ESSAY ON THE VELOCITY OF LIGHT. 155 
In the circumstances in which the experiment was made, the first eclipse was produced in 
about twelve turns and siz-tenths of a turn in one second. With double the velocity the star 
shines again; with a triple velocity a second eclipse takes place: for a quadrupie velocity 
the point shines again, and so on. 
‘The first telescope was placed on the terrace of a house at Suresnes, near Paris; the 
second on the heights of Montmartre, at an approximate distance of 8,633 metres, (28,516 
feet, or 5.3645 miles.) 
“The.disk, carrying 720 teeth, was attached to wheel-work moved by weights, and con- 
structed by M. Froment; a register gave the number of revolutions. The light was obtained 
from a lamp so disposed as to give a very bright beam.” 
M. Fizeau, by repeating several times his experiment, found for the velocity 
of light a value which differed but little from that previously deduced from 
astronomical phenomena; the mean of twenty-eight observations gave him a 
velocity of 70,948 leagues of 25 to the degree, or of about 315,000 kilometres 
(195,741 miles) in one second of time. 
The publication of this magnificent experiment was an event in science. 
The project developed by Arago before the Academy, in 1838, had shown the 
possibility of rendering appreciable the progressive transmission of light in 
space»and demonstrated that it would have to run over but a very short dis- 
tance on the earth’s surface to determine thus its velocity. The experiment 
of M. Fizeau, formed on an entirely different plan, realized for the first time 
so daring an idea, and furnished at the same time a determination of the ve- 
locity of light, whose agreement with that which astronomers had arrived at 
from sidereal observations was as satisfactory as one could desire for a first 
attempt of the kind.* 
REALIZATION OF THE EXPERIMENT OF ARAGO, ON ONE HAND BY M. FOUCAULT, 
ON THE OTHER BY MM. FIZEAU AND BREGUET. 
For nearly eleven years the proposed experiment of Arago had remained as 
fo) 
an ingenious suggestion when the labors of M. Fizeau awakened the interest 
of the learned world in problems of this nature. Some months later (meeting 
of April 29, 1850) Arago again requested for it the attention of the Academy. 
After having referred to the object of the notice printed in the Comte Rendu, 
of the meeting of December 3. 1838. he added: 
‘*That communication established that, according to readily admitted hypotheses as to 
the angular deviations susceptible of being observed in an ordinary telescope, it would not 
be impossible to determine the comparative velocity of light in bisulphide of carbon and in 
air, without having recourse to an extreme length of tube, or to a mirror, making more than 
1,000 turns in a second.. But the mirror which M. Wheatstone used made only 800 turns in 
the same interval of time. 
“It was evident that in this method of observation, and for a given angular deviation, the 
length of tube containing the liquid ought to be so much the shorter, as the movement of 
rotation of the mirror is more rapid. This is the reason I propose to add to this deviating 
motion of rotation, which cannot surpass certain limits, 2 cumbination of several revolving 
mirrors. 
‘“'The two rays (one having traversed the liquid, the other the air) strike the first mirror, 
and form a certain angle; this angle is doubled when the rays fall upon a second mirror 
turning in the same direction with the same velocity; the angle is tripled if these rays are 
reflected from a third revolving mirror, and soon. We can thus, by the multiplication of 
the revolving mirrors, arrive at the same result given by a single mirror turning with a 
double, triple, &c., velocity of that which it is possible to obtain with the certainty of not 
destroying the teeth of the wheel, or of overheating the axis, 
‘* My triend, M. Bréguet, jr., undertook to accoiuplish this end by means of a mechanism, 
in which the communication of motion was given by wheel-work. He executed a special 
arrangement of cog-wheels, the invention of which is due to White. At one of the former 
industrial exhibitions could be seen the system of these movements. 
* Tt was for this experiment of Suresnes that the Institute of France awarded to M. Fizeau, 
at its annual meeting of 1856, the triennial prize of thirty thousand francs, founded by the 
Emperor for the work or the discovery which, in the opinion of the five academies of the Insti- 
tute, has done most honor ard service to the country. 
