So-called experts had attempted, with more or less satisfaction to themselves, to answer the question addressed by the mad Lear to poor Tom: "Let me talk with this philosopher. What is the cause of thunder?" Pliny thought he had explained it when he called it an earthquake in the air. Dr. Lister announced that lightning was caused by the sudden ignition of immense quantities of fine floating sulphur. Jonathan Edwards, in his diary of 1722, records the popular impression of the day upon this subject: "Lightning," he says, "seem to be an almost infinitely fine combustible matter, that floats in the air, that takes fire by sudden and mighty fermentation, that is some way promoted by the cool and moisture, and perhaps attraction of the clouds. By this sudden agitation, this fine floating matter is driven forth with a mighty force one way or other, whichever way it is directed, by the circumstances and temperature of the circumjacent air; for cold and heat, density and rarity, moisture and dryness, have almost an infinitely strong influence upon the fine particles of matter. This fluid matter thus projected, still fermenting to the same degree, divides the air as it goes, and every moment receives a new impulse by the continued fermentation; and as its motion received its direction, at first, from the different temperature of the air on different sides, so its direction is changed, according to the temperature of the air it meets with, which renders the path of the lightning so crooked." Even this explanation was a daring bit of speculation in Jonathan Edwards, for thunder and lightning were then commonly regarded as the physical expression of God's wrath against the insects He had created. [Illustration: Franklin Entering Philadelphia.]