Page:Popular Science Monthly Volume 30.djvu/836

812 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, the 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