The important Etesian or northerly winds, with all their important local modifications in the Mediterranean basin, must have been a subject of constant observation to the Greeks and Romans. There was a general belief that as these winds reappeared regularly in summer, they were in some way connected with the position of the sun in the firmament. Seneca, after briefly stating this opinion, dissents from it on the ground that, as the sun reduces the strength of the morning or land breeze, it cannot be through his influence that the Etesian winds then begin to blow. But he does not explain how he would himself account for their occurrence. They are now known to be further illustrations of the influence of atmospheric pressure. In summer, when the hot region of the Sahara becomes a vast area of low pressure, the air streams into it from the north across the Mediterranean basin. The account given of cloud winds (203) is an excellent illustration of the utter ignorance of the philosophers of antiquity of the very rudiments of meteorology, and, at the same time, of the confidence with which they offered their explanations of the phenomena of the atmosphere. Even now, after prolonged investigation, the laws that regulate the production of furious winds and gusts connected with clouds are far from being fully understood. The boldest meteorologist of to-day, with all his detailed experience, would hesitate to express his opinion as dogmatically as is done in the text. The idea that air accumulating either above ground or below acquires a vast disruptive force, obtained wide credence in early times. It was this pent-up accumulation which was supposed to burst clouds asunder and produce thunder-storms, while the same energy in caverns under ground led to earthquakes and the eruptions of volcanoes. The occurrence of whirlwinds is explained by Seneca from the analogy of eddies in a river. As the water meets with impediments in its flow, it is driven back and made to whirl round before it can continue the onward current, so the wind, as long as it meets with no obstacle, sweeps on, but when it is thrown back by any projection in its course, or is collected together into a highly inclined narrow pipe, it whirls round upon itself like the eddies of a river. But the cause of the vorticose movement where there is no visible impediment is, of course, left unaccounted for.