OF ARTS AND SCIENCES: APRIL 14, 1863. 115 



Let US glance first at the two astronomical raetliods of measuring the 

 velocity of light. While the senses of touch and taste act only by con- 

 tact, those of hearing and seeing bring the mind into communication 

 with distant objects. The air and the omnipresent ether supply the 

 delicate and ever-ramifying threads by which telegraphic intercourse 

 is maintained with the ear and the eye. "When the origin of the sound 

 or the light is at a large distance, compared with the velocity of the 

 acoustic or luminous wave, allowance must be made for the time taken 

 by the news of an audible or visible event to come to us. Only the 

 vast spaces of Astronomy are commensurable with the great velocity 

 of light, and furnish a sufficiently large theatre for a direct experiment 

 upon it. But in stellar astronomy, the magnificence of the extent of 

 view so far transcends in magnitude even the velocity of light, that the 

 luminous ray, vast as is its speed, seems to loiter upon its long way. 



Hence in Astronomy a distinction exists between the actual interval 

 of successive events and the apparent interval. For example, the 

 first satellite of Jupiter revolves around its primary in about 42^ hours ; 

 and, therefore, enters the shadow of Jupiter and is eclipsed once every 

 42^ hours. As it takes light more than 40 minutes to pass over the 

 average distance of Jupiter, the echpse is not seen until so many minutes, 

 on the average, after it has happened. If this delay were constant, the 

 interval of successive eclipses would not be changed. But in the 

 course of six months the distance of the earth from Jupiter increases 

 by the diameter of the earth's orbit, and in the next six months changes 

 back again ; and when the earth is nearest to Jupiter, the news of an 

 eclipse reaches us in about 32 minutes ; whereas if the earth is at the 

 greatest distance, 50 minutes are required. Consequently, the intervals 

 between successive eclipses, as they exist for our eyes, are variable, 

 beinf sometimes larger and sometimes smaller than the real intervals. 

 This irregularity in the apparent intervals of the eclipses of the same 

 satellite, at first attributed to errors of observation, finally conducted 

 Romer in 1675 to the discovery of the velocity of light. Delambre, 

 after dfscussing 1000 of these eclipses, observed between 1G62 and 

 1802, calculated the velocity of light to be such as to require 493.2 

 seconds to pass over the mean distance of the sun. If this time divides 

 95,360,000 statute miles, which is the sun's distance as given by the 

 transits of Venus in 1761 and 1769 according to Encke's computations, 

 the quotient, or 193350 statute miles, is the velocity of light in a 

 second. 



