ESSAY ON THE VELOCITY OF LIGHT. 163 
toothed disk a uniform motion of one revolution per second, and the revolving 
mirror put in motion by the little air-turbine, or siréne, ought to take rank 
among the numerous mechanical wonders which have proceeded from his hands. 
The most ingenious conceptions remain unrealized if they do not find an artist 
capable of execufing them. Thus history has taken the care to treasure the 
memory of eminent mechanicians, to whom are due the delicate constructions 
which have given important progress to science. ‘The name of the illustrious 
Inglish clock-maker, Graham, remains forever associated with the discovery of 
aberration and of nutation, made by Bradley, with an instrument constructed 
by that artist; in the same manner the name of M. Froment will remain in- 
separable from the beautiful and refined experiment of which we have just 
spoken, and for which he executed the important and ingenious improvements 
added by M. Foucault to the original apparatus devised by Arago. 
Thus science is in possession of two methods, essentially different from each 
other, to effect the measure of the velocity of light on the surface of the earth. 
The first of these two methods, invented by M. Fizeau, was employed by 
him in his memorable experiment of Suresnes; but although the number that 
he thus obtained was very satisfactory from its close agreement with that 
which is deduced from astronomical observations, still we should only regard 
this experiment as having established the possibility of measwaing the velocity 
of light by employing this method. As soon as the success of this method 
was made known the Academy of Sciences, on the proposition of a commission 
of which Arago was a member, determined to construct at its expense an appa- 
ratus of large dimensions to turn the toothed disk destined to measure the 
velocity of light by the method of M. Fizeau, in using every possible precau- 
tion to give to that measure all the precision of which it was susceptible. The 
apparatus ordered of M. Froment was finished, but its adjustment on the ter- 
race of the Observatory was prevented; all the details of the observation to 
be made between this terrace and a distant station, such as Mont-Valérien or 
the tour of Montlhéry, had been studied; it was determined to employ tele- 
scopes of large aperture, in order to increase the luminous intensity of the 
observed image, and thus give all the desirable distinctness to the successive 
eclipses occasioned by the increasing velocity of the disk; it was proposed 
thus to produce twelve or more successive eclipses, &c., when the death of 
Arago postponed the execution of this important operation. Let us hope that 
the postponement will not be indefinite, and that the realization of an experi- 
ment which promises such chances of great success will not be delayed. 
The second method is that of M. Foucault, who has also obtained by its 
means a value for the velocity of light which is not far removed from that 
given by astronomical observation, and agreeing even still closer with it if we 
adopt the increase which it seems we should give to the value of the parallax 
of the sun. ‘This method, much more delicate than that of M. Fizeau, ap- 
pears, on account of its very refinement, more exposed than the latter to the 
influence of constant causes of error capable of falsifying the result. Let us 
also hope that M. Foucault will not delay to complete his labors, by studying 
the influence of these causes of error, so as to deduce a value for the velocity 
of light the most precise that this method can furnish. The simultaneous use 
of these two methods of measurement, whose results mutually control each 
other, is not too much in order to determine with all the exactitude possible 
the value of an element so important’as that of the velocity of the propagation 
of light through space. 
