VOL. v.] PHILOSOPHICAL TRANSACTIONS. 521 



His third thing suggested, the moon's perigaeosis, is so far from being con- 

 trary to my hypothesis, that it is a great part of it. And it is in one of my 

 letters to you expressly mentioned as such. But forasmuch as it does not still 

 fall out at the same time of the day, month, or year, I could not make it a 

 component of any of those noted periods, diurnal, menstrual, or annual. 



The account which I give of the diurnal and menstrual periods (from the 

 common centre of gravity of the earth and moon) he doth allow as very ra- 

 tional : and consequently that any acceleration or retardation of the compound 

 motion of the particular parts in the earth'^ surface, is to give such an accumu- 

 lation of waters as causes a tide ; and the complication of such accelerations and 

 retardations, concurring or interfering from one with another, occasions the 

 perplex varieties in them. 



Now as to the two most signal motions of the e^rth, the diurnal and annual; 

 if we suppose them each in themselves equal, and both perfectly circular and 

 upon parallel axes, though neither of them, singly considered, would give an 

 inequality of motion ; yet the compound of both together, being swiftest at 

 midnight and slowest at noon, would give us two tides in each diurnal revolu- 

 tion; but those always at noon and midnight. 



If to these we add the menstrual, whereby the earth describes a small epicy- 

 cle about the common centre of gravity of the earth and moon : and suppose 

 this also equal in itself and circular, about an axe parallel to the rest; neither 

 would this of itself give any inequality, but compounded with the rest it will, at 

 those times, give us two tides. 



Now because this coming of the moon to the meridian, above and below the 

 horizon, or (as seamen call it) the moon's southing and northing, doth in a 

 month's time pass round the whole circle of 24 hours ; hence it comes to pass 

 that the time of the tides does so also, which I take to be the true account of 

 the menstrual period. And because this composition of the menstrual with the 

 diurnal casts the time at the moon's being in the meridian, and that of the an- 

 nual and diurnal, when the sun is in the meridian ; when both these happen at 

 the same time, as at the full and change of the moon, the tides must needs be 

 the greater; which I take to be the true account of the spring tides and neap 

 tides. And thus far (which is the main of my hypothesis) he concurs with me, 

 as having given at least a very rational and probable account. 



Because it has been almost generally received, that there is an observable an- 

 nual period; I did, for the solving of that, apply not the inequality of the na- 

 tural days, but those causes from whence that proceeds, the eccentricity of the 

 sun or earth's orb, and the obliquity of the zodiac. The former of these, if 

 singly considered, would cast those annual tides in Jqne and December (the 



VOL. I. 3 U 



