74 



A HISTORY OF 



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 whenever 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 con- 

 trary, 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 mo- 

 tions of the moon. But it appeared, likewise, 

 that when the moon was in the opposite me- 

 ridian, 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 greatest 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 part of the globe directly 

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

 globe directly opposite. 



Mankind continued for several ages con- 

 tent with knowing the general cause of these 

 wonders, hopeless of discovering the parti- 

 cular manner of the moon's operation. Kep- 

 ler was the first who conjectured that attrac- 

 tion was the principal cause ; asserting, that 

 the sphere of the moon's operation extended 

 to the earth, and drew up its waters. The 

 precise manner in which this is done, was 

 discovered by Newton. 



The moon has been found, like all the rest 

 of the planets, to attract, and 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 proportion as the distance, when 

 squared, increases. This being premised, 

 Jet us see what must ensue upon supposing 

 tl le moon in the meridian of any tract of the j 



sea. The surface of the water immediately 

 under the moon is nearer the moon than any 

 other part of the globe is ; and, therefore, 

 must be more subject to its attraction than 

 the waters any where else. The waters will, 

 therefore, be attracted by the moon, and rise 

 in a heap ; whose eminence will be the high- 

 est where the attraction is greatest. In or- 

 der to form this eminence, it is obvious that 

 the surface, as well as the depths, will be 

 agitated ; and that wherever the water runs 

 from one part, succeeding waters must run 

 to fill up the space it has left. Thus the 

 waters of the sea, running from all parts, to 

 attend the motion of the moon, produce the 

 flowing of the tide ; and it is high tide at that 

 part wherever the moon comes over it, or to 

 its meridian. 



But when the moon travels onward, and 

 ceases to point over the place where the wa- 

 ters were just risen, the cause here of their 

 rising ceasing to operate, they will flow back 

 by their natural gravity, into the lower parts 

 from whence they had travelled; and this 

 retiring of the waters will form the ebbing of 

 the sea. 



Thus the first part of the demonstration is 

 obvious ; since, in general, it requires no 

 great sagacity to conceive that the waters 

 nearest the moon are most attracted, or rais- 

 ed highest by the moon. But the other part 

 of the demonstration, namely, how there 

 come to be high tides at the same time, on 

 the opposite side of the globe, and where 

 the waters are farthest from the moon, is not 

 so easy to conceive. To comprehend this, 

 it must be observed, that the part of the 

 earth and its waters that are farthest from 

 the moon are the parts of all others that are 

 least attracted by the moon : it must also be 

 observed, that all the waters, when the moon 

 is on the opposite side of the earth, must be 

 attracted by it in the same direction that the 

 earth itself attracts them ; that is, if I may 

 so say, quite through the body of the earth, 

 towards the moon itself. This, therefore, 

 being conceived, it is plain that those waters 

 which are farthest from the moon, will have 

 less weight than those of any other part, on 

 the same side of Ihe globe; because the 

 moon's attraction, which conspires with the 

 earth's attraction, is there least. Now. there- 



