296 KELVIN 



were held together by a rigid bar the water would be drawn 

 to the side nearest to the moon drawn to a prodigious height 

 of several hundred feet. But the earth and moon are 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 in- 

 termediate points of the circle C, D (FiG. 124, p. 298), there 

 is a somewhat complicated action according to which gravita- 

 tion is increased by about one 17-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 disturb- 

 ing 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 al- 

 teration 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 delicacy and 

 consistency and accuracy to indicate these results. 



A most earnest and persevering effort has been made by 

 Mr. George Darwin and Mr. Horace Darwin to detect vari- 

 ations in gravity due to lunar disturbance, and they have 

 made apparatus which, notwithstanding the prodigious small- 

 ness of the effect to be observed, is in point of delicacy 

 and consistency capable of showing it; but when they had 

 got their delicate pendulum their delicate plummet about 

 the length of an ordinary seconds' pendulum and their 

 delicate multiplying gear to multiply the motion of its lower 

 end by about a million times, and to show the result on a 

 scale by the reflection of a ray of light, they found the 

 little image incessantly moving backward and forward on 

 the scale with no consistency or regularity; and they have 

 come to the conclusion that there are continual local varia- 

 tions of apparent gravity taking place for which we know 

 no rule, and which are considerably greater than the lunar 

 disturbance for which they were seeking. That which they 



