THE WAVE THEORY OF LIGHT. 347 



ferous ether ? It is matter prodigiously less dense 

 than air millions and millions and millions of 

 times less dense than air. We can form some sort 

 of idea of its limitations. We believe it is a real 

 thing, with great rigidity in comparison with its 

 density : it may be made to vibrate 400 million 

 million times per second ; and yet be of such 

 density as not to produce the slightest resistance 

 to any body going through it. 



Going back to the illustration of the shoemaker's 

 wax ; if a cork will, in the course of a year, push its 

 way up through a plate of that wax when placed 

 under water, and if a lead bullet will penetrate 

 downwards to the bottom, what is the law of the 

 resistance ? It clearly depends on time. The cork 

 slowly in the course of a year works its way 

 up through two inches of that substance ; give it 

 one or two thousand years to do it and the resist- 

 ance will be enormously less ; thus the motion of 

 a cork or bullet, at the rate of one inch in 2,000 

 years, may be compared with that of the earth, 

 moving at the rate of six times ninety-three 

 million miles a year, or nineteen miles per second, 



