“Momentum,” said Tom; “and pray what is that?” “It is a power,” replied his father, “intimately connected with motion; and, therefore, as your friend, the vicar, justly remarks, may be very properly introduced before we quit that subject.--It is the force with which a body in motion strikes against another body.” “That,” observed Tom, “must of course depend upon the velocity of the body’s motion.” “Undoubtedly, my dear; the quicker a body moves, the greater must be the force with which it would strike against another body; but we also know that the heavier a body is, the greater also will be its force; so that _momentum_, you perceive, must have a relation to both these circumstances, viz. velocity, and weight; or, to speak more correctly, _the momentum of a body is composed of its quantity of matter, multiplied by its quantity of motion_: for example, if the weight of a body be represented by the number 3, and its velocity also by 3, its _momentum_ will be represented by 3 x 3 = 9; so that, in producing momentum, increased velocity will always compensate for deficiency of matter, and a light body may thus be made a more effective force than a heavy one, provided that its velocity be proportionally increased; thus, a small ball weighing only _two pounds_, and moving at the rate of _five hundred feet_ in a second, will produce as much effect as a cannon ball of _ten pounds_ in weight, provided it moved only at the rate of _one hundred feet_ in the same time.” “Let me see,” cried Tom, “whether I understand your statement. We must multiply, as you say, the weight by the velocity; the weight of the small ball you state at two pounds, and it travels at the rate of five hundred feet in a second; then its momentum must be a thousand. The weight of the great ball is ten pounds, its velocity only a hundred feet, then its momentum must also be a thousand; because, in both cases, the sums multiplied into each other will give the same product.” “Exactly: and thus you perceive that the small ball becomes an exact balance to the larger one; the first making out in motion what it wanted in matter, while the latter makes out in matter what it wanted in motion. I wish you to keep this law of _Momentum_ in your remembrance; upon it depends the action of all the _mechanical powers_(8), as they are termed.” “I have heard,” said Louisa, “that a feather might be made to produce as much havoc as a cannon shot, if you could give it sufficient velocity.”