156 ANNUAL OF SCIENTIFIC DISCOVERY. 



uient of Foucault, the same fact is made visible to the eye, and in a 

 manner purely mechanical. The method by which this is effected 

 may perhaps be best explained by the description of Foucault's arrange- 

 ment, as first publicly exhibited in the Pantheon at Paris. To the 

 centre of the dome of the Pantheon a fine wire is attached, from which 

 a sphere of metal, four or five inches in diameter, is suspended so as 

 to hang near the floor of the building. This apparatus is put in vibra- 

 tion, after the manner of a pendulum. Under, and concentrical with 

 it, is placed a circular table, some twenty feet in diameter, the circum- 

 ference of which is divided into degrees, minutes, &c., and the divis- 

 ions numbered. Now, it can be shown, by the principles of mechanics, 

 that, supposing the earth to have the diurnal motion upon its axis 

 which is imputed to it, and which explains the phenomena of day and 

 night, &c., the plane in which this pendulum vibrates will not be 

 aflected by this diurnal motion, but will maintain strictly the same 

 direction during twenty-four hours. In this interval, however, the 

 table over which the pendulum is suspended will continually change 

 its position in virtue of the diurnal motion, so as to make a complete 

 revolution round its centre. Since, then, the table thus revolves, and 

 the pendulum, which vibrates over it, does not revolve, the consequence 

 is, that a line traced upon the table, by a point projecting from the bot- 

 tom of the ball, will change its direction relatively to the table from 

 minute to minute, and from hour to hour ; so that, if such point were 

 a pencil, and paper were spread upon the table, the course formed by 

 this pencil, during 24 hours, would form a system of lines radiating from 

 the centre of the table ; and the two lines formed after the interval of 

 one hour would always form an angle with each other of 15, being the 

 24th part of the circumference. The practised eye of a correct observer, 

 especially if aided by a proper optical instrument, may actually see the 

 motion which the table has in common with the earth under the pen- 

 dulum between two successive vibrations. It is, in fact, apparent that 

 the ball, or, rather, the point attached to the bottom of the ball, does 

 not return precisely to the same point of the circumference of the table 

 after two successive vibrations. Thus is rendered visible the motion 

 which the table has, in common with the earth. It is true that, cor- 

 rectly speaking, the table does not turn round its own centre, but turns 

 round the axis of the earth ; nevertheless, the effect of the motion, rel- 

 atively to the pendulum suspended over the centre of the table, is 

 precisely the same as it would be if the table moved once in 24 hours 

 round its own centre ; for, although the table be turned, in common 

 with the surface of the earth, round the earth's axis, the point of sus- 

 pension of the pendulum is turned also in the same time round the 

 same axis, being continually maintained vertical above the centre of 

 the table. The plane in which the pendulum vibrates does not, however- 

 partake of this motion, and, consequently, has the appearance of revolv- 

 ing once in 24 hours over the table, while, in reality, it is the table 

 which revolves once in 24 hours under it. 



The occurrence from which M. Foucault was led to his discovery is 

 thus related by him : " Having fixed on the arbor of a lathe, and in the 

 direction of the axis, a round and flexible steel rod, it was put in vibra- 



