What causes the wind? William of Conches made one of the most elaborate attempts in many centuries to answer this,[768] for, though borrowing largely from Seneca, he added some significant observations of his own. In the first place he argued that local winds are produced by various local causes, as, for instance, when air enters a cavern, on account of its _labilitas_, or fluidity, it tends to force out the air already there and thus to make a commotion which generates wind. We may be allowed to suppose here that William has in mind a cavern with two entrances, for it is difficult to understand how such an effect could be produced in a cavern with only one. Similarly, William thought that waters entering the hollows of the earth tend to force out the vapors therein contained and thus to produce blasts and even earthquakes. A damp vapor in rising might cause a wind to blow on account of the removal of its weight (_ex ponderatione sua_). William borrowed the idea that winds may result from the destruction and flattening out of clouds directly from the ἐκνέφτα, or “cloud breezes,” of Aristotle and Seneca. Adelard of Bath also attributed the origin of certain winds to local exhalations of vapors off the surface of land and water. “Marshes and valleys give up a great deal of dense air, which in the natural course of things rises upward; further, when they are loosened, they give back to its natural position much moisture of water which they had previously held imprisoned; add to this that I do not exclude from my statement the actual air which is the content of earth” (Gollancz’s translation).[769] _Atmospheric Circulation_