The squall differs from both of them in its extent, the clouds being more properly rent asunder than broken into pieces. A black cloud is formed, resembling a great animal, an appearance much dreaded by sailors. It is also called a pillar, when the moisture is so condensed and rigid as to be able to support itself. It is a cloud of the same kind, which, when drawn into a tube, sucks up the water[392]. CHAP. 51. (50.)—OF THUNDER[393]; IN WHAT COUNTRIES IT DOES NOT FALL, AND FOR WHAT REASON. Thunder is rare both in winter and in summer[394], but from different causes; the air, which is condensed in the winter, is made still more dense by a thicker covering of clouds, while the exhalations from the earth, being all of them rigid and frozen, extinguish whatever fiery vapour it may receive. It is this cause which exempts Scythia and the cold districts round it from thunder. On the other hand, the excessive heat exempts Egypt; the warm and dry vapours of the earth being very seldom condensed, and that only into light clouds. But, in the spring and autumn, thunder is more frequent, the causes which produce summer and winter being, in each season, less efficient. From this cause thunder is more frequent in Italy, the air being more easily set in motion, in consequence of a milder winter and a showery summer, so that it may be said to be always spring or autumn. Also in those parts of Italy which recede from the north and lie towards the south, as in the district round our city, and in Campania, it lightens equally both in winter and in summer, which is not the case in other situations. CHAP. 52. (51.)—OF THE DIFFERENT KINDS OF LIGHTNING[395] AND THEIR WONDERFUL EFFECTS. We have accounts of many different kinds of thunder-storms. 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[396], 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[397]. 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. Marcia, a lady of high rank at Rome, was struck while pregnant; the fœtus was destroyed, while she herself survived without suffering any injury[398]. Among the prognostics which took place at the time of Catiline’s conspiracy, M. Herennius, a magistrate of the borough of Pompeii, was struck by lightning when the sky was without clouds[399]. CHAP. 53. (52.)—THE ETRURIAN[400] AND THE ROMAN OBSERVATIONS ON THESE POINTS.