1013] Gyrostats and Gyrostatic Action 631 



WEEKLY EVENING MEETING, 



Friday, February 14, 1913. 



Sir James Crichton Browne, J.P. M.D. LL.D. F.R.S., Treasurer 

 and Vice-President, in the Chair. 



Professor Andrew Gray, M.A. LL.D. F.R.S. 



Gyrostats and Gyrostatic Action. 



We are accustomed in daily life to handle non-rotating- bodies, 

 and their dynamical properties excite little attention, though it 

 cannot be said that they are commonly understood. It is different, 

 however, with rotating bodies. These, when handled, seem to be 

 endowed with paradoxical, almost magical properties. I have here 

 an egg-shaped piece of wood. I place it on the table and it rests, as 

 we expect it to do, with its long axis horizontal. Our experience 

 tells us that this is the natural and correct position of the body. 

 But I set it spinning rapidly on the table, as you see, with the long 

 axis horizontal, and you observe that after an apparently wobbling 

 motion it erects itself so that its long axis is vertical. It was started 

 spinning about a shortest axis, but the body has of itself changed the 

 spin, and it is now turning about the long axis. In taking this 

 position it has actually raised itself against gravity, through a height 

 equal to half the difference between the lengths of the long and 

 short axes. This seems paradoxical, but the man who is in the habit 

 of spinning tops knows that this is the proper position of the body, 

 that it must stand up in this way when spinning rapidly on a rough 

 horizontal plane. 



This experiment may be performed at the breakfast table with an 

 egg as the spinning body. But the egg must be solid within — that 

 is, it must be hard-boiled ; a raw or soft-boiled egg will not spin. 

 Perhaps this is why Columbus did not adopt this method for his 

 celebrated experiment ; there may, of course, have been other 

 reasons. 



It is thus made clear that by causing a body to rotate rapidly we 

 endow it with new and strange properties. Between a top when 

 spinning and the same top when not spinning* there is a difference 

 which reminds us of that between living and dead matter ; and this 

 will strike us still more forcibly when we consider some more compli- 

 cated cases of rotational motion. The top, the ordinary spinning- 

 top of the schoolboy, stands on its peg and " sleeps " in the upright 

 Vol. XX. (No. 107) 2 u 



