“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?”
“That point cannot be supported,” answered his father, “unless the ring be so held that the line of direction shall fall within the base of the support, which will be the case whether you poise the ring on the tip of your finger, or suspend it by a string, as represented in the figures which I have copied from the ‘Conversations on Natural Philosophy.’ I need scarcely add, that it will be more stably supported in the latter position, because the centre of gravity is below the point of suspension; whereas in the former the base is extremely narrow, and it will, consequently, require all the address of the balancer to prevent the centre of gravity from falling beyond it. As you are now in possession of all the leading principles upon which the operations of the centre of gravity depend, I shall put a few practical questions to you, in order that I may be satisfied you understand them. Tell me, therefore, why a person who is fearful of falling, as, for instance, when he leans forward, should invariably put forward one of his feet, as you did the other day, when you looked into Overton well?”
“To increase his base,” answered Tom; “whenever I lean greatly forward, I should throw the line of direction beyond it, did I not at the same instant put out one of my feet, so as to extend my base, and thus to cause the line to continue within it.”