“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. “To prevent confusion, therefore, in our ideas, it became necessary to distinguish these two kinds of motion from each other by appropriate terms; and, accordingly, where there has been an actual change of place, in the common meaning of the term, the motion which produced it is termed ABSOLUTE motion; whereas, on the contrary, when the situation has been only relatively changed, by an alteration in the position of surrounding bodies, the motion is said to be RELATIVE.”