It is such a balanced top as this that we must call to our aid in explaining the action of Mr. Brennan's gyroscopes. The explanation will involve the use of a diagram perhaps rather unpleasantly suggestive of the days when you studied geometry, and I fear I cannot hope to make interesting reading of the explanation. But it will be worth your while to follow it, that you may understand the action of one of the most remarkable and ingenious of inventions. Figure 1 represents a kind of top called a Foucault gyrostat. It is merely a top or gyroscope in gimbal frames, such as I have already referred to. With certain slight modifications, the diagram that represents it might also be a diagram of one of the gyroscopes in Mr. Brennan's car. Indeed, it was such a top as this that led Mr. Brennan to his discovery. Once while on a visit to Cannes, he purchased a top like this of a street vender--and the gyrocar is the outcome of the studies he made with it. This is also the kind of top with which Foucault, after whom it is named, proved that the earth revolves; but we shall come to that story in another connection. [Illustration: FIG. 1.] Reverting to the diagram, the gyroscope or top proper is at the centre, revolving on the axis _O A_. It is pivoted on the frame _B A C_, which frame is in turn pivoted so that it can rotate on the axis _B C_. Lastly, the outer frame _B D C E_ is pivoted on the axis _D E_. Thus the apparatus as a whole is capable of revolving on each of its three principal axes. But under ordinary conditions it is only the inner wheel that is spinning. As this wheel is perfectly balanced, it will maintain steadily any position that it chances to have when it is set spinning, and the outer frames will remain stationary unless a disturbing force is applied to them. Suppose, now, that the wheel has been set spinning on its axis _O A_ in the direction indicated by the arrow, while its axis is horizontal, as represented in the diagram. The wheel will then tend to maintain its position and resist any attempt to displace it. But its resistance will be shown in a very peculiar way--whereby hangs our tale. If you apply a steady downward pressure to the frame _B A C_ at point _A_, attempting thus to deflect the axis of the spinning wheel of the gyroscope, the frame will not tip down as you expect it to do (and as it would do if the top were not spinning) but instead, it will move in a horizontal plane along the arc _C A B_, the entire mechanism rotating on the axis _D E_. This motion is equivalent to the wabble of the top, and it is called "precession." Please remember the word and its meaning, for we must use it repeatedly.