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 (6), 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." Unquestionably but there is a practical difficulty in the attempt, from the resistance of the air, which increases, as you have already seen in the experiment of the paper and pennypiece (p. 64.) as the weight of a body decreases. Were it not for this resistance of the air, a hailstone falling from the clouds would acquire such descend like a bullet from a gun, and destroy every thing before it ; even those genial showers which refresh us in the spring and summer months, would, without such a provision, destroy the herbage they are so well calculated to cherish. From this view of the subject of Momentum," continued Mr. Seymour, "you will easily understand why the immense battering rams, used by the ancients, in the arts of war, should have given place to cannon balls, of but a few pounds in weight. Suppose, for example, that the battering ram of Vespasian weighed 100,000 pounds, and was moved, we will admit, with such a velocity, by strength of hands, as to pass through 20 feet in one second of time; and that this was found sufficient to demolish the walls of Jerusalem, can you tell me with what velocity a 32-pounder must move to do the same execution?" "I will try," said Tom. "The momentum of the battering ram must be estimated by its weight, multiplied into the space passed over in a second of time; which is 100,000 multiplied by 20; that will give 2,000,000. Now, if this momentum which must also be that of the cannon ball, be divided by the weight of the ball, it will give the velocity required, which I make out to be 62,500 feet." 66 Admirably calculated," said Mr. Seymour; " and I will take care, my dear Tom, that your ingenuity shall be suitably rewarded." Mr.Twaddleton here observed, that he thought "his young friends and playmates" must have received, for that day, as much philosophy as they could conveniently carry away without fatigue. Mr. Seymour concurred in this observation; and the more readily, as the path they had to travel was rugged, and beset with difficulties. "I will, therefore,” said he, "not impose any farther burthen upon them; but I will assist them in tying, into separate bundles, the materials which they have collected in their progress, in order that they may convey them away with greater ease and security. Although their labour has been considerable, I trust, at our next meeting, that its produce will be turned to their gratification, and that they will depart with a satisfaction like that which lightens the burthen of the sportsman, who, laden with the spoils of the day, returns with the cheering prospect of the feast which they are designed to furnish. Know then, my dear children," said the affectionate parent, "that G you have, this day, been instructed in the three great Laws of Motion, viz. I. That every body will continue in a state of rest, until put into motion by some external force applied to it, and if that force be single, the motion so produced will be rectilinear, i. e. in the direction of a straight line. II. Change of Motion is always proportional to the moving force impressed, and is always made in the direction of the right line in which the force acts. III. Action and Reaction are equal, in equal quantities of matter, and act in contrary directions to each other. CHAP. V. A SAD ACCIDENT TURNED TO A GOOD ACCOUNT. JUST as Mr. Seymour was, on the following morning, stepping upon the lawn, with the intention of joining his children, Rosa and Fanny both made their appearance completely drenched with water, and dripping like mermaids. 66 Heyday!" exclaimed their father, "how has this misfortune happened?" "Do not be angry, papa," said Tom; "indeed, indeed, it was an accident. Fanny, observing the water-cart in the garden, had just begun to wheel it forward, when the water rushed over her like a wave of the sea, and, upon stopping the cart, it flew over with equal force on the opposite side, and deluged poor Rosa, who was walking in front of it." |