in the Air and in the Sea. 



169 



from L to L', all the points in the arc eca! are moved somewhat 

 nearer to the moon, and therefore the attraction of the moon on 



Fig. 4. 



-£ — j 



3 









?;'~^_ 





\ ^ 







1 1 ^~"~~^^-\ 











\ E 





\a 







A 





/ /*>~^--^^^ 







s «. 





J/^—" ^^"^"^ 







^^r~ 



^^^ 











C "^ 







/ 



all these points is increased; while every point in the arc e' da 

 has removed a little further from the moon, and is consequently 

 less attracted. We will represent the attraction of the moon by 

 two threads L c and L d fastened to the circle. We will gradu- 

 ally more and more draw the thread L c, to represent the con- 

 stantly augmenting attraction of the point c. We will constantly 

 let the thread L d give way, to imitate the diminution of the 

 attraction of the point d. Of course, through greater tension of 

 the thread Lc and continual yielding of the thread Ld, the 

 points c and d receive a motion in the direction of the arrows C 

 and D. This motion will be the quicker the greater the circle 

 to which the points belong, because in greater circles the change 

 of distance from the moon, and consequently the alteration of 

 her attraction, is more considerable for every point than in 

 smaller circles. 



All that we have just said of the moon holds good also for the 

 sun, with only this difference — that the motions of air and water 

 produced by its attraction will be somewhat less than those pro- 

 duced by the moon. 



We see therefore that the attractions of the sun and moon 

 must each present two reciprocally counteracting developments 

 of force. The one, which calls forth an east-to-west current and 

 corresponds to high water, we will henceforth name the flood- 

 current force ; the other, corresponding to the ebb and impelling 

 air and water from west to east, we will call the ebb-current 

 force. 



If these two forces are of equal intensity, they will balance 

 each other and produce no current ; but as soon as one of the 

 two is greater, the water and air will be subject to the action of 

 the greater force and move onward with the velocity correspond- 

 ing to the difference between the two forces. 



