573 



the velocity of electricity, and had shown by numerical calculation 

 that with a high velocity of rotation the result would be sensible to 

 observation. But the experiment proposed by M. Arago, though 

 the execution of it might not have been impossible, would at any 

 rate have been highly inconvenient for observation, because the 

 result depended on the aspect of a momentary image appearing 

 casually at an unknown instant in an unknown part of the field of 

 view, and it was never carried out. The velocity of light had recently 

 been measured directly for the first time by M. Fizeau ; but in this 

 case the observed time was that which light took to travel some 

 miles in air. But by a highly ingenious contrivance, in which, by 

 the introduction of a concave mirror, he was able to produce a fixed 

 image of a revolving image, M. Foucault actually solved experiment- 

 ally the question proposed for solution by M. Arago, though in a 

 manner totally different from that suggested by that distinguished 

 astronomer, and thus proved, for the first time by direct experiment, 

 that light is propagated more quickly in air than in water. 



On the 3rd of February, 1851, M. Foucault communicated to the 

 French Academy* an experiment which has attracted more public 

 attention than perhaps any other of modern times, namely his cele- 

 brated experiment of the pendulum. The phenomena of astronomy 

 have long since proved the rotation of the earth ; but we, living on 

 it, are not sensible of that rotation. In one experiment only had 

 the effect of the rotation been manifested, namely in the easterly 

 deflection of a body let fall from a great height. But in this case 

 the experiment could only be attempted in particular places, and the 

 result was only a small deflection, which was liable to be interfered 

 with by a variety of disturbing causes. But M. Foucault showed 

 that a pendulum suspended so as to oscillate in all vertical planes 

 alike, withdrawn from the vertical, and then left to itself, became by 

 the very fact of its motion in some sense a celestial body ; and while 

 the plane of motion tended to remain parallel to itself, and would 

 actually do so at either pole, the earth turned round under it, and 

 thus the plane of motion was affected with an apparent rotation in 

 the direction, in our hemisphere, of the hands of a watch, the velo- 

 city of rotation decreasing from one turn in twenty-four hours at the 

 pole to zero at the equator, in proportion to the sine of the latitude, 

 * Comptes Rendus, torn, xxxii. p. 135. 



