Page:Popular Science Monthly Volume 91.djvu/382

 Popular Science Monthly

��torily explained about seven years ago by Dr. George Simpson, of the Indian Meteor- ological Service. The first stage in the pro- duction of a thunder- storm is a violent and turbulent uprush of air, resulting in rapid condensation of mois- ture in the form of the immense clouds that characterize such a storm. The drops of water constituting these clouds are re- peatedly broken up by the air currents, and it has been proved by laboratory experi- ments that this process in- volves the separation of posi- tive from negative electricity. The drops become positively charged; i. e., they retain a greater number of positive than of negative ions. The latter are set free and carried aloft to the upper part of the cloud, giving it a strong nega- tive charge; while the positive ions are carried down with rain. If the process continues long enough, a strong potential gradient is set up between

���A house equipped with properly grounded lightning rods is practically safe from lightning

���cloud and earth, until ultimately the tension is relieved by an electrical discharge. Lightning occurs in many forms and pre- sents many curious features, not all of which are fully un- derstood. Besides the ordinary linear flashes, we have the familiar phenomenon of "sheet lightning," which is, as a rule, merely the reflection on the clouds of dis- tant linear lightning, but may sometimes be an actual diffuse discharge. Dr. Walter Knoche, director of the meteorological service of Chile, has recently described a re- markable form of sheet light- ning that occurs on a vast scale along the crest of the Andes during the warm season. The mountains seem to act as gigantic lightning-rods, giving rise to more or less continuous diffuse discharges between themselves and the clouds, with occasional outbursts sim- ulating the beams of a vast searchlight. These displays

��How a woman was killed when a bolt struck the tin roof of an unprotected house

���The' modern method of fastening lightning rods to buildings. Insulators in clamps are no longer considered necessary except in localities where prejudice in their favor still remains

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