192 



OF THE EBB AND FLOW OF THE SEA. 



THE investigation of the causes of the ebb and flow of the 

 sea, attempted by the ancients and then neglected, resumed 

 by the moderns, but rather frittered away than vigorously 

 agitated in a variety of opinions, is generally with a hasty 

 anticipation, directed to the moon, because of certain cor 

 respondences between that motion, and the motion of that 

 orb. But to a careful inquirer certain traces of the truth 

 are apparent, which may lead to surer conclusions. Where 

 fore, to proceed without confusion, we must first dis 

 tinguish the motions of the sea, which, though thought 

 lessly enough multiplied by some, are in reality found to 

 be only five ; of these one alone is eccentric, the rest 

 regular. We may mention first the wandering and various 

 motions of what are called currents: the second is the 

 great six-hours motion of the sea, by which the waters 

 alternately advance to the shore, and retire twice a day, 

 not with exact precision, but with a variation, constituting 

 monthly periods. The third is the monthly motion itself, 

 which is nothing but a cycle of the diurnal motion periodi 

 cally recurring: the fourth is the half monthly motion, 

 formed by the increase of the tides at new and full moon, 

 more than at half moon : the fifth is the motion, once in 

 six months, by which, at the equinoxes, the tides are in 

 creased in a more marked and signal manner. 



It is the second, the great six-hours or diurnal motion, 

 which we propose for the present as the principal subject 

 and aim of our discourse, treating of the others only in 

 cidentally and so far as they contribute to the explanation 

 of that motion. 



First then, as relates to the motion of currents, there is 

 no doubt that to form it the waters are either confined by 

 narrow passages, or liberated by open spaces, or hasten as 

 with relaxed rein, down declivities, or rush against and 

 ascend elevations, or glide along a smooth level bottom, or 

 are ruffled by furrows and irregularities in the channel, or 

 fall into other currents, or mix with them and become sub 

 ject to the same influences, or are affected by the annual or 



