286 KELVIN 



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 sec- 

 ond; 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 shoemakers' 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 bot- 

 tom, 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 resistance 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, through the luminiferous ether; but when we can 

 have actually before us a thing elastic like jelly and 

 yielding like pitch, surely we have a large and solid 

 ground for our faith in the speculative hypothesis of an 

 elastic luminiferous ether, which constitutes the wave theory 

 of light. 



