Page:Popular Science Monthly Volume 91.djvu/383

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Popular Science Monthly

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��are visible far out at sea. Dr. Knoche once saw them from a distance of three hundred miles. Something akin to "Andes light- ning" has occasional- ly been reported from other mountainous regions of the world. "Ball" lightning takes the form of a fiery mass (not al- ways globular) , which generally moves quite deliberately through the air or along the ground, and in many cases disappears with a violent detonation. Many ingenious ex- planations of this strange phenomenon have been offered, but none of them is wholly satisfactory. An immense amount of statistical and de- scriptive information concerning ball light- ning has been gath- ered in recent years; notably by Dr. Ignazio Galli, in Italy.

Do Lightning-Rods Help?

Turning, now, to the subject of light- ning-rods, it may be stated emphatically

���that the reputation of these devices has been fully rehabilitated in this country. The Weather Bureau has always strongly advocated their use, and Prof. J. Warren Smith, of that Bu- reau, has recently as- sembled some im- pressive statistics in their favor, compiled from the records of fire insurance com- panies.

Damage by light- ning is comparative- ly rare in towns, where metal roofs, steel frames of build- ings, tall chimneys and other objects constitute an assem- blage of conductors capable of dissipat- ing the electrical charge ofl passing clouds without ' dis- ruptive discharges between the clouds and the earth. On the other hand, light- ning is a thing to be reckoned with in the rural districts. It is estimated that the total property loss from this cause aver-

��A discharge of lightning of some duration, which is photographed as a broad ribbon of fire

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��The correct location and distribution of lightning rods on various kinds of buildings

��ages $>», 000,000 a year in this country; also that about 1,500 persons are struck by lightning in the same period. Nine- tenths of these accidents occur in rural localities.

At present, according to an estimate of the Bureau of Standards, not more than fifteen to twenty per cent of the buildings in the United States which are liable to damage by lightning are protected in any manner against it. Yet, to quote from the same authority, "such evidence as is available on the effectiveness of lightning-rods indicates that, taking rods as they come in the general run of installations, they reduce the fire hazard from lightning by eighty to ninety per cent in the case of houses, and by as much as ninety-nine per cent in the case of barns." In Technologic Paper No 56 the whole question of protection against lightning is

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