[Illustration: Fig. 45. A Represents the egg in its natural state, and, therefore, in unstable equilibrium; B, another egg, with the surface, S, flattened, by which the centre of gravity is lowered, and if not disturbed beyond the extent of the point of support the equilibrium is stable.] The most philosophical mode of making the egg stand on its end and without disturbing the exterior shell is to alter the position of the yolk, which has a greater density than the white, and is situated about the centre. If the egg is now shaken so as to break the membrane enclosing the yolk, and thus allow it to sink to the bottom of the smaller end, the centre of gravity is lowered; there is a greater proportion of weight [Page 36] concentrated in the small end, and the egg stands erect, as depicted at fig. 46. It is this variable position of the centre of gravity in ivory balls (one part of which may be more dense than another) that so frequently annoys even the best billiard-players; and on this account a ball will deviate from the line in which it is impelled, not from any fault of the player, but in consequence of the ivory ball being of unequal density, and, therefore, not having the centre corresponding with the centre of gravity. A good billiard-player should, therefore, always try the ball before he engages to play for any large sum. [Illustration: Fig. 46.--No. 1. Section of egg. C. Centre of gravity. Y. The yolk. W. The white. No. 2. C. Centre of gravity, much lowered. Y. The yolk at the bottom of the egg.] The toy called the "tombola" reminds us of the egg-experiment, as there is usually a lump of lead inserted in the lower part of the hemisphere, and when the toy is pushed down it rapidly assumes the upright position because the centre of gravity is not in the lowest place to which it can descend; the latter position being only attained when the figure is upright. [Illustration: Fig. 47.--No. 1. C. Centre of gravity in the lowest place, figure upright. No. 2. C. Centre of gravity raised as the figure is inclined on either side, but falling again into the lowest place as the figure gradually comes to rest.] There is a popular paradox in mechanics--viz., "a body having a tendency to fall by its own weight, may be prevented from falling by adding to it a weight on the same side on which it tends to fall," and the paradox is demonstrated by another well-known child's toy as depicted in the next cut. [Page 37] [Illustration: Fig. 48. The line of direction falling beyond the base; the bent wire and lead weight throwing the centre of gravity under the table and near the leaden weight; the hind legs become the point of support, and the toy is perfectly balanced.] [Illustration: Fig. 49.--No. 1. Sword balanced on handle: the arc from C to D is very small, and if the centre, C, falls out of the line of direction it is not easily restored to the upright position.