QQ PrilLOSOPHICAL TRANSACTIONS. [aNNO 1666. 



casioned by the greatest acceleration or retardation, which the diurnal arch gives 

 to that of the menstrual ; which seems to be the true cause of the daily tides. 

 And withal gives an account, not only why it should be every day, but like- 

 wise why at such a time of the day, and why this time should in a month run 

 through the whole 24 hours, viz. because the moon's coming to the meridian 

 above and below the horizon, or as the seamen call it, the moon's southing 

 and northing, doth so ; as likewise of the spring tides and neap tides. For when 

 it so happens that the menstrual and diurnal accelerations or retardations are 

 coincident, as at new moons and full moons, the effect must needs be the 

 greater. And although this happen but to one of the two tides, that is the 

 night tide at the new moon, when both motions do most of all accelerate, and 

 the day tide at full moon, when both do most retard the annual motion, yet 

 this tide being tlms raised by two concurrent causes, though the next tide have 

 not the same cause also, the impetus contracted will have influence upon the 

 next tide; for a like reason as a pendulum, let fall from a higher arch, will 

 make the vibration on the other side, beyond the perpendicular, to be also 

 greater; or, of water in a broad vessel, if it be so jogged as to be cast forward 

 to a good height above its level, will upon its recoiling, by its own gravity, 

 mount so much the higher on the hinder part. 



But here also we are to take notice, that though all parts of the earth by its 

 diurnal motion do turn about its axis, and describe parallel, yet not equal cir- 

 cles, but greater near the equinoctial, and lesser near the poles; which may be 

 a cause why the tides in some parts may be mudi greater than in others. But 

 this belongs to the particular considerations, not to the general hypothesis. 



Having thus endeavoured to give an account of the diurnal and menstrual 

 periods of tides, it remains that I endeavour the like as to the annual. Of 

 which there is at least thus much agreed, that at some times of the year, the 

 tides are noted to be much higher than at other times. But here I have a 

 double task; flrst, to rectify the observation, and then to give an account of it. 



As to the first, it having been observed that those high tides have used to 

 happen about the spring and autumn ; it has been generally taken for granted 

 that the two equinoxes are the proper times to which these annual high tides 

 are to be referred, and such causes sought for as might best suit with such a 

 supposition. 



But it is now the best part of twenty years since I have had frequent occasions 

 to converse with some inhabitants of Romney-marsh in Kent; where the sea 

 being kept out by great earthen walls, that as at high water not to overflow the 

 level ; and the inhabitants' livelihood depending most on grazing, or feeding 

 .sheep, they are very vigilant and observant at what times they are most in 



