1858.] on Rotatory Stability, 487 



Now let us suppose that the observer (carried on the inner 

 frame), by some slight movement, displaces the centre of gravity of 

 the whole ; or what is the same thing, let us suppose a small weight 

 added to one end of the frame, — suppose, so as to give a tendency 

 to turn about (p), then the disk (jo) simply retains its position, but 

 the disk {q) is affected — not, however, directly ; but the effect 

 being compounded with the rotation, gives a precessional motion to 

 that particular disk, which merely causes it to turn on the pivots of 

 its own inner ring. If the small weight were placed so as to pro- 

 duce motion about (q), precisely the same effect would take place 

 on the disk (/>), while {q) would retain its position. 



In one word, the precessional disturbance is transferred from 

 the general frame, to the particular revolving disk. 



Thus on the whole, as Prof. Smyth observes, " If any want of 

 balance is produced on the frame, it is not immediately altered 

 thereby, the first effect being only to make the appropriate wheel 

 turn on its gymbal pivots. If, too, the wheel is heavy and the 

 speed great, this precessional turning will be so slow that there will 

 be time for an attendant to press in the opposite direction on the 

 general frame, and correct the effect of the observer's want of 

 balance, before the plane of rotation coincides with the direction of 

 the disturbing force, when the virtue of resistance ceases.'* 



To strengthen the effect, and at the same time to give a more 

 convenient balance to the whole, the disks {p) and {q) respectively, 

 are reinforced each by a corresponding revolver at the other end 

 of the same diameter of the frame, and similarly mounted in the 

 inner general frame. 



Thus all angular disturbance in vertical planes being got rid of, 

 it only remains, on the principle before described, to add a fifth 

 revolver in the middle, in a vertical plane, to counteract angular 

 horizontal motion. The inner general frame supports a vertical 

 frame, which carries this last revolver. Thus we have the con- 

 struction of " the compound precessional free revolver stand," as 

 the inventor designates it. But he has found the simple form so 

 effective by itself in the hands of a practised observer, as to render 

 this more complex construction of no immediate necessity. 



To complete the whole, Prof. Smyth has carefully investigated 

 the best form of a train of wheels for communicating the rotatory 

 motion : and has also considered the question of the best moving 

 power to be used ; which he finds, after many trials, to be that of 

 water ; which is best brought to bear by a peculiarly beautiful and 

 simple form of the turbine. 



Thus taking a summary view of the whole subject ; — by direct 

 consequence, from the simplest acknowledged mechanical principles, 

 the gyroscope, when its equilibrium is slightly disturbed, demon- 

 strates the precession of equinoxes ; explains the boomerang ; — 

 and sustains itself in the air against gravitation. When its 

 equilibrium is undisturbed it exhibits to the eye the actual rotation 



