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.”_ [Illustration: Two men on the ice, falling in opposite directions.] ------------------------------------------------------------------------ CHAPTER IV. _A sad accident turned to a good account.--One example worth a hundred precepts.--The Centres of Magnitude and Gravity.--The Point of Suspension.--The Line of Direction.--The stability of bodies, and upon what it depends.--Method of finding the centre of gravity of a body.--The art of the Balancer explained and illustrated.--Various balancing toys._ 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. “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.” “Well, well, lose no time in changing your clothes, and meet me again in half an hour.” At the appointed time the children reassembled on the lawn. “And so then,” said their father, “I perceive that my philosophical lesson of yesterday has been entirely lost upon you.” The children were unable to comprehend the meaning of this rebuke; but Mr. Seymour proceeded:-- “I trust, however, that the accident of this morning will serve to impress it more forcibly upon your memory: one example is better than a hundred precepts.” Tom was more puzzled than ever.