lb6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1666. 



give an account of the menstrual periods from the earth's carrying the moon 

 about itself as Jupiter doth his satellites, which together with them is carried 

 about by the sun as one aggregate ; and that the earth with its moon is to be 

 supposed in like manner to be carried about by the sun as one aggregate, cannot 

 be reasonably doubted, by those who entertain the Copernican hypothesis, and 

 do allow the same of Jupiter and his satellites. But though he would thus 

 have the earth and moon looked upon as two parts of the same moved aggre- 

 gate, yet he does still suppose, as Galilaeo had done before him, that the line 

 of the mean motion of this aggregate, (or, as he calls, motus equahilis et viluti 

 medius), is described by the centre of the earth, about which centre he supposes 

 both its own revolution to be made, and an epicycle described by the moon's 

 motion, not by another point, distinct from the centres of both, about which, 

 as the common centre of gravity, as well that of the earth as that of the moon, 

 are to describe several epicycles. And for that reason fails of giving any clear 

 account of this menstrual period. And in like manner, he proposes the con- 

 sideration as well of the earth's aphelium and perihelium as of the equinoctial 

 and solstitial points, in order to find a reason of the annual vicissitudes ; but 

 does not fix upon any thing in which he can acquiesce : and therefore leaves it 

 in medio as he found it. 



It had been more agreeable to the laws of statics, if he had, as I do, so con- 

 sidered the earth and moon as two parts of the same moveable, (not so, as he 

 doth, alium in centra et sequentem precise revolutionem axis, alium remotius ac 

 velut in circumferentia, but) so as to make neither of them the centre, but both 

 out of it, describing epicycles about it : like as, when a long stick thrown in 

 the air, whose one end is heavier than the other, is whirled about, so as that 

 the end which did first fly foremost becomes hindmost; the proper line of mo- 

 tion of this whole body is not that which is described by either end, but that 

 which is described by a middle point between them ; about which point each 

 end in whirling describes an epicycle. And indeed, in the present case, it is not 

 the epicycle described by the moon, but that described by the earth, which 

 gives the menstrual vicissitudes of motion to the water; which would, as to this, 

 be the same if the earth so move, whether there were any moon to move or 

 not; nor would the moon's motion, supposing the earth to hold on its own 

 course, any whit concern the motion of the water. 



But now, after all our physical or statical considerations, the clearest evidence 

 for this hypothesis, if it can be had, will be from celestial observations, as for 

 instance, (see fig. 5) supposing the sun at S, the earth's place in its annual orb 

 at T, and Mars (in opposition to the sun or near it) at M, from whence Mars 

 should appear in the zodiac at 7, and will at full moon be seen there to be; 



