An exaggerated idea of the differences in temperatures north and south of the equator led Robert,[726] and after him Roger Bacon,[727] to doubt the validity of the theory of the precession of the equinoxes. This phenomenon would inevitably produce a gradual shifting of the climatic conditions of the southern hemisphere to the northern, and, as a result, the latter would presumably in the course of time become uninhabitable. Since this seemed incredible to Grosseteste and Bacon, they were impelled to deny the possibility of its cause. _The Polar Regions_ In discussing the climate and habitability of the polar regions,[728] Robert cites a work, _De vegetabilibus_ (erroneously ascribed to Aristotle in the Middle Ages) and a commentary upon it. Here the extraordinary view was expressed that no plants or animals could survive in the polar zone because the heat of the sun would burn them up! This view originated in the known fact that the sun shines continuously for half the year at the pole and at no time sinks very far below the horizon. The commentator pointed out that the sun never retires more than 23° out of sight and that it is capable of illuminating and heating the atmosphere at 18° below the horizon. The theory, however, failed to take into account the very important fact that the sun’s rays reach the polar regions at a sharply oblique angle and that consequently their powers of generating heat are limited. This circumstance together with the “observations and reasoning of Aristotle, Ptolemy, and other authorities” led the Bishop of Lincoln conclusively to reject the singular theory of the _De vegetabilibus_ and to attribute to the polar zones a climate that, in so far as it was dependent upon the disposition of the heavens, rendered these regions uninhabitable on account of the cold. Nevertheless, he recognized that there might be accidental local conditions, such as the presence of mountains of peculiar shape, capable in the polar regions of producing areas of intense heat or of delightfully temperate climate. But to this subject we shall revert in a later section devoted to the influence of mountains on climate.[729] CHAPTER VII THE ATMOSPHERE