Page:Popular Science Monthly Volume 2.djvu/133

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fused the lead-pipe connections, and set fire to the gas. In another instance, that of a church, provided with a lightning-rod, a lightning discharge left the rod at a point in close proximity to the gas-pipes, ignited the gas in the vestry, and the church was consumed. In a third case, the discharge descended a rod on a church-steeple, and, when within five feet of the ground, left the conductor, pierced a wall four feet thick, and disappeared in the gas-pipe under the floor of the church. Sillimau's "Physics" gives a similar example, where, in a church in New Haven, the lightning has twice penetrated a twenty-inch brick wall at a point opposite a gas-pipe twenty feet above the earth, although the conductor, of three-quarter-inch iron, was well mounted, but its connection with the earth was less perfect than that of the gas-pipe.

It being established that the lightning will take the easiest track into the ground, it follows, from what occurred in each of the above cases, that the least-obstructed path was by way of the gas-pipes, with their extended ground connections. In the first example, although there was a lightning-rod on the chimney, the lightning took to the rope, and, instead of leaving it at the lower end for the rod, which was near by, found an easier passage through the air to the gas-pipes of the cotton-factory, which differed from the rod in having an extensive ground contact. The same was true in the other cases the gas-pipes furnishing a readier path to the ground than the rods themselves. On account of the great surface contact with the ground, which gas and water pipes present, it has been recommended that lightning-rods be connected with these, as affording an excellent means for the escape of the electric discharge. At first glance, this might seem a dangerous expedient so far as gas-mains are concerned, the accidents above mentioned pointing to the danger of setting fire to the gas. This accident arose from the use of lead-pipe in making the connections with the meter. Had the gas-pipe throughout been of iron or brass, nothing of the kind could have occurred. Unmixed with atmospheric air, gas will not burn, and it was only through the melting of the lead-connections by the lightning that the gas was liberated and then ignited. Brass or iron pipes would have carried off the discharge without becoming fused, no gas would have been liberated, and no fire could have occurred. Commenting upon these accidents, Mr. Wilde says: "In my experiments on the electrical condition of the terrestrial globe, I have already directed attention to the powerful influence which lines of metal, extended in contact with moist ground, exercise in promoting the discharge of electric currents of comparatively low tension into the earth's substance, and also that the amount of the discharge from an electro-motor into the earth increases conjointly with the tension of the current and the length of the conductor extended in contact with the earth. It is not, therefore, surprising that atmospheric electricity, of a tension sufficient to strike through a stratum of air several hundred yards thick, should find an easier path to the earth by leaping from a lightning-conductor through a few feet of air or stone to a great system of gas or water mains, extending in large towns for miles, than by the short line of metal extended in the ground which forms the usual termination of a lightning-conductor."

But in the country no such system of gas and water pipes is at hand the connection of the rod with the earth must therefore be made in some other way. On this point Mr. Brooks remarks: "Unless a hundred square feet of metal can be laid in the bed of a spring or body of water, I believe the building is safer without the lightning-rod." The advice generally given is to bury the lower end of the rod in charcoal or coke. Prof. Phiu says, use coke, not charcoal; and, "whether iron or copper is employed, it will be well to sprinkle the coke copiously with a strong solution of washing-soda, for the purpose of neutralizing any acids that might corrode the metal. If a trench ten feet long be sunk to the depth of permanent moisture, and filled to a depth of twelve inches with coke, it will be ready to receive the end of the rod, and will furnish a path for all the electricity that will ever tend to escape from the clouds to the earth."


Foul Air.—The condition of the air commonly breathed in the workshop and school-