To understand this we need to realise that all movement is relative. In ordinary language, when we say a thing is still we mean that it is still in relation to the surface of the earth, but since the earth is moving the stillest thing, apparently, is really travelling at enormous speed. Saint Paul's Cathedral in London, or a tall sky-scraper in New York, would usually be regarded as supreme instances of immobility. It would be hard to find better examples of stationariness, as we ordinarily look at things. Each stands, firm and strong, upon a horizontal base. Yet each is really turning a somersault every twenty-four hours. The plateau upon which St Paul's stands, though it seems still and motionless beneath our feet, is continually tilting; its eastern edge is continually going downwards and its western edge upwards, as the earth performs its daily spin. It is only a north and south line which does not share in some degree this continual tilting action. Every plane, large or small, so long as it remains horizontal, is being tilted thus, down at the eastern edge and up at the western. And the plane in which the axle of a gyroscope with "two degrees" is free to move is a horizontal plane. Owing to its being held between the prongs of the fork, while it can swing round to point north, south, east or west, or towards any point between them, it cannot deviate from the horizontal plane. Therefore such axle is always being tilted by the motion of the earth, _except when it happens to be lying exactly north and south_. Now for a reason which is too complex to go into here a gyroscope strongly objects to having its axle tilted in this manner. If it be compelled by superior force to submit to tilting, it tries to wrench itself round sideways. Anyone who has a gyroscope top and cares to try the experiment will feel this action quite easily. Hold the spinning-top in your hand and turn it over so as to tilt the axle, when it will, if you are not careful, twist itself out of your grasp. So a gyroscope of the kind we are considering, when the motion of the earth tilts its axis, turns itself round in its socket until at last it reaches the north and south position, when the tilting, and therefore the twisting, ceases. Hence the axle of the gyroscope if left to itself (the rotation of the wheel being maintained the while) will place itself in a north and south direction. And, moreover, it will keep in that direction. It will take some force to slew it round into any other. And if moved into any other by some extraneous means it will restore itself to the old position again. Hence a wheel thus arranged has all the attributes which we need for a mariner's compass. But unfortunately there are mechanical difficulties in the way of using such a simple contrivance for that purpose.