the undulation of the ground displacing objects, cracking walls, and shattering glass much like an earthquake in miniature. Sometimes this may possibly prove the source of the principal destruction.
Looking directly at the destruction itself, the results of the explosion appeared as follows: The buildings nearest the wrecked magazine were all crushed together, and, so far as could be determined from the ruins themselves, were pushed away bodily from the demolished building for a short distance, not more than one or two feet. This shows that the explosives instantly produced a very large volume of gas, which forced itself against the surrounding air, and condensed it very quickly, until it gave way in the direction of the least resistance, which would necessarily be upward. This condition was confined to a small circle, for, while such a condensation would produce a wave of air, the mass bodily displaced must be confined within comparatively narrow limits. Displacement would not appear beyond. Fortunately, at Brighton, no other magazine stood within this area, so that the dynamite in the others was unaffected by the shock, while the rain prevented the fire from spreading by means of dry powder. Outside of this area there was a narrow ring or circular strip of ground with a radius of not far from fifteen rods where comparatively little injury was done. One or two magazines stood in this region, and they escaped almost without injury, only being slightly battered by flying stones. Here the air was not moved as a mass either way. The changes of density to which it was subjected were of the nature of molecular movements rather than motion of any great mass of air. Beyond that area the movement of the air was toward the point of the explosion. This was shown by the forcing of the glass outward in all of the more distant buildings, while the walls of at least one dwelling-house and of several of the magazines left standing were thrown down toward the wrecked magazine. Furthermore, tbe roof of one magazine was clearly lifted, and allowed to drop, besides being riddled with stones. These phenomena pointed clearly to diminished pressure of the external air produced by the explosion, as is noticed in a small way when any gun is fired. Since most of the magazines stood in this region, no blow was struck upon them, and there was nothing to explode the dynamite stored within, else the first explosion would have been followed by others in a series, and the damage multiplied.
These phenomena taken together seem to indicate the following as the steps by which the destruction was produced, though they followed so quickly that only delicate instruments would have distinguished them: First, the lightning exploded some of the black powder. The blow produced by this explosion detonated the dynamite, tearing up the ground to make the hole and breaking the foundation-stone into small pieces. Then the rest of the powder exploded, sending the fragments away in all directions.
It is very strange that when the danger from lightning is so well known—one of the same group of magazines was exploded by lightning in 1880—no precautions are taken by the owners for protection. The magazines are low structures, some of them roofed with slate, others with thin metal, in all cases very light, that they may offer but little resistance in case of explosion. The total neglect of precautions against lightning indicates a disregard of the known laws of electricity, or else the mistaken notion that a lightning-rod, by furnishing a good conductor, attracts the lightning, and thereby increases the danger in place of being a safe path for the current. When such buildings stand upon level ground, in open areas, they necessarily become the path of any descending flash. If the electricity goes through the building it becomes a source of danger, because it is likely to meet sufficient resistance to raise the temperature above the igniting-point of powder, and it must be carried completely around the powder to insure safety. A network of metal rods carried over the top of those whose roofs are slated, and given a sufficient ground connection, would be a complete protection; it would carry away all the electricity, usually silently. To protect those with metal roofs, nothing more would be required than wide strips of metal from the roof itself to the ground. Of course, in either case, great care must be taken to prevent scattering powder on the ground within reach of the electricity as it leaves the conductors. The problem of protection in this case has sometimes been compared with that of the protection of tanks in which petroleum is stored. This is a complete misconception. Protection of powdermagazines simply requires a proper conductor to carry off the electricity, 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 glass near him was cracked at the moment when he felt the shock, not when he heard the report, which was a little later. This shows that the earth-wave moved faster than the air-wave which produced the sound. There is also reason to believe that this earth-wave traveled much farther than the air-wave. A self-recording barometer in the laboratory of Lake Forest University, about thirty miles distant from Brighton village, showed a sudden movement of the mercury at about that time, which could be accounted for only by referring it to the wave of the explosion. Probably this was not the limit of the movement. Unfortunately, there were no means of determining the rate at which this earth-wave moved. All these conditions combine to surround this remarkable explosion with peculiar interest.