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George. When a body readily yields or bends under a force applied to it, it is said to be flexible; but if, instead of bending, it breaks, it is brittle.

13. John. I have seen knives and other tools very brittle when new, but very flexible when they had been heated, and have heard people say they had lost their temper.

Mr. M. Yes; when a piece of steel is heated, and then slowly cooled, it is flexible; but if cooled suddenly, it is brittle. You can try this experiment by heating and cooling a needle.

14. John. I recollect my father had a quantity of iron wire which he could not use because it was so brittle, and that he heated it red-hot, and cooled it slowly. I think he called it annealing.12

15. Mr. M. That is the name of the process. Glass can be annealed; and the value of glassware depends much on the manner in which it is cooled. Malleability is the property which allows bodies to be hammered or rolled into thin sheets. Do you know what is the most malleable substance?

16. George. Gold is considered the most malleable.

Mr. M. And what is most ductile, or can be drawn to the finest wire?

Frank. Platinum 13 has been drawn into finer wire than any other substance I ever heard of.

17. Mr. M. What is tenacity?

Frank. I believe the word is derived from the Latin word teneo, I hold; and if so, it must mean the property of holding together.

John. I think I have heard that iron is the most tenacious of the metals.

18. Mr. M. Yes; iron wire is so tenacious that when only the sixteenth of an inch in diameter it will support 540 pounds, while a similar wire of lead will only sustain 27 pounds. The tenacity of iron makes it valuable in the construction of suspension bridges and other structures.

19. Here Mr. Maynard remarked that, although their hour had not quite expired, he would close this conversation, as he had promised to accompany a number of the younger pupils in a long ramble immediately after dinner, for the purpose of aiding them in making a map of the stream which, flowed

through the valley. He informed the class that the subject of their next conversation would be MOTION AND ITS Laws.

20. The means which Mr. Maynard adopted for interesting his pupils in a great variety of subjects of study were wisely contrived for combining amusement and instruction. With a view to such results, the excursion referred to had been planned for his younger pupils; and by such means they were early grounded in the principles of geographical knowledge, and interested in learning more of a subject whose very rudiments had proved to them a delightful recreation.

21. Thus, before his youthful pupils were aware that they were studying geography, they could tell the direction by the compass, and the distance from the old mansion, of every grove, fountain, and hillock for two miles around, the windings of the stream which flowed through the valley, the various ravines which entered the glen below, and could accurately trace on a slate or paper a map of the whole, the boundaries of the estate on which they resided, and a profile outline of the hills which separated their little world of tranquil beauty from the great and noisy world around them.

22. But they had visited many of the neighboring villages beyond the hills, and could locate11 them also; and as they inquired about distant cities and countries, and read, or heard related, interesting accounts of them and their people, they not only learned more of geography, but began the acquisition of historical knowledge. Thus, though secluded15 for a while from the noisy scenes of life, they were taught that, if their lives should be spared, they were soon to mingle with the moving throng, and that the duties of life required of them a knowledge of that world on whose stage of action they were soon to enter.

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10 DUC-TIL-I-TY, that property of bodies which renders them capable of being extended by drawing without breaking.

11 TE-NAC-I-TY, that property of bodies which keeps them from being parted without considerable force.

12 AN-NEAL-ING, the process of applying heat for removing brittleness.

13 PLAT-I-NUM, the heaviest of the metals. 14 Lo'-CATE, determine the place of; arrange on a map.

15 SE-CLU'-DED, shut out; living in retire

ment.

LESSON V.

MOTION AND ITS LAWS.

1. Mr. M. WHEN Plato, an ancient philosopher, was asked for a definition of motion, he is said to have arisen, and to have walked back and forth in the presence of his interrogators, as much as to say, "You see it, but I can not tell you." Who of you will define motion?

1

Ida. Motion is a continued change of place with regard to a fixed point.

2. Mr. M. I would prefer to call motion simply the act of changing place, and consider it the condition of matter opposed to rest. It is enough for our purpose, however, that we understand the usual meaning of the term. Can you define uniform motion?

John. When a body passes over equal spaces in equal successive portions of time, its motion is called uniform.

3. Mr. M. Very well; now tell me when it is accelerated,2 and when retarded.3

John. When the spaces passed over in equal times continually increase, the motion is called accelerated; and when such spaces decrease, it is called retarded; as a stone thrown up in the air is retarded in ascending, and accelerated in descending.

4. Mr. M. A good definition, and I think you understand it. Can you define velocity?

George. It is the swiftness of the motion, and is measured by the space passed over in a given time.

Mr. M. Now tell me what momentum1 is.

5. George. Momentum is the quantity of motion, and is the products of the quantity of matter by the velocity.

Mr. M. If that is so, a small body moving swiftly may have as much momentum as a large one moving slowly.

George. Yes, sir. A 32-pound ball, moving 1000 feet in a second, will have as much momentum as a battering-ram weighing 2000 pounds, and moving 16 feet in a second.

6. Mr. M. I am pleased to see you so promptly make the practical calculations illustrative of the principles of motion.

Suppose you were on the deck of a steam-boat going north at the rate of ten miles an hour, and that, when you were in the same straight line with two trees on shore, you were to run toward the stern of the boat with a speed of ten miles an hour, would you change position in respect to the trees?

7. John. Certainly not; for in the time occupied in running south, the boat would have advanced as far north as the space I passed over, and I should have remained at the same point in space.

Ella. Running ten miles an hour and standing still at the same time is strange enough. How is it explained?

8. Mr. M. The motion was absolute in regard to the boat, but not in respect to the trees on the shore. If you had remained at the same place on the boat, you would have been in motion in relation to objects on shore; and if you had started from the stern of the boat and run in the direction of the boat's motion with the velocity stated, you would have been going twenty miles in respect to the trees, and ten miles in respect to the boat.

9. George. This seeming paradox" of motion and rest, or rest and motion, or motion twenty and ten miles at the same time, seems very plain to me when we use the terms relatively, or in comparison with other bodies.

10. Mr. M. If now you all understand what is meant by uniform, accelerated, retarded, absolute, and relative motion, as I think you do, I will only add that we do not know of any thing in a state of absolute rest. The earth on which we live is in rapid motion around the sun, and the sun itself is in motion around some central body, which, for aught we know, is itself revolving around some remote centre.

There are three principles of motion, known as Newton's Laws. Can you tell me the first?

11. Frank. "Every body continues in its state of rest, or of uniform motion in a straight line, unless acted upon by some external force." This, I suppose, means that a body at rest can not put itself in motion, and that a body in motion can not stop itself.

John. Then every thing at rest must always continue so unless moved by some force; and every thing in motion

must always continue moving unless stopped by some resist

ance.

12. Mr. M. You understand the exact meaning of the first law; now what forces tend to put bodies in motion, and what to stop them?

John. Mills are put in motion by horse-power, water-power, wind-power, and steam-power; so I would call the momentum of falling water, or of moving air, or the muscular power of animals, or the expansive force of steam, moving powers, or forces. Bodies in motion are stopped by the rubbing together of the surfaces in contact, and by the weight of the parts that have to be lifted.

13. Mr. M. That is, they are stopped by friction and gravity.10 George, can you give me the second law of motion?

George. "Change of motion is proportional to the force impressed,11 and is in the direction of the line in which that force acts." When I load my gun with a charge adapted to the distance I wish to shoot, and aim in the direction of the object I wish to hit, I act upon the second law of motion, and did so before I ever heard of such a law.

14. Mr. M. I hope you would not insinuate that you are no wiser for your pains in learning the law you have repeated so correctly. I am very glad the Laws of Newton, as they are called, are self-evident to you, and I shall expect to hear some very sensible replies from you to the questions I shall have occasion to ask in the course of these lessons. There is another law for Frank to state and illustrate.

15. Frank. "Action and reaction are equal, and in opposite directions." If I understand what it means, it is that when I pull the reins in riding, I push as much as I pull.

George. If that is the case, how does he stop the horse? 16. Frank. Allow me to suggest that the soft reins do not hurt my hands as much as the same force, exerted by the small and hard bit, hurts the horse's mouth; so he concludes to stop.

Mr. M. A very sage conclusion, truly; but tell me, Frank, if you were sitting in the stern of a boat, and pulling by a rope attached to the bow, could you move the boat?

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