BOOK V

The movements of the atmosphere form the subject of discussion in this part of the treatise. In the first chapter the author seeks for an exact definition of the term “Wind” (_ventus_), and ends by adopting one which is obviously inaccurate--“wind is air flowing in one direction,”--for as he afterwards speaks of whirlwinds he was well aware that the movement may be in every direction, or vorticose. Dismissing the opinion of Democritus as to the origin of wind, he states that in his judgment wind may arise from four different causes. First; The earth itself breathes forth a vast amount of air from its interior, where there are large rivers and lakes, and where the moist air naturally gives rise to blasts of wind. Second; Long-continued evaporation carries the terrestrial emanations aloft, where the intermingling of the breath results in wind. Third; Much more important is the fact that the air in its very constitution possesses an innate power of motion; we cannot imagine that while we ourselves are endowed with a capacity of movement--and water has this power also--the atmosphere should be left inert and immovable (197). Fourth; Sometimes the sun is itself the cause of wind, when he loosens and expands the thick air (198).

In this enumeration allusion is made to one or two features of natural history which the author appears to accept as fact. He thinks there must be some vital force in water, otherwise it could not bring forth animals and plants, as we know it does. But not only water; fire, too, which devours everything, possesses this generative capacity, for, unlikely as it might be thought, it is nevertheless true that fire gives birth to some animals. The air, too, has some vital energy, as it alternately thickens, contracts, and expands, and rids itself of its impurities. The portion of it contained within the earth is asserted in a later part of the volume to be the source of the life of the vegetation at the surface (244).

The local winds, now known as “land and sea breezes,” are next discussed (198). Instead of the simple explanation which in our own day has shown these aerial currents to be beautiful examples of the results of diurnal variations of atmospheric pressure, the ancient theory represented that during the day the exhalations from the land are borne on high to supply the sun with nourishment, while at night, as they are not needed for that purpose, they accumulate until they have filled up a given space enclosed by mountains. When in such a space there is no more room, they move towards the quarter to which they can most easily escape; hence the wind. It is curious, however, to note that Seneca only describes the land breeze, which falls away as the morning advances. He does not specially refer to the equally characteristic sea breeze, which springs up after the other dies down, and continues during the day, until in the evening it is again replaced by the land breeze.

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.

In the fifteenth chapter of this Book a story is told of Philip of Macedon, who sent down a party of miners to examine an old mine. The men brought back to daylight a wonderful tale of vast caverns with high over-arching roofs, and filled with huge rivers and vast lakes. If the author’s intention was to connect the spaciousness of these underground chambers with the operations of ancient miners, he was sadly mistaken, since at no time has metal-mining led to the excavation of huge caverns; on the contrary, it has always been pursued in narrow shafts and passages. If the report brought back to the king was veracious, his emissaries had only come upon a series of natural grottos and tunnels, such as are of common occurrence in limestone districts, and which have no connection whatever with mining.[117] But the narrative served Seneca’s purpose, since it furnished him with the occasion for a diatribe against the cursed love of gold, which had apparently been rampant in days long before those of Philip, and allowed him to supply from his own imagination some additional lurid horrors of the underground world.

[117] It is possible that these ancient mines were driven in search of metal seams or veins traversing limestone, like those of lead among the caverned limestones of Derbyshire.

When he gets back to his subject, he enters upon an enumeration of the various winds known to the ancients. He himself thinks that as the heavens are divided into twelve sections, so there are twelve distinct winds, not all felt everywhere, but never exceeding that number. He does not attempt, however, to account for them. In his reference to the names given to the various winds, he gives a quotation from Ovid’s _Metamorphoses_, in which the more conspicuous winds from the different quarters are mentioned. To this quotation he adds a line from Virgil’s graphic picture of the storm in the first book of the _Aeneid_, where Aeolus opens his cave and the south-east, south, and south-west winds rush out in fury upon the sea. Seneca remarks, in passing, that such a collocation of winds as Virgil enumerates could never have happened in a single tempest. The poet, however, has made no mistake. In a great cyclonic storm the wind veers round with the compass from south-east by south to south-west. And even if Virgil had added the north wind, which the philosopher says he left out, he would only have followed the invariable course of the winds in the cyclones of the northern hemisphere, which circle round towards the north as the storm area is passing eastward.

In conclusion, the author points out the teleological significance of the winds, and is thence led to repeat the time-honoured reproach against human iniquity which turns the winds from their beneficent intention to purposes of war.