VOL. XXIII.] PHILOSOPHICAL TRANSACTIONS. 15 



he gives the way of finding* both the heliocentric and geocentric places of a 

 comet for any given time, and describes tables that are necessary for the more 

 ready performance of it. 



The 6th and last book is upon comparative astronomy ; where he considers 

 the phaenomena that would appear to a spectator placed in each of the heavenly 

 bodies, and compares them with those that are seen from the earth. The chief 

 of those he takes notice of, are their vicissitude and duration of days and nights, 

 the apparent size, intenseness of heat, and light of the sun, the length of their 

 years, and variety of seasons, the phases of the moon, and the eclipses of the 

 luminaries, the number, directions, stations and retrogradations of the planets 

 that are to be seen, and the helps they may afford to discover the true system 

 of the universe. After having given an account of all these appearances in each 

 of the heavenly bodies, he shows that of all the primary planets, the earth is 

 the fittest place where an observer may discover the true system of the world, 

 and Mercury the most unfit for that purpose; and that therefore an inhabitant 

 in any one of the planets may on better reasons suppose his own habitation 

 immoveable, and that the whole heavens turn round about him, than these 

 which our terrestial observers h^ve for their immobility ; and having fewer 

 arguments, it will be harder to convince them of their own motion. We 

 may reasonably then imagine, that an astronomer in any of the other planets, as 

 for example in Venus, would suppose his own habitation immoveable, and would 

 endeavour to make a system to satisfy all phaenomena on that scheme. This, 

 in his proposition on the astronomy in Venus, he says may be effected from the 

 true system two ways: first, if Venus were brought from its place in the trye 

 system down to the centre, and the sun and the rest of the planets, together 

 with their orbits, advanced as far the same way in parallel Imes. Such a system 

 in Venus would be perfectly Tychonic, in which the sun would be made to turn 

 round Venus in an orbit, equal and like to that in which Venus does really turn 

 round the sun, in the true system; and all the planets, together with their 

 satellites, besides their being carried round Venus by the sun, would have all 

 the same motions round the sun, as in the true system. The orbit of the earth 

 in this system would cut the sphere of the sun after the same manner, as in the 

 orbit of Mars it intersects the orbit of the sun in the Tychonic system, which is 

 framed for our earth ; whereas the orbit of Jupiter would not cut it, but surround 

 it. The second way, by which an astronomer in Venus may contrive a system 

 to satisfy appearances, and make himself unmoveable in the centre, is by bring- 

 ing Venus to the centre, and advancing the sun and all the planets just as far in 

 parallel lines, as was done before; but leaving their orbits immoveable, and the 

 very same which the planets described round the sun in the true system. If 



