Royal Society. 203 
If the earth were alone, that is to fay, not affe^^ed by the 
a£lions of the fun and moon, it is not to be doubted, but the 
ocean, being equally prefled by the force of gravity to^A/ards the 
centre, would continue in a perfeft ftagnation, always at the lame 
height, without either ebbing or flowing ; but it being here dc- 
monftrated that the fun and moon have a'^like principle of gravi- 
tation towards their centres, and that the earth is within the a61i- 
vity of tlieir attra6tions, it will plainly follow, that the equality 
of the preflure of gravity towards the centre will thereby be dii- 
turbed • and tho' the fmallnefs of thcfe forces, in refpedl of the 
gravitation towards the centre of the earth, renders them altoge- 
ther imperceptible by any experiments we can devife, yet the 
ocean being fluid and yielding to the leaft force, by its rifing 
ihews where it is lefs prefled, and where it is more preflTed by 
its finking. Now if we fuppofe the force of the moon's attrac- 
tion to decreaie, as the fquare of the diftance from its centre en- 
creafes, as in the earth and other celeftial bodies, we Ihall find, 
that where the moon is perpendicular either above or below the 
horizon, in the Zenith or ]<!adir, there the force of gravity is 
moft of all dimin idled, and confequently the ocean muilt there 
neceflarily fwell by the coming in of the water from thoie parts, 
where the prefllire is greateft j viz. in thofe places, where the 
moon is near the horizon. Thus, let M. Fig. 3. Plate VI. be 
the moon, E the earth, C its centre, and Z the place where the 
moon is in the Zenith, N where in the :Nadir : Now by the hy- 
pothefis it is evident, that the water in Z, being nearer, is more 
attra6i:ed by the moon than the centre of the earth C, an J that 
again more than the water in N5 wherefore, the water in Z hath 
a tendency towards the moon, contrary to that of gravity, being 
equal to the excefs of the gravitation in Z, above that in C 5 and 
in the other cafe, the wate'r in N, tending lels towards the moon 
than the centre C, will be lefs prefled, by as much as is the diffe- 
rence of the gravitations towards the moon in C and N. This 
being rightly underliood, it plainly follows, that the^fea, which 
otherwife would be fpherical, upon the prefllire of the moon, 
muft form itlelf into a fpheroidal or oval figure, whofe longcll 
diameter is where the moon is vertical, and ihorteit where flie is 
in the horizon 5 and that the moon Ihifting her pofitjon as (lie 
turns round the earth once a day, this oval of water fliifts with 
her, occafioning thereby the two floods and ebbs obierva'^le in 
each 25 hours. And this may fuffice as to the general caufe of 
the tides : It remains now to ihew how naturally this motion ac- 
counts for all the particulars that have been obferved about them. ; 
C c 2 fo 
