VOL. I.] PHILOSOPHICAL TRANSACTIONS. Ql 



accidental as they have been thought to be, but might from the regular laws of 

 motion, if well considered, be both well accounted for, and even foretold;) 

 these three notorious observations are made of the reciprocation of tides. 



1. The diurnal reciprocation; whereby twice in somewhat more than 24 

 hours, we have a flood and an ebb; or a high-water and low-water. 1. The 

 menstmal ; whereby in one synodical period of the moon, suppose from full- 

 moon to full-moon, the time of those diurnal vicissitudes moves round through 

 the whole compass of the ^^jy^y\^i^oM ^ or natural day of 24 hours; as for instance, 

 if at the full moon the full sea be at such or such a place just at noon, it shall 

 be the next day at the same place somewhat before one of the clock ; the day 

 following, between one and two ; and so onward, till at the new moon it shall 

 be at midnight ; the other tide, which in the full moon was at midnight, now at 

 the new moon coming to be at noon ; and so forward, till at the next full moon 

 the full sea shall at the same place come to be at noon again : Again, that of the 

 spring tides and neap tides ; about the full moon and new moon the tides are 

 at the highest, at the quadratures the tides are at the lowest; and at the times 

 intermediate, proportionably. 3. The annual, whereby it is observed, that at 

 some part of the year, the spring tides are yet much higher than the spring 

 tides at others, which times are usually taken to be at the spring and autumn, 

 or the two equinoxes ; but I have reason to believe, as well from my own ob- 

 servations for many years, as of others who have alike observed it, that we 

 should rather assign the beginnings of February and November, than the two 

 equinoxes. 



Now in order to give account of these three periods, according to the laws of 

 motion and mechanic principles, we shall first take for granted, what is now 

 pretty commonly entertained by those who treat of such matters, that a body 

 in motion is apt to continue its motion, and that in the same degree of cele- 

 rity, unless hindered by some contrary impediment, like as a body at rest is apt 

 to continue so, unless by something acting on it put into motion ; and accord- 

 ingly, if on a board or table some loose incumbent weight be for some time 

 moved, and have thereby contracted an impetus to motion at such a rate ; if that 

 board or table chance by some external obstacle or otherwise to be stopped or 

 considerably retarded in its motion, the incumbent loose body will shoot for- 

 ward upon it; and contrarywise, in case that board or table chance to be acce- 

 lerated or put forward with a considerably greater speed than before, the loose 

 incumbent body, not having yet obtained an equal impetus with it, will be left 

 behind, or seem to fly backward upon it. Or, which is Galilaso's instance, if 

 a broad vessel of water, for some time evenly carried forward with the water in 

 it, chance to meet with a stop, or to slack its motion, the water will dash for- 



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