RESULT OF THE EXPERIMENT. 35 



that, from the degree of speed which it would be found 

 necessary to give to the wheel in order to bring the teeth 

 up rapidly enough into the path to stop every flash on its 

 return, it would be easy to determine the time required 

 for making the journey. You will see, by a careful inspec- 

 tion of the figure, that there are two little indexes at the 

 ends of the fourth and fifth axles, by which the speed of 

 the wheels in this instrument is registered, so that the com- 

 putation can easily be made. The result of a trial made 

 with the apparatus near Paris corresponded very nearly, 

 in respect to the velocity of light, with those which had 

 been obtained by the astronomical calculations. 



I have described this contrivance in detail, both because 

 it is very useful to learn to understand tho nature and ac- 

 tion of mechanism from engravings and descriptions, and 

 also because this case is a striking instance of the ingenu- 

 ity and skill which have been exercised by scientific men 

 in discovering secrets of nature which we might have 

 thought it hopeless to attempt to unfold. The idea of at- 

 tempting to find any means of actually measuring, with- 

 in a space of a few miles, the velocity of a motion swift 

 enough to pass seven times round the earth in a second, 

 would have seemed to every one, at first view, to be utter- 

 ly chimerical. 



Do not forget the result, which is, that the velocity with 

 which light moves is such as to carry it about 175,000 

 miles, or seven times round the earth, in a second. The 

 rays require about eight minutes to come to us from the 

 sun. 



