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ADDRESS OF PROFESSOR LOVERING. 621 
spiration from Arago, they owed their success to nothing except 
their own skill in devising and executing. Having tried the tem- 
per of their steel on this easier problem, they were ready for the 
grand attack, which was to measure the absolute velocity of light. 
The instrumental arrangements of these two experimentalists 
agreed only in the part which each borrowed from Poggendorff: 
the details differed so widely as to give to whatever agreement 
might appear in their results the force of an irresistible argument 
for their accuracy. ` The velocity of light, as found by Fizeau in 
1849 by the artificial eclipses which the teeth of his revolving 
wheel produced, exceeds by about six per cent. the velocity which 
Foucault obtained, in 1862, with the moving mirror. The arith- 
metical mean of the two values comes very close to the astrono- 
mer’s estimate of the velocity of light. But this simple average 
I8 precluded unless it can be proved that the two experiments are 
entitled to equal weight. The internal evidence, expressed by 
what mathematicians call the probable error, manifested a decisive 
_ preference for Foucault’s result, and it has met with general accep- 
ce. The soundness of the scientific judgment in this case has 
been placed beyond all cavil by Cornu, who has recently repeated 
izean’s experiment, with additional precautions, and resolved the 
discord into a marvellous accord. Fizeau’s experiment, in spite 
of the numerical defect, was hailed as one of the grandest triumphs 
of experimental skill. In 1856, he received the prize of 30,000 
francs which the Emperor of the French had founded, to be given 
the work or the discovery, which, in the opinion of the five 
academies of the Institute, had conferred the greatest honor and 
meee upon the nation. Hitherto, it had been supposed that 
nothing short of an interstellar or an interplanetary space was a 
match for the enormous velocity of light. And yet one physicist, 
by using a distance of less than six miles, and another, without 
_ Boing outside of his laboratory, have discovered what astronomers 
had searched heaven and earth to find out. 
_ By these capital experiments the science of optics has achieved 
lts own independence. Let us see what they have done, at the 
Same time, for astronomy. The sequences in the eclipses of Jupi- 
ter’s moons are modified by the velocity of light. The aberration 
x Starlight is a measure of the ratio between the velocity of light 
4nd the Velocity of the earth. For nearly two centuries our 
Knowledge of the velocity of light leaned upon. one or the other 
