ture's boundless parterres,17 and gather sweets from all of them. 47. "Nature," said Mr. Maynard, "does not teach us the whole of one science before she imparts some knowledge of another. She has given us five senses, and it is our duty so to educate them that they may be faithful and 'swift-winged messengers' to convey to the mind perceptions18 of the surrounding world. The more of these well-assorted mental stores are garnered up in the chambers of thought, ready to respond to the call of memory, the greater the amount of material which the mental powers will have to work upon. 48. "Let no one," said he, "compare the mind of the child, thus educated, to a reservoir filled by art. While every system of education should be based upon thorough discipline of the mental powers, I would place before them an abundance of the materials of knowledge; and as ideas are recollected perceptions, we may expect, other things being equal, to find the most ideas in those who have had the most thorough education of the senses. 49. Let us profit by such suggestions. Indeed, what extent and variety of knowledge are required in the teacher of children! To be a perfect specimen - a model teacher- all science and literature should be at his command: he should be a master of the art of instruction, and fascination should dwell upon his lips. 1 ME-CHAN'-IC-AL, pertaining to machines 11 Ax'-Is or PIV'-OT, the point of suspension and the principles of mechanics. Me- on which the lever turns. chanics is that science which treats of the 12 AR-CHI-ME-DES, a famous mathematician doctrines of motion. 2 CE'-SAR'S BRIDGE. This refers to the famous bridge which Cæsar built for crossing the Rhine into Germany. 3 CAT-A-PUL'-TA or ¤ÃT'-A-PULT, BAL-LIS'- 4 AP-PRE-CIATE, set a proper value upon. and mechanician of antiquity, born at Syracuse, in Sicily. He declared that if he had another earth on which to place his machines, he could move that which we inhabit. By the invention of machines he for a long time defended Syracuse on its being besieged by the Romans under Marcellus. 13 TEN'-SION, the straining, tightness or stretching caused by the weight or power. IN-CLINED', tending downward in direction. 14 6 IN-FLEX'-I-BLE, that can not be bent. 9 MAN'-U-AL, a small hand-book containing 16 17 PÄR-TERRES' (pär-tārs), flower-gardens. 18 PER-CEP'-TION, the notice which the mind takes of external objects. 7 Fig. 31. LESSON VIII. MECHANICAL POWERS-Continued. 1. Mr. M. The wheel and axle is, as you see, a lever continually acting, and, of course, its powers will be estimated on the same principle as the lever. If the circumference of the wheel is 10 feet, and that of the axle 2 feet, what power, applied at the circumference of the wheel, will balance 500 pounds suspended from the axle ? 2. Frank. The rope attached to the weight will go two feet in one revolution, which will raise the weight two feet; and if we multiply the weight, 500, by its distance, 2 feet, we have 1000. But as the power, multiplied by its distance, 10, must equal this, we have 100 for an Fig. 32. answer. Mr. M. That is very handsomely explained. What modifications of the wheel and axle can you name? John. The windlass seems to be, in real ity, the same thing. Fig. 35. 4. Mr. M. It is a wheel and axle made to revolve by the weight of several persons stepping constantly on the circumference of a long wheel. The treadwheel is often employed in prisons as a means of driving machinery, where the rogues, as they turn the wheel, can sing, "They've built us up a noble wall, To keep the vulgar out: But just to walk about." Next we must talk about the wedge, which is reckoned as two inclined planes. Give me some examples of the use of this modified mechanical power. 5. Ida. Nails, knives, needles, axes, hatchets, chisels, razors, swords, and scissors. Ella. Then scissors are both levers and inclined planes. Mr. M. You have named but few of the applications of this power. The wedge is commonly employed in splitting wood, rocks, etc. A thin wedge requires less power to move it forward than a thick one. Can you give me one of the most surprising instances of the power gained by the use of the wedge? Fig. 36. 6. Frank. I think its use in the supporting and launching of ships is certainly very surprising. Ella. Oh! I should be so delighted to understand how a ship is launched. I have just read and learned Longfellow's beautiful description of the "Building of the Ship." Mr. M. Then you may repeat it if you please. Ella. I am not certain that I shall get all the words right, but I will try. All around them, and below, The sound of hammers, blow on blow, She starts-she moves--she seems to feel And, spurning with her foot the ground, She leaps into the ocean's arms." 8. Mr. M. The very lines seem almost enough to start the vessel from her ways,' but the reality is a little more prosaic.2 The weight of the ship you see Fig. 37. in the picture is supported on blocks and wedges under the keel.3 Along the sides are smooth timbers, at an inclination sufficient to enable the vessel to slide when the weight comes on to the sliding planks, by means of a frame or cradle fitted to the form of the ship. 9. The ways being well greased, the blocks and wedges which had been supporting the ship are driven out from under the keel, until the whole weight gradually rests upon the sliding ways or cradle, when the noble structure, from its own weight, glides into the water. The screw alone remains of the mechanical powers, and this is only a spiral1 inclined plane. 10. John. Is the screw a simple machine? Mr. M. The screw is placed under the head of simple machines, but can not be used without the application of a lever or some other contrivance, when it becomes a compound engine of great power, either in pressing bodies closer together, or in raising great weights. George. I do not see how the screw is an inclined plane. Will you please explain it ? 11. Mr. M. That will be evident if you wind the triangular piece of paper, b, b, around the pencil or cylinder, a, a. Do you not see that the upper edge of the paper continues around the pencil from bottom to top? Fig. 38. b George. Yes, sir, it is very plain. Ella. The upper edge of the paper, which certainly represents an inclined plane, coincides with the screw. And now I see that our winding stairs may be called a large screw. 12. Mr. M. True, common stairs may be considered as an inclined plane, with notches to keep the feet from slipping, and the winding of the plane makes a screw. In this case people walk on the threads of the screw, but commonly the screw itself is turned. It con sists of two parts. In the figure which I here show you, a, a, is called the screw; c, c, the nut; and b, the lever. 13. John. I see, if the screw is turned one way, it will be raised from the nut a distance equal to that between the turns or threads, while if it is turned the other way it will be lowered the same distance. I would like to know how the power gained by its use is calculated. Fig. 39. 14. Mr. M. Well, suppose the distance between the threads to be one inch, and the length of the lever five feet, what pressure can be exerted if a power of ten pounds be applied at the end of the handle? George. Surely this can not be difficult, for the distance in inches which the handle moves, multiplied by the power, ten pounds, must be equal to the one inch which the screw moves, multiplied by the number of pounds' pressure.* Fig. 40. * 15. Mr. M. I am glad you understand the beautiful simplicity of the formula I gave you in the last conversation. The distance described by the power must be ten feet in diameter, or 31.416 feet in circumference, which, multiplied by the power 10, will give 314.16. Divide this by one inch, or one twelfth of a foot, and we obtain 3769.92 pounds for the pressure exerted by a power of ten pounds. |