48 



FERGUSON'S LECTURES. 



LECT. 

 n. 



p. A third thread m is tied to the line h, c'o e 

 , above the ball e, and passing by the other side of the 

 moon M, through the bar N, has the weight r hung to 

 it, which is lighter than the weight q. 



The use of these three unequal weights is to repre- 

 sent the moon's unequal attraction at different distances 

 from her. With whatever force she attracts the center 

 of the earth, she attracts the side next her with a great- 

 er degree of force, and the side farthest from her with a 

 less. So, if the weights are left at liberty, they will 

 draw all the three balls towards the moon with different 

 degrees of force, and cause them to make this 

 appearance ; by which means 

 they are evidently farther 

 from each other than they 

 would be if they hung at liber- 

 ty by the lines h, i, k ; because 

 the lines would then hang per- 

 pendicularly. This shews that 

 as the moon attracts the side 

 of the earth which is nearest her with a greater de- 

 gree of force than she does the center of the earth, 

 she will draw the water on that side more than she 

 draws the center, and so cause it to rise on that 

 side ; and as she draws the center more than she 

 draws the opposite side, the center will recede farther 

 from the surface of the water on that opposite side, and 

 so leave it as high there as she raised it on the side 

 next to her. For, as the center will be in the middle 

 between the tops of the opposite elevations, they must 

 of course be equally high on both sides at the same 

 time.* 8 



Note 28. The united operations of the sun and moon in producing the 

 phenomena of the tides are more amply, as well as more accurately, 

 described in our author's Treatise on Astronomy, of which a new edi- 

 tion in now ready for the press. 



