44 EQUILIBRIUM THEORY. 



waters of the globe is only about 1 1 J Q of the force 

 of terrestrial gravity opposed to it ; the moon's attrac- 

 tion being nearly as 1J grain to 1683 Ibs., the weight 

 of a cubic yard of water. This minute force, however, 

 would be sufficient to raise or depress the surface of 

 such a universal ocean as we have supposed, to the ex- 

 tent of about 58 inches, vertical height. 



The " equilibrium theory," * commonly explained as 

 above, may be much more forcibly stated by separating 

 or "resolving" the attractive force into its component 

 parts, and considering these separately. For this pur- 

 pose, let M (fig. 3) be the position of the moon, and 

 the circle E the earth ; then if we take any point A, on 



FIG. 3. 



the surface, the force which attracts the water at A, in 

 the direction AM, may be shown to be composed of two 

 forces, one pulling against gravity in the direction E A, 

 and the other at right angles to it, pulling towards the 

 line M E. The first we shall, for convenience, call the 

 normal, and the latter the tangential force. The respec- 

 tive amounts of these forces at any point may be indi- 

 cated by lines, drawn as follows. At the point A (fig. 

 4), drop the perpendicular A c, and measure off on the 



* For the substance of what follows, I am indebted to my friend 

 the Rev. Prof. Haughton, T.C.D. 



