Everyone is familiar with the fact that a round block of wood will support itself upon a comparatively tall peg so long as it is rapidly rotating. And that is but one of the curious things which a rotating body will do. For example, imagine a wheel mounted upon an axle the ends of which are supported inside a ring, while the ring again is supported on pivots between the two prongs of a fork, the fork being free to swivel round in a socket. The wheel is then free to move in any direction. Technically, it is said to have "three degrees of freedom." It can spin round, its axle can turn over and over with the pivoted ring inside which it is fixed, while it can also swing round and round as the fork turns in its socket. Assuming that the joints are all perfectly free, that the pivots move in their sockets with perfect freedom--which, of course, they do not--then a wheel so mounted could move in any direction under the influence of any force that might act upon it. Now a wheel so mounted if left alone remains in precisely the same position so long as it goes on rotating. If it be turning sufficiently quickly its tendency to remain will be strong enough to overcome the friction of any ordinarily well-made instrument. Consequently a wheel of that description has been used to demonstrate the rotation of the earth, it remaining still (except, of course, for its rotating movement) while the earth has moved under it. Could we entirely eliminate the effects of friction that might be used as a compass, for it could be set, say with its axle pointing north and south, at the commencement of the voyage, and it would remain so despite all the evolutions through which the ship might go. But there is a better scheme even than that, based upon the peculiar behaviour of a revolving wheel when it has only two degrees of freedom. Suppose that we dispense with the ring employed in the previous arrangement, pivoting the ends of the axle between the prongs of the fork. The wheel is then free to rotate, and its axle can slew round through a complete circle by the turning of the fork in its socket, but there can be no tilting of the axle. Being thus deprived of one of its movements the gyroscope with three degrees becomes a gyroscope with two degrees of freedom, and in that form it supplies the need for an efficient and reliable compass. The secret of the whole thing is the curious fact that a gyroscope with two degrees of freedom exhibits a keen desire to place its axis parallel with the axis of the earth. Owing to the shape of the earth, a device such as has been described, with its fork standing up vertically, cannot possibly have its axis really parallel with that of the earth, except on the Equator. Still it gets as nearly parallel as possible. To be scientifically accurate, we ought to say that it places it own axis "in the same plane" as that of the earth.