280 



NOTE ON THE CAUSE OF TIDES. 



shrinkage of course becomes gx*eater, and the tide higher, "when both 

 sun and moon take part in the counter-pull, whether acting on the 

 same side of the earth or on opposite sides. It may be assumed, 

 however, from the known height of the tidal wave where the march 

 of this wave is unopposed, that the maximum amount of contraction 

 does not exceed a foot for each thousand miles of the earth's radius — 

 being thus, in round numbers, less than one part in five millions. In 

 the tremendous pull of the earth upon the moon, by which the moon 

 is kept upon its course, a passing contraction of this comparatively 

 slight amount may be easily conceived to follow. According to the 

 commonly adopted theory, one tide is assumed to result from the 

 withdrawal of the earth, locally, from the waters above it : in the 

 view now proposed, both tides are assumed (although on a difierent 

 principle) to be thus caused. 



