“I now remember,” cried Tom, “that the centre of gravity is that point, about which all the parts of a body exactly balance each other.” “Now you are right; it is, in other words, that point in which the whole weight, or gravitating influence, of a body is, as it were, condensed or concentrated, and upon which, if the body be freely suspended, it will rest with security; and consequently, as long as this centre is supported, the body can never fall; while, in every other position, it will endeavour to descend to the lowest place at which it can arrive.” “Have all bodies, whatever may be their shape, a centre of gravity?” asked Louisa. “Undoubtedly.” “And you say,” continued Louisa, “that every body will fall, if this point is not supported.” “Infallibly. And now, Tom,” said Mr. Seymour, “can you tell me what is meant by the _line of direction_?” The young philosopher was unable to answer this question, and his father, therefore, informed him that, if a perpendicular line were drawn from the centre of gravity of a body to the centre of the earth, such a line would be termed the _line of direction_; along which every body, not supported, endeavours to fall; and he was also informed that, if this said line fell within the base of a body, such a body was sure to stand; but never otherwise. Louisa observed that she was not quite sure she understood her papa’s meaning, and therefore begged for further explanation. [Illustration: Fig. 10. Overloaded cart with plumb line hanging from its center of gravity to outside its base of support.] [Illustration: Fig. 11. Properly loaded cart with plumb line hanging from its center of gravity to inside its base of support.] “I will exemplify it then,” replied Mr. Seymour, “by a drawing. Fig. 10 represents a load of stones in a cart moving upon the sloping road C D E; this load, being low down in the cart, B will represent its centre of gravity, and B F its line of direction, which, you perceive, falls much within the supporting or lower wheel G; and there cannot, therefore, be any danger of such a cart being overturned; but in fig. 11, the centre of gravity is raised from its former position to H, and H I is now the line of direction; which, falling without the base, or wheel K, the load will not be supported, and must consequently fall. These figures,” added Mr. Seymour, “will also explain a fact which you must have frequently observed, that a body is stable or firm in proportion to the breadth of its base; hence the difficulty of sustaining a tall body, like a walking stick, upon its narrow base; of that of balancing a hoop upon its edge, or a top upon its point; while, on the contrary, it is almost impossible to upset the cone or the pyramid, since, in the latter cases, the _line of direction_ falls within the middle of the base, the centre of gravity of the body being necessarily low.”