382 PBOCEEDINGS OF THE AMERICAN ACADEMY 



that an astronomer, without going out of his observatory, should be 

 able to determine exactly the size and figure of the earth, and its 

 distance from the sun and moon, simply by comparing his observa- 

 tions with analysis ; the knowledge of which formerly demanded long 

 and laborious voyages into both hemispheres." 



The ancients supposed that light came instantaneously from the 

 stars ; a consolation for those who believed that the heavens revolved 

 around the earth in twenty-four hours. Galileo and the academicians 

 of Florence obtained even negative results. 



While the number of physical sciences has received numerous addi- 

 tions during the last half-century, new affiliations and a more inti- 

 mate correlation have been manifested. In this mutual helpfulness 

 light has played an imi^ortant part. The optical method of studying 

 sound, and the many varieties of flame apparatus, have made acoustics 

 as intelligible through the eye as through the ear. 



Velocity being expressed by space divided by time, it is evident 

 that, in measuring an immense velocity, we must have at our com- 

 mand an enormous distance, such as we find only in astronomy, or 

 else possess the means of measuring fractions of time as small as one 

 millionth of a second. The first successful attempt to measure such 

 a velocity was made by Wheatstone in 1834. Discharges from a 

 Leyden jar were sent through a wire, having two breaks in it, |- of a 

 mile apart. The wire was in the form of a loop, so as to bring the 

 breaks into the same vertical line. The sparks, seen at these breaks, 

 were reflected by a mirror, at the distance of 10 feet, and revolving 

 800 times per second. The images of the two sparks were relatively 

 displaced in a horizontal direction. As the displacement did not ex- 

 ceed one half of an inch, the time taken by electricity to go from one 

 break to the other was less than a millionth of a second. Since the 

 distance was one quarter of a mile, the electricity travelled, in that 

 case, at the rate of 288,000 miles a second. If this experiment is 

 interpreted to mean that electricity would go over 288,000 miles of 

 similar wire in one second, as it probably often was at that time, the 

 conclusion is fallacious. The velocity of electricity, unlike that of 

 sound or liojht, diminishes when the lensth of wire increases. 



In 1838, Wheatstone suggested a method for measuring the velocity 

 of light, which he thought was adequate for giving, not only the abso- 

 lute velocity, but the ditference of velocity in different media. 



In that year Arago communicated to the French Academy the 

 details of an experiment which he thought would give the velocity of 

 light in air or a vacuum. As his own health was broken down (he 



