“What honours, then,” continued Louisa, “shall we decree to Tom’s ball, if it instructs us in the first principles of philosophy?”
“We are trifling,” observed Mr. Seymour, and so saying, he took the ball from Tom’s hand, and rolling it along the ground, exclaimed, “there it goes, performing, as you may perceive, two different kinds of motion at the same time; it turns round, or revolves on its axis; and goes straight forward, or, to speak more philosophically, performs a rectilinear motion.”
Tom said that he did not exactly comprehend what was meant by the axis.[(7)] His father, therefore, informed him that the axis of a revolving body was an imaginary line, which was itself at rest, but about which all its other parts turned, or rotated: “But,” continued he, “can you tell me whether you understand what is meant by the word motion?”
“If he can,” exclaimed the vicar, “he is a cleverer fellow than the wisest philosopher of antiquity, who, upon being asked the very same question, is said to have walked across the room, and to have replied, ‘You see it, but what it is I cannot tell you.’”
“Your ancient acquaintances,” observed Mr. Seymour, “entertained some very strange notions touching this said subject of motion. If I remember right, Diodorus denied its very existence; but we are told that he did not himself remain unmoved, when he dislocated his shoulder, and the surgeon kept him in torture while he endeavoured to convince him, by his own mode of reasoning, that the bone could not have moved out of its place: we have, however, at present, nothing to do with the ancients; the philosophers of our own times agree in defining motion to be ‘the act of a body changing its situation with regard to any other;’ and you will therefore readily perceive, that this may actually happen to a body while it remains absolutely at rest.”
“Well, that beats all the paradoxes I ever heard,” cried Tom; “a body then may be in motion, while it is at rest?”
“Certainly,” replied Mr. Seymour; “it may be relatively in motion, while it is absolutely at rest.”
“How can a body change its place,” said Louisa, “except by moving?”
“Very readily,” answered her father; “it may have its relative situation changed with respect to surrounding objects; there is your ball, and here is a stone, has not each of them a particular situation with respect to the other; and by moving one, do I not change the relative situation of both?”
“I perceive your meaning,” said Tom.