It cannot be denied, that fire proceeding from the stars which are above the clouds, may fall on them, as we frequently observe on serene evenings, and that the air is agitated by the impulse, as darts, when they are hurled, whiz through the air. And when it arrives at the cloud, a discordant kind of vapor is produced, as when hot iron is plunged into water, and a wreath of smoke is evolved. Hence arise squalls. And if wind or vapor be struggling in the cloud, thunder is discharged; if it bursts out with a flame, there is a thunderbolt; if it be long in forcing out its way, it is simply a flash of lightning. By the latter the cloud is simply rent, by the former it is shattered. Thunder is produced by the stroke given to the condensed air, and hence it is that the fire darts from the chinks of the clouds. It is possible also that the vapor, which has risen from the earth, being repelled by the stars, may produce thunder, when it is pent up in a cloud; nature restraining the sound whilst the vapor is struggling to escape; but when it does escape, the sound bursting forth, as is the case with bladders that are distended with air. It is possible also that the spirit, whatever it be, may be kindled by friction, when it is so violently projected. It is possible that, by the dashing of the two clouds, the lightning may flash out, as is the case when two stones are struck against each other.

We have accounts of many different kinds of thunderbolts. Those which are dry do not burn objects, but dissipate them; while those which are moist do not burn, but blacken them. There is a third kind, which is called bright lightning, of a very wonderful nature, by which casks are emptied, without the vessels themselves being injured, or there being any other trace left of their operation. Gold, copper, and silver are melted, while the bags which contain them are not in the least burned, nor even the wax seal much defaced. Among the prognostics which took place at the time of Catiline’s conspiracy, Marcus Herennius, a magistrate of the borough of Pompeii, was struck by lightning when the sky was without clouds.