198 AN EMPIRICAL STUDY OF GYRATING BODIES. 



the point p with each vibration of the pendulum. In 

 this we have a key to the movements of the instrument 

 under consideration. 



Supr>ose, then, the wheel in fig. 48 being in motion 

 around its axis, that it be drawn to the right. To avoid 

 unconscious direction, I use a fine cord, attach one end 

 to m, and the other to some fixed point, and when all 

 is steady, I cut the string with the blaze of a match. 

 If the wheel were at rest, the vibration would be in a 

 fixed plane — at least till the rotation of the earth, or 

 some imperfection in the support at A, had time to 

 manifest itself. Leaving these out of consideration — 

 for we shall be through with our experiment before 

 they have time to show themselves — we fix our atten- 

 tion upon a molecule, d. As the pendulum moves 

 from its high position at the right towards the centre, 

 d tilts downward a very little, but enough to impart 

 to it a certain momentum. When, therefore, it comes 

 to d' (90° from its first position) it pushes vertically 

 downward. The effect of such a force is to push the 

 gyroscope away from the spectator. Turning to the 

 molecule, b, opposite to d, we find that it goes up- 

 ward while d was going downward, and, consequently, 

 when it gets around to b', it pushes up. Hence, a 

 and b work together, and tend to send the instrument 

 from the observer (i. e., behind the plane of the paper), 

 What is true of d and b is true of every other opposite 

 pair of molecules, and so is true of the whole. As the 

 amount of the push depends upon the velocity of b's 

 and d's motions, the lateral movement will be zero when 

 the vibration commences, will reach its maximum at 

 bottom of arc, and decrease to zero as the bob ascends to 

 the limit of the arc. 



When the bob begins to return from the left to right, 

 d will move up from the horizontal, and b down from it, 

 consequently d' and b' will reverse their previous action, 



182 



