Although the old philosophers say that Eurus is warm and dry, and Zephyrus warm and wet, and that Boreas is cold and dry, and Auster cold and wet, some of the doctors declare that neither Zephyrus nor Auster are so, but that Zephyrus is cold and wet, and Auster warm and wet; nor do I know whether they said so with reference to the general nature of the winds, or with reference to the nature of the winds in certain countries, since, we perceive a difference in the winds in various countries, because Eurus and Zephyrus are wet in some countries, and dry in others216. However, I shall relate the generalities concerning the nature of all the winds.
When the air has been heated by the sun it expands and becoming extended, dilates, and a black dark vapour rises from the sea up into the air and is converted into a cloud above, and, when that mist comes in contact with the cold air above, it suddenly contracts, which causes it to flow and dissolve, and converts it into rain. Moreover, when that sea vapour and the air come in contact with each other above in the warm dry atmosphere, and both together are drawn up to the frost region or to the domain of cold, they there become contracted and remain in the atmosphere. It is the nature of the warm air and that of the cold region to be opposed to each other, and they do not endure to remain in the same place, and, consequently, the cold space drives out the air, and being continually expelled, it runs from place to place setting the atmosphere in motion. That motion of the air is the wind, and the greater the cause whence the motion arises, the greater the wind.
Another cause of wind: When a battle or conflict is being fought by large hosts and vast troops, with the movements and panting of the men, some of them fleeing and others in pursuit, the rarefied air flies before them, and raises wind.
If you wish to prove clearly what we have said concerning the rising of the wind into the cold air after it has been heated, take a basin and put water into it to a depth of two or three inches, and place an empty glass vessel in it, and leave it there during the night until morning in some cool place; and in the morning you will find that vessel full of cold condensed air. Turn it mouth downwards in the water which is in the basin, and place them both in some place exposed to the heat of the sun, when it has risen; and when the condensed air in the glass becomes heated, it expands and dilates, and spreads and seeks a larger space, and since it has no way of escape except through the mouth of the vessel down into the water, it goes down into the water, and lifts it up to the mouth of the basin. It appears then like the full tide, gradually
p.201growing until, sometimes, it overflows the basin. It is that which proves that the air which was in the vessel we have mentioned, increases and expands. Leave it so again until the following night, and as the heat of the day departs, and the cold of the night comes, that cold will collect the air that was in the vessel into the same vessel again, and will condense it there and the water will subside into its own place again. Now, since this small volume of air has expanded so much, it is certain that the entire air, or a part of it, increases greatly in its own sphere.