-Ci LECTURE VXl. 



for although the bucket seems suspended by its handle, yet if the handle be- 

 gan to descend, the centre of gravity would be obliged to rise ; consequently 

 the whole will retain its position, and remain at rest. (Plate III, Tig. 45.) 



The apparent ascent of a loaded cylinder on an inclined plane, and tjie mo- 

 tion of a roller composed of two united cones, with a coynnpu axis, resting on 

 the edge of a triangle which is inclined to the horizon, may bq easily under- 

 stood from the same consideration. (Plate III. Fig. 46.) 



We may also observe, in tJie equilibrium of animals, many circumstances il- 

 lustrative of the properties of the centre of gravity. W hen a person stands on 

 one foot, and leans forwards, in the attitude which is usually exhibited in the 

 statues of Mercury, the other foot is elevated behind, in order to bring back 

 the centre of gravity, so as to be vertically over some part of the foot on which 

 he stands. But on account of the convex, and irregular form of the foot, the 

 basis that it affords is really very narrow; hence when we attempt to stand on 

 one foot, we find it often necessary to use a muscular exertion, in order to bring 

 the point of support to that side towards which we are beginning to fall ; and 

 when the basis is still more contracted, the body never remains at rest, but, by 

 a succession of actions of this kind, sometimes too minute to be visible, it is 

 Jcept in a state of perpetual vibration, without ever attaining such a position as 

 would give it any degree of positive stabiUty; and thus it! may be, conceived to 

 be supported even on a single point, recovering its position, from time to time, 

 by means of a slight degree of rotatory motion, which is produced by its flex- 

 ure, and by the changes of the position of the extremities : hence, b}' habit, the 

 arts of ropedancers and balancers are acquired. Sometimes, however, the po- 

 sition of the balancer ^is not so dilhcult to be preserved as it appears, the cur- 

 vature of the wire in contact with the foot tending materially to assist him. 



When we attempt to rise from a scat, we generally draw our feet inwards, 

 in order to bring the point of support into, or near, the vertical line passing- 

 through the centre of gravity, and to create a tottering equilibrium, which is 

 favourable for the beginning of motion. And before we rise, we bend the 

 upper part of the body forwards, in order to procure a momentum, capable of 

 carrying the centre of gravity beyond the vertical line, passing through the 

 point of support. 



