186 ANNUAL OF SCIENTIFIC DISCOVERY. 



ined, will be seen to share the same apparent displacement, and so 

 will give a manifest sign of the movement of the earth. To obtain 

 this effect in the highest perfection, we should give to our axis of 

 rotation a perpendicular direction, and we should be very careful not 

 to select the pole-star as our point of direction ; for, as that star has no 

 apparent motion, one instrument would ' refuse to exhibit.' To verify 

 the earth's rotation by the aid of such an instrument is equally possible 

 at the poles and at the equator. The only necessary condition to this 

 experiment is, that we shall support the revolving body by its centre 

 of gravity, without imposing upon it any bond attaching it to the 

 earth, so that it shall be as it were more free than a planet, a kind of 

 isolated little globe lost in space and disengaged from aU perturbatory 

 action. 



" But who, we may ask, will undertake to furnish us with the means 

 of fulfilling this solitary condition '? The question might have gone 

 unanswered, had not that incomparable artificer, M. Froinent, come to 

 our rescue. ' Mount,' we said to him, ' mount upon a steel axis a crown 

 of bronze, in such wise that it may turn swiftly within a circle, which shall 

 rest, by means of blades of steel, upon a second exterior circle ; which 

 again shall be itself suspended in the air, by means of a thread, without 

 torsion. Arm the whole system with screw-balance weights, in order to 

 secure a perfect equilibrium ; and then, if the earth really does turn, we 

 shall see it with our microscopes.' After eight months of steady applica- 

 tion, M. Froment has furnished us with an instrument so perfect that 

 the massive pieces which compose it move upon each other at the 

 faintest breath. This admirable mobility, however, disappears as soon 

 as the bronze crown begins to revolve ; for then, in virtue of the fixity 

 of the plane of rotation, the whole system consolidates itself with sur- 

 prising force. When in this condition, the revolving body ceases to 

 take part in the earth's diurnal motion ; and, although the steel axis, 

 in consequence of its shortness, seems to preserve its primary direc- 

 tion relatively to terrestrial objects, we have only to bring the micro- 

 scope to bear upon it, in order to discover an apparent uniform and 

 continuous movement, which causes it to follow exactly the movement 

 of the celestial sphere. Thus, with a deviation of a new kind, we 

 obtain a new proof of the earth's rotation ; and this with an instru- 

 ment of no great size, and easy of transportation, - - an instrument, 

 too, which offers us an image of the continuous movement of the globe 

 itself. In the case of the pendulum, you have before you only the 

 progressive displacement of an ideal plane, more or less well defined 

 by the curve described by an oscillating mass. But, in our new instru- 

 ment, you have material masses really withdrawn from the influence 

 of the diurnal motion, and which return under the general law only 

 in consequence of the decay in the velocity of the moving body." 



M. Foucault now goes on to describe another set of experiments 

 which may be made with this new instrument, the result of which is to 

 make apparent in bodies revolving on the surface of the earth what 

 he calls " a force of orientation," a power, that is, of " taking their 

 bearings," as we may say. These experiments are carried on by sup- 



