It is hardly necessary to state, that the children instantly assented to these propositions. The vicar had placed them in so clear and popular a point of view, as to be intelligible to the lowest capacities.
“With these admissions, then, my dear children,” said their father, “I shall have but little difficulty in convincing you of the truth of the other laws by which the direction of moving bodies is governed. At present, however, it is not my intention to enter upon this subject; you have some preliminary knowledge to acquire before you can understand what is termed the Composition and Resolution of Forces.”
“I shall not easily forget,” said Louisa, “that matter is perfectly passive, and that it can neither put itself in motion when at rest, nor stop itself when in motion.”
“This indifference to rest or motion,” replied Mr. Seymour, “has been termed the Vis Inertiæ of matter.”
“A very objectionable term,--a very puzzling expression,” exclaimed the vicar;--“to denote a mere state of passive indifference by the term Vis, or power, does appear to me, who have been in the habit of connecting words with ideas, as excessively absurd.”
“I allow,” said Mr. Seymour, “that the simple word Inertia would have been more correct; but we are bound to receive an expression which has been long current. I suppose, however, you know that the addition of Vis originated with Kepler, who, like my boy Tom, could not help thinking that the disposition of a body to maintain its motion, or state of rest, indicated something like power; but we will not waste our time upon verbal disquisitions. It is clear, that matter, at rest, resists being put in motion; the degree of that resistance is always in proportion to the degree of force applied to put it in motion; or, to speak more philosophically, that Action and Reaction are equal, and in opposite directions.”
“You, surely, do not mean to say,” exclaimed Tom, “that if I strike my marble, the marble strikes my hand, with the same force in return?”
“Precisely; that is my meaning.”
“What!” cried Louisa, “if a man strikes another on the face with his hand, do you seriously maintain that both parties suffer the same pain?”
“Oh, no, no,” said Tom, “papa can never intend to say that; I am quite sure, if it were the case, Mr. Pearson would not be so fond of boxing our ears.”