ON GEOLOGICAL TIME. 33 



action on the earth disturbs, by re-action, the 

 moon. The tidal deformation of the water exer- 

 cises the same influence on the moon as if she 

 were attracted not precisely in the line towards the 

 earth's centre, but in a line slanting very slightly, 

 relatively to her motion, in the direction forwards. 

 The moon, then, continually experiences a force 

 forward in her orbit by re-action from the waters 

 of the sea. Now, it might be supposed for a 

 moment that a force acting forwards would quicken 

 the moon's motion ; but, on the contrary, the 

 action of that force is to retard her motion. It 

 is a curious fact easily explained, that a force 

 continually acting forward with the moon's 

 motion will tend, in the long run, to make the 

 moon's motion slower, and increase her distance 

 from the earth. On the other hand, the effect of a 

 resisting force on, for instance, the earth would 

 undoubtedly be, in the course of ages, to make the 

 earth go faster and faster round the sun. The 

 reason is, that the resistance allows the earth to 

 fall in a spiral path towards the sun, whose 

 attraction generates more velocity than frictional 

 VOL. II D 



