“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.” Mr. Seymour answered this question, by observing that, if the hand possessed the same degree of feeling as the face, they would both suffer equally under the conflict. “If,” continued he, “you strike a glass bottle with an iron hammer, the blow will be received by the hammer and the glass; and it is quite immaterial whether the hammer be moved against the bottle at rest, or the bottle be moved against the hammer at rest, yet the bottle will be broken, though the hammer be not injured; because the same blow which is sufficient to shiver the glass is not sufficient to break or injure a lump of iron. In like manner, the blow that is sufficient to pain your sensitive face, and make your ears tingle, will not occasion the least annoyance to the obtuse hand of your preceptor. The operation of this law,” continued Mr. Seymour, “will be exemplified in every step of our progress. When the marble, as it rolls along, strikes any obstacles, it receives, in return, a corresponding blow, which will be found to influence its subsequent direction. The peg of the top, as it rubs on the ground, is as much influenced by the friction, as if a force were actually applied to it when in a state of rest; and when we consider the forces by which the kite is made to ascend into the air, you will learn, from the same law, the nature of that advantage which you derive from running with it.” The vicar observed that the subject of _Momentum_ might be introduced, and advantageously explained, upon this occasion.