VOL. XIX.] PHILOSOPHICAL TRANSACTIONS. . I45 



the equality of the pressure of gravity towards the centre will thereby be dis- 

 turbed ; and though the smallness of these forces, in respect of the gravitation 

 towards the earth's centre, renders them altogether imperceptible by any expe- 

 riments we can devise, yet the ocean, being fluid and yielding to the least force, 

 by its rising shows where it is less pressed, and where it is more pressed by its 

 sinking. Now if we suppose the force of the moon's attraction to decrease, as 

 the square of the distance from its centre increases, as in the earth and other 

 celestial bodies, we shall find that where the moon is perpendicularly either 

 above or below the horizon, in zenith or nadir, there the force of gravity is 

 most of all diminished, and consequently that there the ocean must necessarily 

 swell by the coming in of the water from those parts where the pressure is 

 greatest, viz. in those places where the moon is near the horizon. Thus, let 

 M, fig. Q, pi. 3, 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 hypftthesis, it 

 is evident that the water in z, being nearer, is more drawn by the moon than 

 the centre of the earth c, and that again more than the water at n ; therefore 

 the water in z has a tendency towards the moon, contrary to that of gravity, 

 being equal to the excess of the gravitation in z, above that in c : and in the 

 other case, the water at n, tending less towards the moon than the centre c, 

 will be less pressed, by as much as is the difference of the gravitations towards 

 the moon in c and n. This rightly understood, it plainly follows, that the sea, 

 which otherwise would be spherical, by the pressure of the moon must form 

 itself into a spheroidal or oval figure, whose longest diameter is where the 

 moon is vertical, and shortest where she is in the horizon ; and that the mcon 

 shifting her position as she turns round the earth once a day, this oval of water 

 shifts with her, occasioning thereby the two floods and ebbs observable in each 

 25 hours. 



And this may suffice as to the general cause of the tides. It remains now to 

 show how naturally this motion accounts for all the particulars that have been 

 observed about them ; so that there can be no room left to doubt but that this 

 is the true cause of them. The spring tides at the new and full moons, and 

 neap tides at the quarters, are occasioned by the attractive force of the sun in 

 the new and full, conspiring with the attraction of the moon, and producing a 

 tide by their united forces: whereas in the quarters, the sun raises the water 

 where the moon depresses it, and the contrary ; so as the tides are made only 

 by the difference of their attractions. That the force of the sun is no greater 

 in this case, proceeds fro:Ti the very small proportion the semidiameter of the 

 earth bears to the vast distance of the sun. 



It is also observed that, caeteris paribus, the equinoctial spring tides in 



VOL. IV. U 



