“The figure,” said Mr. Seymour, “is made of the pith of the elder-tree, which is extremely light, and is affixed to the half of a leaden bullet; on account, therefore, of the disproportion between the weight of the figure and that of its base, we may exclude the consideration of the former, and confine our attention to the latter. The centre of gravity of the hemispherical base is, of course, in its axis; and therefore tends to approach the horizontal plane as much as possible, and this can never be accomplished, until the axis becomes perpendicular to the horizon. Whenever the curved surface is in any other position, the centre of gravity is not in the lowest place to which it can descend, as may be seen by the diagram which I have just sketched. If the axis _a b_ be removed to _c d_, it is evident that the centre of gravity will be raised, and that, if left alone, it would immediately descend again into its original position.” [Illustration: Fig. 14. Hemisphere shown in two positions using solid and dotted lines.] “I understand it perfectly,” said Tom. “When the axis _a b_ is perpendicular, the centre of gravity will be in its lowest point, or as near the earth as it can place itself; when, therefore, the figure is pressed down, the centre of gravity is raised, and, consequently, on the removal of that pressure, it will descend to its original position, and thus raise the figure.” [Illustration: Toy man with sword, turban, and shield, mounted on a candlestick with weights jutting out and down from both sides.] “I see you understand it. Here, then,” continued Mr. Seymour, “is another toy in further illustration of our subject. It consists of a small figure, supported on a stand by a ball, which is quite loose; and yet it is made to turn and balance itself in all directions, always recovering its erect position, when the force applied to it is removed. The two weights, in this case, bring the centre of gravity considerably _below_ the point of suspension or support, and therefore maintain the figure upright, and make it resume its perpendicular position, after it has been inclined to either side; for the centre of gravity cannot place itself as low as possible, without making the figure stand erect.” “That is very evident,” cried Louisa. “I shall next exhibit to you,” continued Mr. Seymour, “a toy that furnishes a very good solution of a popular paradox in mechanics; viz. _A body having a tendency to fall by its own weight, how to prevent it from falling, by adding to it a weight on the same side on which it tends to fall._” “That is indeed a paradox!” exclaimed Louisa. “The next time I see the gardener sinking under the load of a heavy sack, I shall desire him to lighten his burden by doubling its weight.” [Illustration: Toy horse with its back legs on a ledge, with a weight attached to its belly with a wire which curves back and down under the ledge.]