[Footnote 368: Another notion, less easily explicable and less commonly entertained, but interesting for its literary associations, was the notion of a mountain of loadstone in the Indian ocean, which prevented access to the torrid zone by drawing the nails from ships and thus wrecking them. This imaginary mountain, with some variations in the description, is made to carry a serious geographical argument by the astrologer Pietro d' Abano, in his book _Conciliator Differentiarum_, written about 1312. (See Major, _Prince Henry the Navigator_, p. 100.) It plays an important part in one of the finest tales in the _Arabian Nights_,--the story of the "Third Royal Mendicant."] [Sidenote: Going downhill.] Besides this dread of the burning zone, another fanciful obstacle beset the mariner who proposed to undertake a long voyage upon the outer ocean. It had been observed that a ship which disappears in the offing seems to be going downhill; and many people feared that if they should happen thus to descend too far away from the land they could never get back again. Men accustomed to inland sea travel did not feel this dread within the regions of which they had experience, but it assailed them whenever they thought of braving the mighty waters outside.[369] Thus the master mariner, in the Middle Ages, might contemplate the possible chance of being drawn by force of gravity into the fiery gulf, should he rashly approach too near; and in such misgivings he would be confirmed by Virgil, who was as much read then as he is to-day and esteemed an authority, withal, on scientific questions; for according to Virgil the Inhabited World descends toward the equator and has its apex in the extreme north.[370]