NATURAL PHILOSOPHY. 187 



pressing the play of one or the other of the articulations, which leave 

 the crown or circular plate of bronze at liberty to move around its 

 centre of gravity. If we stop the play of the blades which represent 

 the horizontal diameter, about which one of the concentric circles 

 moves, the axis of the revolving body is brought down into the hori- 

 zontal plane, where it may " take its bearings," in the manner of the 

 ordinary declination compass. Supported by the vertical suspension, 

 the body, turning swiftly, shows itself to be solicited by a directing 

 force, which tends to recall its axis into the plane of the meridian, the 

 plane of the pole-star, of the midday sun, of the axis of the earth. 

 And when it has reached its position of equilibrium, the body turns, 

 like the earth, from west to east. Thus we may almost find our meri- 

 dian without lifting a glance to the heavens. If, on the contrary, we 

 check the suspending thread, we restore the blades to their function, 

 and permit the axis of the revolving body to incline only in some ver- 

 tical plane arbitrarily chosen. We launch it into motion, and watch 

 the effect. Soon the axis inclines in one direction or in the other ; 

 and, when it takes a definite position, we find that the axis points to 

 the poles of the earth, and thus we discover the inclination of the axis 

 of the earth. These facts can be seen without the microscope. After 

 mentioning as a means of verification inherent in the instrument, the 

 artificial acceleration of the compounding efficacy of movement com- 

 municated to the instrument through its base by the earth, whereby 

 the phenomena of " orientation" are exaggerated without being 

 altered, M. Foucault goes on to express the general principle, that, 

 when a force, or system of forces, acting upon a revolving body tends 

 to produce a new rotation not parallel to the first, the resulting effect 

 is a progressive displacement of the axis of primary rotation, which 

 directs itself towards the axis of new rotation by a path which tends 

 to make them parallel. " By this principle," he says, " we may 

 equally well explain the precession of the equinoxes, and the pirou- 

 ettes of a teetotum on a parlor table. We account thus for the singu- 

 lar reactions we experience when we agitate with the hand a swiftly 

 revolving body. We may foresee in the motion of a railway train 

 along a curved line, a cause of dislodgement added to the centrifugal 

 force, and which tends to spin off all the wheels driven by so rapid a 

 rotatory motion. We discover, that contrary to the received opinion, 

 the whirligig which, with the help of a mirror, has been thought to 

 furnish an artificial horizon, does not tend exactly towards the verti- 

 cal position, but leans northward or southward, according as it turns to 

 the right, or left ; and that always on a perfectly horizontal surface it 

 must advance slowly to the east, so that really to demonstrate experiment- 

 ally the motion of the earth, we shall soon only have to choose among 

 a thousand ways of doing so. Meanwhile, thus much is acquired, viz: 

 the apparent deviation of the plane of rotation, and the phenomenon 

 of orientation, which reveal the direction of the earth's axis. As all 

 these facts depend on the earth's rotation, we propose to give this 

 new instrument which has served to establish them, the name 

 gyroscope" To-Day. 

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