VOL. I.] PHILOSOPHICAL TRANSACTIONS. QS 



which he gives us no account. For though he takes notice of a menstrual 

 period, yet he does it only as to the quantity of the tides, greater or less ; not as 

 to the time of the tides, sooner or later. 



To supply this,* Jo. Baptista Balianus makes the earth to be but a secondary 

 planet ; and to move not directly about the sun, but about the moon, the 

 moon meanwhile moving about the sun ; in like manner as we suppose the 

 earth to move about the sun, and the moon about it. 



But this, though it might furnish us with the foundation of a menstrual 

 period of accelerations and retardations in the compound motion of several parts 

 of the earth's surface ; yet I am not at all inclined to admit this as a true hypo- 

 thesis, for divers reasons, which if not demonstrative are yet so consonant to the 

 general system of the world, as that we have no good ground to disbelieve 

 them. For, 1st, The earth being undeniably the greater body of the two, it 

 cannot be thought probable that this should be carried about by the moon, 

 less than itself; the contrary being seen not only in the sun, which is larger 

 than any of the planets which it carries about; but in Jupiter larger than any of 

 his satellites ; and Saturn than his. 2d, As the sun, by its motion about its 

 own axis is with good reason judged to be the physical cause of the primary 

 planets moving about it, so there is the like reason to believe that Jupiter and 

 Saturn moving about their axes is the physical cause of their satellites mov- 

 ing about them; which motion of Jupiter has been of late discovered, by the 

 lielp of a fixed spot discerned in him ; and we have reason to believe the like 

 of Saturn. Whether Venus and Mercury, about whom no satellites have been 

 yet observed, be likewise so moved, we have not yet the like ground to deter- 

 mine : but we have of Mars, from the observations of Mr. Hook, made in 

 February and March last, consonant to the like observations of Jupiter made 

 by him in May l664. Now that the earth has such a motion about its own 

 axis, whereby it might be fitted to carry about the moon, is evident by its 

 diurnal motion. And it seems as evident that the moon has not, because of 

 the same side of the moon always turned towards us ; which could not be if the 

 moon carried the earth about : unless we should say, that it carries about the 

 earth in just the same period in which it turns upon its own axis : which is 

 contrary to that of the sun carr)'ing about the planets ; the shortest of whose 

 periods is yet longer than that of the sun's moving about its own axis. And 

 the like of Jupiter, shorter than the period of any of his satellites. Of Saturn 

 we have not yet any period assigned, but it is likely to be shorter than that of 

 his satellites. And therefore we have reason to believe, not that by the moon's 



♦ Vid. Riccioli Almagest, novum, Tom. I. lib. 4. cap. JO. n. Ill, p. 21 6", 2. 



