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| THE OPEN LETTER |
"What is lightning and what causes it?" The question came to us a few days after we had made announcement of a "Weather" number of The Mentor. It was a natural question, for lightning is the most sensational of all weather phenomena. It has always had a fearful sort of fascination for humanity. To the ancients it came as a bolt of wrath from the hand of Jove. To the fire-worshipers it was a warning message. To parched travelers it was a bright promise, for it heralded the coming of rain. To the superstitious it was a signal flash from the spirit world. And to those of nervous temperament it was a highly disturbing phenomenon producing emotions varying from uneasiness and alarm to hysteria. The question then, "What is lightning and what causes it?" has an interest for all. I referred it to Mr. Talman, the author of the Mentor article on "The Weather." His reply follows.
"Not so many generations ago 'natural philosophers' thought that inflammable gases, exhaled from the earth, took fire spontaneously in the air, and that this was lightning. The idea also prevailed—and it is not yet quite extinct—that a stroke of lightning involved the hurling down from the sky of a mass of rock, called a 'thunderbolt.' In the eighteenth century people became quite familiar with the process of generating, by friction, a mysterious something called 'electricity,' which, when it passed from one body to another through a small layer of intervening air, produced sparks. Several philosophers noticed the resemblance between these sparks and lightning. It remained, however, for Benjamin Franklin to prove that lightning was really an electrical discharge on a large scale. The experiments by which he proposed to demonstrate this were successfully performed, first by others, in France, and then, by Franklin himself, at Philadelphia. With the aid of his famous kite he drew down from a thundercloud a little of the 'electrical fluid' (as it was then called), and produced tiny sparks from an iron key at the lower end of the wet kite-string.
"We do not even yet know what electricity is, but we know a great deal about the way it behaves and the effects it produces. There are two kinds of electricity, which we call positive and negative. A body is said to be charged when it has an excess of either kind, and the two kinds have a tendency to unite and neutralize each other's effects. Thunderclouds become heavily charged with electricity. We are not quite sure how this happens, but it is now commonly believed that the strong uprising currents of air that occur in the storm, in the process of breaking up the water-drops in the cloud also separate positive from negative electricity; leaving the former in excess in the part of the cloud next to the earth, and carrying the latter far aloft.
"By a process called 'induction' the positive charge in the cloud draws an excess of negative electricity to the surface of the ground underneath. The stronger the contrast between these opposite charges, the harder they try to break through the interposing barrier of the air (which is a poor conductor of electricity) and to neutralize each other. At length they succeed in doing so. A powerful stream of electricity flows for an instant between cloud and earth. Its passage heats the air and makes it luminous—just as the passage of an electric current heats the filament of an electric lamp and makes it luminous. This is lightning.
"These discharges occur not only between the clouds and the earth, but also, and probably more often, between clouds charged with opposite kinds of electricity.