742 SCIENTIFIC RECREATIONS. 



tion, it will be easily seen that the more rapid the movement given to the 

 top, that due to gravity remaining constant, the nearer will be the axis of 

 the resultant movement to its real axis, and consequently the slower will be 

 the movement of rotation of the whole round the pivot. Thus this apparently 

 incomprehensible phenomenon is easily explained by gravity, vertical force 

 producing a movement of rotation in a horizontal plane. One can also 

 explain by analogous reasoning, and calculation of passive resistance, why 

 the axis of the top gradually inclines in proportion as the speed of the latter 

 diminishes, and the speed of rotation round the fulcrum increases ; why it 

 falls immediately if an obstacle is opposed to -the latter movement, and 

 finally, why it produces on the hand which holds it movements which astonish 

 persons so intensely who behold it for the first time. 



The principle we have just described is often enunciated, by saying 

 that every body in rapid rotation rests in its plane, and can only be driven 

 out by a considerable force ; this, however, is a defective formula. The 

 principle should be stated in the following manner. A body in rapid rota- 

 tion tends to remain in its plane ; that is, its axis rests parallel with itself, 

 and instead of obeying any force tending to divert its direction, it produces 

 in consequence of the combination of two simultaneous movements, a dis- 

 placement of the axis, generally much feebler and of a different kind from 

 that which this force exercises on the same body in repose. One of the 

 most charming applications of this theory is due to M. Foucault. The 

 Gyroscope, which bears his name, is a heavy disc, the axis of which is supported 

 by a " Cardan " balance, so that, whatever is the position of the contrivance, 

 it is possible to preserve it in a constant direction. Therefore if the disc is, 

 by means of special mechanism, put in rapid rotation, we may give it all 

 kinds of possible displacement without changing the plane in which the 

 gyroscope moves. Supposing then that its connection with the suspension 

 is fixed in a relatively immovable manner, but attracted by a movement 

 towards the ground, the plane of rotation of the disc will not entirely 

 participate in this movement. It is true, it will be carried into the move- 

 ment of general removal, but it will remain constantly parallel with itself, 

 and only appears displaced in comparison with the surrounding objects, 

 which obey more completely than itself the movement of the globe's rota- 

 tion round its poles. Thus can we demonstrate the movement of our 

 planet. In virtue of the same principle, we see every day passing before 

 our eyes a crowd of phenomena with which we are so familiar that they do 

 not excite our attention. Thus it is because the hoop tends to remain in 

 its plane of rotation that it rolls on without falling or deviating, and for the 

 same reason that tops rotate vertically on their points, or when they are 

 running down, describe a series of concentric circles ; and for the same 

 reason again, a juggler is able so easily to hold on the point of a stick a 

 plate which he puts in rapid rotation, etc. It is also owing to this property 

 of rotating bodies that we have been enabled to make use of cylindrical or 

 conical projectiles in artillery. The coiled riflings of the cannon causing the 



