ASTRONOMY. 



attractive power. But the point A being 

 nearer to the moon than the point C, the 

 attraction at A will be greater than at C : 

 and because the points B and D are at 

 equal distances from the moon, the at- 

 traction at those points will also be equal; 

 and so at any other intermediate points 

 the attractive force will be different, ac- 

 cording to their different distances from 

 the moon. 



From this example, then, it is sufficient- 

 ly evident, that the attractive force of the 

 moon, acting unequally upon different 

 parts of the ocean, must occasion it to 

 assume a different figure from what it 

 would otherwise have, if there were no 

 such unequal attractions. And since this 

 attractive force is greatest on the part of 

 the ocean which lies immediately under 

 the moon, the waters will of course flow 

 constantly to that part, and be elevated or 

 depressed at different places, according 

 as her situation changes with respect to 

 those places. But, as the earth turns 

 round on its axis, from the moon to 

 the moon again, in about twenty-four 

 hours and three-quarters, the flux and re- 

 flux will be necessarily retarded from day 

 to day about three quarters of an hour, 

 which is agreeable to experience. It re- 

 mains now to be explained, why they 

 ebb and flow twice a day, or in the space 

 of about twenty-four hours. When the 

 moon passes the meridian of any place, 

 or is at her greatest height above the 

 horizon of that place, she will evidently 

 attract and elevate the waters which lie 

 immediately under her : but what is the 

 reason, that, twelve hours afterwards, 

 -when she passes the meridian below the 

 horizon, the waters at the same place are 

 then also elevated ? We know, from ex- 

 perience, that, whether the moon be in 

 the zenith or nadir, the phenomenon is 

 nearly the same ; it being high water with 

 us at the same time that it is high water 

 with our antipodes. 



Let M, (fig. 4.) represent the moon as 

 before ; O, the centre of the earth ; and 

 Z and N, those parts of the surface which 

 are the nearest to the moon, and the 

 farthest from her. Then, because the 

 point Z is nearer to the moon than any 

 other part of the hemisphere H Z R, it 

 is evident that the waters will be more 

 strongly attracted by her, about that 

 point, than at others which are more re- 

 mote ; and since this attraction acts in a 

 contrary direction to that of the earth, 

 the waters in all parts, from H R to Z, 

 must have their gravity or tendency to- 

 wards the centre O diminished : and as 

 this tendency is the least at the point Z, 



they will consequently stand higher there 

 than in any other part of the hemisphere. 

 Agdin, in the opposite hemisphere H N R, 

 al iiough the attraction of the moon con- 

 spires with that of the earth, yet, as it is 

 known to decrease in proportion as the 

 squares of the distances increase, it is 

 plain that the joint influence of the two 

 forces, taken together, will be less at the 

 point N, on the side opposite to the moon, 

 than at those parts which lie nearer to H 

 R ; and consequently, as the gravity of the 

 waters, or their tendency towards ihe 

 centre, is also the least at that point, they 

 will be more elevated there than in any 

 other part of the hemisphere ; so that the 

 attractive force of the moon will evident- 

 ly raise the waters, both at that point of 

 the surface which is nearest to her, and 

 at that which is farthest from her, at the 

 same time, as was to be shown. 



Following this system, then, it is to be 

 observed, that at any port or harbour 

 which lies open to the ocean, the action 

 of the moon will tend to elevate the 

 waters there, when she is on the meridi- 

 an of that place, whether it be above the 

 horizon or below it. But the water can- 

 not be raised at one place, without flow- 

 ing from and being depressed at another; 

 and these elevations and depressions will 

 obviously be the greatest at opposite 

 points of the earth's surface. When the 

 moon raises the waters at Z and N, they 

 will be depressed at H and R ; and when 

 they are raised by her at H and R, they 

 will be depressed at Z and N. And as 

 the moon passes over the meridian, and is 

 in the horizon twice every day, there will 

 therefore be two tides of flood, and two 

 of ebb, in that time, at the interval of 

 about six hours and eleven minutes each; 

 which is exactly conformable to theory 

 and experience. 



From what has been hitherto said, it 

 may be supposed, that the moon is the 

 sole agent concerned in producing the 

 tides. But it will be necessary to observe, 

 before we quit the subject, that the influ- 

 ence of the sun would also produce a 

 similar effect, though in a much less de- 

 gree, than from his superior magnitude 

 we should naturally be led to imagine. 

 For it is not the entire actions of those 

 bodies upon the whole globe of the earth 

 that is here to be considered, but only 

 the inequalities of those actions upon dif- 

 ferent parts of it. The whole attractive 

 force of the sun is far superior to that of 

 the moon ; but as his distance from the 

 earth is nearly 400 times greater, the 

 forces, with which he acts upon different 

 parts of it, will be much nearer to equali- 



