The author states that he cannot pursue his calculations further, and condescends to acknowledge that it is very difficult to know accurately what is the thickness of the ninth and of the crystalline heavens! Perhaps, however, these reckonings are better than those of the Egyptians, who came to the conclusion that Saturn was only distant 492 miles, the sun only 369, and the moon 246. These numerous variations and adaptations of the Ptolemaic system, prove what a firm hold it had taken, and how it reigned supreme over all minds. Nor are we merely left to gather this. They consciously looked to Ptolemy as their great light, if we may judge from an emblematic drawing taken from an authoritative astronomical work, the _Margarita Philosophica_, which we give on the opposite page. [Illustration: FIG. 17.] In all the systems derived from Ptolemy, the order of the planets remained the same, and Mercury and Venus were placed nearer to the earth than the sun is. According to many authors, however, Plato made a variation in this respect, by putting them outside the sun, on the ground that they never were seen to pass across its surface. He had obviously never heard of the "Transit of Venus." This arrangement was adopted by Theon, in his commentary on the _Almagesta_ of Ptolemy, and afterwards by Geber, who alone among the Arabians departed from the strict Ptolemaic system. [Illustration: FIG. 18.--EGYPTIAN SYSTEM.] The Egyptians improved upon this idea, and made the first step towards the true system, by representing these two planets, Mercury and Venus, as revolving round the sun instead of the earth. All the rest of their system was the same as that of Ptolemy, for the sun itself, and the other planets and the fixed stars all revolved round the earth in the centre. This system of course accounted accurately for the motions of the two inferior planets, whose nearness to the sun may have suggested their connection with it. This system was in vogue at the same time as Ptolemy's, and numbers Vitruvius amongst its supporters. [Illustration: FIG. 19.--CAPELLA'S SYSTEM.] In the fifth century of our era Martian Capella taught a variation on the Egyptian system, in which he made Mercury and Venus revolve in the same orbit round the sun. In the treatise entitled _Quod Tellus non sit Centrum Omnibus Planetis_, he explains that when Mercury is on this side of the orbit it is nearer to us than Venus, and farther off from us than that planet when it is on the other side. This hypothesis was also adopted in the middle ages. We have here indicated the time of the revolution of the various planets, and notice that the firmament is said to move round from west to east in 7,000 years; the second heaven in 49,000, while the _primum mobile_ outside moved in the contrary direction in twenty-four hours.