ON PRESSURE AND EQUILIBRIUM. 49 



fectly erect, and the centre of gravity of the carriage with its passengers, 

 will be somewhat more elevated than it would be on this supposition. 



The direction of the initial motion of the centre of gravity readily ex- 

 plains the suspension of a weight or a bucket of water, on a rod resting on 

 the end of a table, when another rod is employed to keep the bucket at 

 such a distance from the end of the first, that the centre of gravity may be 

 under the table ; for although the bucket seems suspended by its handle, 

 yet if the handle began to descend, the centre of gravity would be obliged 

 to rise ; consequently the whole will retain its position, and remain at rest. 

 (Plate III. Fig. 45.) 



The apparent ascent of a loaded cylinder on an inclined plane, and the 

 motion of a roller composed of two united cones with a common axis,* 

 resting on the edge of a triangle which is inclined to the horizon, may be 

 easily understood from the same consideration. (Plate III. Fig. 46.) 



We may also observe in the equilibrium of animals many circumstances 

 illustrative of the properties of the centre of gravity. When 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, wheji 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 kept in a state of perpetual vibra- 

 tion, without ever attaining such a position as would give it any degree of 

 positive stability ; 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 flexure and by the 

 changes of the position of the extremities : hence, by habit, the arts of 

 rope-dancers and balancers are acquired. Sometimes, however, the position 

 of the balancer is not so difficult to be preserved as it appears, the curva- 

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



When we attempt to rise from a seat, 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. 



When a horse is walking, the centre of gravity is sometimes supported 

 only by two feet of the same side, yet for a time so short that its declension 

 towards the other side is easily recovered, after the legs on that side have 



* Krafft on the apparent Ascent of a Double Cone, Nov. Com. Petrop. vi. 389. 

 Kastner on a Cylinder appearing to roll upwards. Deutsche Schriften Soc. Gott. 1 13. 

 On the motion of a double cone, see also Kostonov. Nov. Act. Petr. 1789, vii. 229. 

 Brunings Hind. Arch. ii. 321. 



