"The gravitation of Tom's ball," said Mr. Seymour, "furnished an ample subject for our morning's diversion, let us try whether its other motions will not suggest further objects of enquiry."

"I well remember," observed Louisa, "that Mrs. Marcet extols that apple, the fall of which attracted the notice of Sir Isaac Newton, above all the apples that have ever been sung by the poets and she declares, that the apple presented to Venus by Paris; the golden apples by which Atalanta won the race; nay, even the apple which William Tell shot from the head of his own son, cannot be brought into comparison with it."

"Well said! Mrs. Marcet," exclaimed Mr. Seymour; "upon my word, had the mother of mankind used but half such eloquence in praise of an apple, we cannot wonder at its influence on Adam."

"What honours, then," continued Louisa, "shall we decree to Tom's ball, if it instructs us in the first principles of philosophy?"

"It shall be duly honoured," replied Mr. Seymour. "We will decree it a prominent station amidst the precious archives of the vicar; the

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relics of war shall recede, and afford a space for an emblem of science, but we are trifling:" 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."

His

Tom said that he did not exactly comprehend what was meant by the axis. 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, 66 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 exist

ence; 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 surpasses all the paradoxes I ever heard," cried Tom; "a body then may be in motion, while it is at rest."

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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 acceptation 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."

"Surely, papa," said Louisa, "no person can ever mistake relative for absolute motion; whence then is the necessity of such frivolous distinctions? When a body really moves, we can observe it in the act of changing its place, and no difficulty can arise about the matter."

"Nothing, my dear, is more fallacious than our vision; the earth appears motionless, and the sun and stars seem as if they revolved round it; but it is scarcely necessary for me to inform you that our globe is constantly moving with considerable velocity, while the sun remains at

rest. - Mr. Sadler, the famous aëronaut," continued Mr. Seymour, "informed me, that he was never sensible of the motion of the balloon in any of his excursions, but that, as he ascended into the air, the earth always appeared as if sinking beneath him."

Mr. Twaddleton here observed, that he had heard a very curious anecdote, when he was last in London, which fully confirmed the truth of Mr. Sadler's statement. "An aeronaut," said he, "whose name I cannot at this moment recollect, had recently published a map of his voyage, and, instead of proceeding in any one line of direction, his track absolutely appeared in the form of circles, connected with each other like the links of a chain: this occasioned considerable astonishment, and, of course, some speculation, until it was at length discovered, that his apparent journey was to be attributed to the rotatory motion of the balloon, which the voyager, not feeling, had never suspected."

"And what," asked Tom, "could have been the reason of his not having felt the motion?" His father explained to him, that we are only conscious of being in motion when the