THE TIDES. 157 



not so connected. We may imagine the earth as 

 falling towards the moon, and the moon as falling 

 towards the earth, but never coming nearer ; the 

 bodies, in reality, revolving round their common 

 centre of gravity. A point nearest to the moon is as 

 it were dragged away from the earth, and thus the 

 result is that apparent gravity differs by about one 

 four-millionth at the points nearest to and farthest 

 from the moon. At the intermediate points of 

 the circle C, D (Fig. 20, p. 161), there is a somewhat 

 complicated action according to which gravitation 

 is increased by about one I /-millionth, and its 

 direction altered by about one I /-millionth, so that 

 a pendulum 17,000 feet long, a plummet rather 

 longer than from the top of Mont Blanc to sea level, 

 would, if showing truly the lunar disturbing force, 

 be deflected through a space of one thousandth of 

 a foot. It seems quite hopeless by a plummet to 

 exhibit the lunar disturbance of gravity. A spring 

 balance to show the alteration of magnitude, and 

 a plummet to show the change of direction are 

 conceivable ; but we can scarcely believe that 

 either can ever be produced, with sufficient deli- 



