638 Professor Andrew Gray [Feb. 14, 



makes the aeroplane rear up in front. If the aeroplane is kept 

 horizontal such couples have to be l)alanced by stresses in the frame- 

 work. These considerations show that sudden turning of aeroplanes 

 should if possible, be avoided. Manoeuvres calling for such turning 

 are accompanied by very considerable danger. No doubt aviators 

 are aware of the existence of gyrostatic action, but there is con- 

 siderable haziness in people's minds as to its direction in the various 

 possible cases. The peculiar properties of rotating bodies need not, 

 of course, be understood theoretically by aviators, though it is well 

 to know something about them. But the aviator, like a person 

 walking or swimming, must know instinctively what to do in an 

 emergency, and what motions must be avoided. The gyrostatic 

 action he has to contend with lies hid, as it were, until he tries some 

 new and violent manoeuvre ; and then it brings him to grief. 



I now pass on to some special experiments which can be carried 

 out with these motor-gyrostats. First take one or two old experi- 

 ments (see Thomson and Tait's " Natural Philosophy," § 345"' et seq.), 

 which are more effectively performed with these fast-running instru- 

 ments. Here is a skate attachment (Fig. 4) on which I place the 

 gyrostat after its speed has been adjusted to the moderate value of 

 about 6000 revolutions per minute. The plane of the flywheel is 

 inclined to the vertical, and you see that the top does not fall down, 

 but precesses round on the table. I increase the inclination and the 

 precession becomes more rapid. Now I attempt to hurry the pre- 

 cession, and the gyrostat stands up erect ; I try to resist the pre- 

 cession and the gyrostat falls over. 



I mount the gyrostat with its wheel horizontal over a flexible sup- 

 port, in the present case a universal joint (Fig. 5). Without rota- 

 tion the instrument would fall over at once, but you see that it stands 

 stably erect when the flywheel is spinning, and has a precessional 

 motion when disturbed from the upright position. 



Again, here is a two-stilt support (Fig. 6). One of the stilts is 

 held by a long socket, at one side of the case, and may be regarded 

 as rigidly attached. The other stilt is simply a bit of wire pointed 

 at both ends ; one end rests on a table, the other, the upper end, 

 rests loosely in a hollow in the upper-side of this projecting piece 

 attached to the case. The gyrostat is thus supported between 

 two stilts, one fixed the other quite loose, and its axis is at right 

 angles to the plane of these when the arrangement stands upright. 

 It would be hard to devise a more unstable support. You see that 

 there is no possibility of making the arrangement stand up without 

 spin. But you see, on the other hand, that there is a fair amount of 

 stability with the flywheel spinning if the arrangement is allowed to 

 oscillate, or, as one might say, wriggle, backwards and forwards, 

 horizontally. 



In the next experiment (due originally, I have been told, to the 

 late Prof. Blackburn) the gyrostat is rigidly clamped to this metal bar, 



