PROPAGATION OF LIGHT. 19 



the principles of the wave-theory, it seemed necessary to sup- 

 pose that the ether which encompasses the Earth does not par- 

 ticipate in its motion, so that the ethereal current produced 

 by their relative motion pervades the solid mass of the Earth 

 " as freely," to use the words of Young, " as the wind passes 

 through a grove of trees." Fresnel has developed this hy- 

 pothesis, and has shown that it suffices to explain other 

 phenomena also, in which the Earth's motion is concerned. 

 Professor Stokes has lately shown that the same results may 

 be deduced from a more plausible hypothesis relative to the 

 mutual dependence of the ether and of the Earth. 



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