THE WONDERFUL CENTURY. 



surfaces, and this greater warmth is communicated to the 

 air-molecules, and causes them to rebound with greater 

 rapidity from the dark surfaces, and back again from the 

 glass of the vessel, and the reaction, being all in one 

 direction, causes the arms to revolve. The near ap- 

 proach to a vacuum is necessary, both to diminish resist- 

 ance, and by greatly reducing the number of molecules, 

 in the vessel, to allow the very small differential action 

 to produce a sensible effect. Sir William Crookes has 

 found that there is a degree of rarefaction where the 

 action is at a maximum, and that when a nearer approach 

 to a perfect vacuum is attained the motion rapidly di- 

 minishes. A proof is thus given of the correctness of 

 the explanation; and the instrument may, therefore, be 

 considered to afford us an experimental illustration of 

 the molecular theory of gases. 



The velocity of light, as is well known, was first deter- 

 mined by irregularities in the time of the eclipses of 

 Jupiter's satellites, which were found to occur earlier or 

 later than the calculated times, according as we were 

 near to, or far from, the planet. It was thus found that 

 it required eight minutes for light to travel from the sun 

 to the earth, a distance of a little more than ninety mil- 

 lions of miles: so that light travels about 196,000 miles 

 in a single second of time. It would seem at first sight 

 impossible to measure the time taken by light in travel- 

 ling a mile, yet means have been discovered to do this, 

 and even to measure the time taken for light to traverse 

 a few feet from one side of a room to the other. Yet 

 more, this method of measuring the velocity of light has, 

 by successive refinements, become so accurate that it is 

 now considered to be the most satisfactory method of 



