386 PKOCEEDINGS OF THE AMERICAN ACADEMY 



In IMarch, 1879, Congress had voted an appropriation of ^5,000 for 

 experiments on the velocity of light, to be made under the direction 

 of Professor Newcomb. All the delicacy of instrumental construction, 

 all the skill of scientific observation, and all the resources of mathe- 

 matical discussion, were enlisted in this service. Tlie method adopted 

 was that of the revolving mirror. The movable mirror was mounted 

 at Fort Myer. Two different locations were selected for the fixed 

 mirror, viz. the Naval Observatory, and the Washington Monument. 

 In one case the distance was 2,550.95 meters, or about 8,367.12 feet ; 

 in the second case, 3,721.21 meters, or about 12,205.57 feet. Mr. 

 Michelson assisted in the observations until his removal to Cleveland, 

 in the autumn of 1880. The observations began in the summer of 

 1880, and were continued into the autumn of 1882, the most favorable 

 days in spring, summer, and autumn being selected. In all 504 sets 

 of measurements were made; viz. 276 by Professor Newcomb, 140 

 by Professor Michelson, and 88 by Mr. Plolcombe. After a full 

 discussion of all the observations and the possible sources of error, 

 Professor Newcomb decided to rest the final result on the 132 sets 

 of observations made iu 1882 over the long distance between Fort 

 Myer and the Washington Monument. The velocity then obtained 

 was 186,282 miles. The velocity deduced from the three sets of 

 observations was 186,251 miles. The probable error of the first 

 result was about 19 miles. 



For some future attack upon this problem Professor Newcomb 

 suggested a prism for the reflector with a pentagonal section, and 

 placed at such a distance that it could revolve through an arc of 36° 

 while the light was going and returning : 500 turns a second and a 

 distance of 19 miles would fulfil this condition. In the Rocky 

 Mountains, or the Sierra Nevada, stations from 20 to 30 miles 

 distant could be found, and with no greater loss of light from absorp- 

 tion than is produced by 2 or 3 miles of common air. 



The first experiments made in Great Britain for the measurement 

 of the velocity of light were published by James Young and Professor 

 G. Forbes in the Philosophical Transactions of 1882. They adopted 

 the method of Fizeau. In 1878, between 600 and 700 observations 

 were made ; but the number of teeth in the rotating wheel was insuffi- 

 cient. New experiments were made in 1880-81 across the river 

 Clyde. Two reflectors were used at unequal distances, and the time 

 was noted when an electric light after the two reflections was at its 

 maximum. The corrected distances for the two mirrors were 18,212.2 

 and 16,835 feet. After an elaborate mathematical discussion of the 



