194 F.BB ANH FLOW. 



and quit again a certain extent of shore, both vary at various 

 hours, and according to the momentum and quantity of 

 the water ; whence these three other motions are rendered 

 more perceptible. 



We must therefore contemplate, singly and specifically, 

 as we purposed, the motion of ebb and flow. And first 

 it is necessary to grant that this motion, the subject of 

 inquiry, is one of these two: either the motion of an^ 

 elevation and depression, or the motion of a progression of 

 the waters. The motion of elevation and depression we 

 understand to be such as is found in boiling water, mount 

 ing and subsiding alternately in a caldron : the motion of 

 progression to be such as is observed in water carried in a 

 basin, which quitting the one side, is projected to the oppo 

 site. Now that the motion we treat of is not of the former 

 sort, is in the first place suggested by this fact, that in dif 

 ferent parts of the world the tides vary according to the 

 times, so that in certain places there are floods and accu 

 mulations of the mass of waters, in others at the same 

 hours ebbs and diminutions. Now the waters, if they did 

 not travel from place to place, but rose ebullient from 

 the bottom, ought to rise every where at once, and to 

 subside together. For we see those two other motions, 

 the monthly and half monthly, in full movement and 

 operation, at the same periods throughout the globe. For 

 the waves increase at the equinoxes in all parts, not in 

 certain places under the equator, or in others under the 

 tropics : and the same is true of the half monthly motion. 

 For every where over the world, the waters are elevated at 

 new moon and full moon, no where at half moon. The 

 waters therefore are manifestly raised, and again depressed 

 in these two motions, and like the heavenly bodies have 

 their apogees and perigees. But in the ebb and flow of 

 the sea, which we now discuss, the contrary takes place, an 

 unequivocal sign of progressive motion. Besides ere we 

 set down the flow of the sea as an elevation of the waters, 

 we ousht to consider a little more carefully how that ele 

 vation can take place. For the swelling must either be 

 produced by an augmentation of the mass of waters, or 

 from an extension or rarefaction of fluid in that mass, or 

 from simple elevation of the mass or body. The third sup 

 position we must dismiss entirely. For if the water united 

 in the same body were lifted up, a vacuum would neces 

 sarily be left between the earth and the under face of the 

 water, there being no body ready to succeed and supply 



