“I now,” said he, “suspend the kite by the loop at its bow, and since it is at rest, we know that the centre of gravity must be exactly below the point of suspension; if, therefore, we draw a perpendicular line from that point, which may be easily done by a plumb-line, with a weight attached to it, such a line will represent the _line of direction_ (as indicated by A B in fig. 13)”. “It is clear enough,” said Tom, “that the centre of gravity must lie in the line A B, but how are we to find in what part of it?” “By suspending the kite in another direction,” answered Mr. Seymour, who then hung it up in the position represented at fig. 14, “and then by drawing another perpendicular from the new point of suspension.” [Illustration: Fig. 14. Kite viewed at a diagonal.] “The centre of gravity,” said Louisa, “will in that case be in the line _c d_, as it was before in that of _a b_.” “In both the lines!” exclaimed Tom, with some surprise; “it cannot be in two places.” “And therefore,” added Mr. Seymour, “it must be in that point in which the lines meet and cross each other:” so saying, he marked the spot _g_ with his pencil, and then told his little scholars, that he would soon convince them of the accuracy of the principle. He accordingly placed the head of his stick upon the pencil mark, and the kite was found to balance itself with great exactness. “True, papa,” said Tom, “that point must be the centre of gravity, for all the parts of the kite exactly balance each other about it.” “It is really,” observed Louisa, “a very simple method of finding the centre of gravity.” “It is,” said Mr. Seymour; “but you must remember that it will only apply to a certain description of bodies: when they are not portable, and will not admit of this kind of examination, their centres of gravity can only be ascertained by experiment or calculation, in which the weight, density, and situation of the respective materials must be taken into the account. Having proceeded thus far, you have next to learn that the centre of gravity is sometimes so situated as not to be _within_ the body, but actually at some distance from it.” “Why, papa!” exclaimed Tom, “how can that possibly happen?” “You shall hear. The centre of gravity, as you have just said, is that point about which all the parts of a body balance each other: but it may so happen that there is a vacant space at this point. Where, for example, is the centre of gravity of this ring? Must it not be in the space which the ring encircles?” “I think it must,” said Tom; “and yet how can it be ever supported without touching the ring?” [Illustration: One ring supported by a string, and another ring balanced on a finger.]