GYROSCOPE (Greek) [from Supplement] is the name given to an instrument for the exhibition of various properties of rotation and the composition of rotations. It differs from a top in having both ends of its axis supported. The invention is probably French or German, and in some of its forms it dates from about the end of the last century; but no certain information can be obtained on these points. We will con sider only two of its many applications.
First, if a mass be set in rotation about its principal axis of inertia of greatest or less moment, it will continue to revolve about it; and, unless extraneous force be applied, the direction of the axis will remain unchanged. Such, for instance, would be the case with the earth, were it not for the disturbances (see NUTATION and PRECESSION) produced by the sun and moon; the direction of the axis would remain fixed in space (i.e., the pole-star would be always the same star), in spite of the earth's motion in its annual orbit. It is for this very reason that modern artillery is rifled, so that the pro jectile revolves about its axis. If, then, a mass of metal, as, for instance, a circular disk, loaded at the rim, and revolving in its own plane, be made to rotate rapidly about its axis of greatest moment of inertia, and if it be freely supported (in gimbals, like the box of a compass), the direction of its axis will be the same so long as the rotation lasts. It will therefore constantly point to the same star, and may, of course, be employed to show that the apparent rotation of the stars about the earth is due to a real rotation of the earth itself in the opposite direction. This application was made by Foucault shortly after his celebrated pendulum (q.v.) experiment, and he is generally looked upon as the inventor. The Transactions of the Royal Scottish Society of Arts, how ever, show that this application of the gyroscope was made many years before (Mar., 1836), by Mr. E. Sang, C.E. It is, in practice, by no means so perfect a mode of prov ing the earth's rotation as the Foucault pendulum; but this arises solely from unavoidable defects of workmanship and materials—the mass of the gimbals, and the friction on the pivots. Prof. Smyth, the Scottish astronomer-royal, has recently applied this property of the gyroscope to the improvement of our means of making astronomi cal observations at sea. A telescope, mounted on the same support as the ends of the axis of the gyroscope, will, of course, be almost unaltered in position by the rolling or pitching of a vessel; and a steady horizon, for sextant observations of altitude, is pro cured by attaching a mirror to the support of the gyroscope, and setting it once for all by means of spirit-levels. The mechanical difficulties of construction have not yet been
quite got over, but there seems to be little doubt that this application will some day be of very great practical value.
But the most singular phenomena shown by the gyroscope are those depending on the co/was/Von of rotations. We have already seen (lloTATioN) that any motion what ever of a body which has one point fixed is of the nature of a rotation about an axis passing through that point. Hence, simultaneous rotations about any two or more axes, being a motion of some kind, are equivalent to a rotation about a single axis. The effect, then. of impressing upon the frame in which the axis of the gyroscope is suspended a tendency to rotate about some axis, is to give the whole instrument a rota tion about an intermediate axis; and this will coincide more nearly with that of the gyroscope itself, as the rate of its rotation is greater. It is hardly possible to explain to the non-mathematical reader the exact nature of the compound motion,which consists in the rolling of an imaginary cone fixed in the gyroscope upon another fixed in space; but the rotation of the axis of a top round the vertical (when it is not "sleeping" in an upright position), and the precession of the earth's axis, are precisely similar phenomena. Thus, when the gyroscope is spin ning, its axis being horizontal, a weight attached to the framework at one endof the axis (fig. b) makes the whole rotate about the vertical; at tached to the other end, the rotation takes place in the opposite direction. And the framework may be lifted by a stritig attached near one end of t.:•e axis (fig. a) without the gyroscope's falling. Its axis still projects hori zontally from the string, but it re volves as a whole round the string. Various other singnlar experiments may be made with this apparatus; and others, even more curious, with the gyrostat of W. Thomson (v. which is simply a gyroscope inclosed in a rigid case, by which the ends of its axis are supported. When a (Mum under certain conditions, Foucault's celebrated experiment, gyrostat is made the bob of a pen the plane of vibration of the pendulum turns, as in but in general at a much greater rate.