THE EARTH. 103 



ally destroys the number of these viler creatures ; its currents and its 

 tides oroduce continual agitations, the shock of which they are not 

 able to endure ; the parts of the fluid rubbing against each other, de 

 stroy all the viscidities ; and the ocean, if I may so express it, ac 

 quires health by exercise. 



The most obvious motion of the sea, and the most generally ac- 

 knowledged, is that of its tides. This element is observed to flow foi 

 certain hours, from south towards the north ; in which motion or flux, 

 which lasts about six hours, the sea gradually swells ; so that entering 

 the mouths of rivers, it drives back the river-waters to their heads. 

 After a continual flux of six hours, the sea seems to rest for a quarter 

 of an hour ; and then begins to ebb, or retire back again, from north 

 to south, for six hours more ; in which time the waters sinking, the 

 rivers resume their natural course. After a seeming pause of a quar- 

 ter of an hour, the sea again begins to flow as before : and thus it has 

 alternately risen and fallen, twice a-day, since the creation. 



This amazing appearance did not fail to excite the curiosity, as it 

 did the wonder of the ancients. After some wild conjectures of the 

 earliest philosophers, it became well known in the time of Pliny, that 

 the tides were entirely under the influence, in a small degree, of the 

 sun ; but in a much greater of the moon. It was found that there was 

 a flux and reflux of the sea, in the space of twelve hours, fifty minutes, 

 which is exactly the time of a lunar day. It was observed, that when- 

 ever the moon was in the meridian, or, in other words, as nearly as 

 possible over any part of the sea, that the sea flowed to that part, and 

 made a tide there ; on the contrary, it was found, that when the moon 

 left the meridian, the sea began to flow back again from whence it 

 came ; and there might be said to ebb. Thus far the waters of the 

 sea seemed very regularly to attend the motions of the moon. But as 

 it appeared, likewise, that when the moon was in the opposite meridi- 

 an, as far off on the other side of the globe, that there was a tide on 

 this side also ; so that the moon produced two tides, one by her great- 

 est approach to us, and another by her greatest distance from us : in 

 other words, the moon, in once going round the earth, produced two 

 tides, always at the same time ; one on the other part of the globe 

 directly under her ; and the other, on the part of the globe directly 

 opposite. 



Mankind continued for several ages content with knowing the gene- 

 ral cause of these wonders, hopeless of discovering the particular 

 manner of the moon's operation. Kepler was the first who conjec- 

 tured that attraction was the principal cause, asserting, that the sphere 

 of the moon's operation extended to the earth, and drew up its wa- 

 ters. The precise manner in which this is done, was discovered by 

 ISewton. 



The moon has been found, like all the rest of the planets, to at' 

 tract aud to be attracted by the earth. This attraction prevails 

 throughout our whole planetary system. The more matter there is 

 Contained in any body, the more it attracts; and its influence de 

 creases in nroportion as the distance, when squared, increases. This 

 being premised, let us see what must ensue upon supposing the moon 

 -a the meridian of any tract of the sea. The surface of the water 



