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33. Mr. M. Perhaps the pulley is of more advantage to the sailor than o man in any other occupation. Have you ever heard of a contrivance called a fire escape, by which a man could let himself down in safety from the lofty window of a burning house?
John. I never heard of one, but I can at once see how it could be done.
34. Frank. I have often seen workmen making some slight repairs to the walls of houses, with one end of a rope fastened around their bodies, and passing over a pulley, as you see in this figure.
Ella. I can see how he can balance himself, but I can not understand how he can draw himself
35. Mr. M. By a little exertion, he can throw more than half his weight on one side of the pulley, which
makes that end of the rope descend, while the other Fig. 24 rises; and, as the nursery rhyme says, "so he goes up, up, up," or "down, down, down," as he chooses.
George. I recollect a man went down into our deep well in that way.
36. Mr. M. That was by means of a fixed pulley; but the mechanical power. is a movable pulley, like the one I have here represented. The power, you will at once see, being held by a single rope, passes over twice the space of the weight, which is held by a double rope.
John. Then, by the way we calculate the powers of levers, the weight is double the power in this case. 37. Mr. M. I shall have to name you John Archimedes, 12 for I verily believe he would have reasoned in the same way. Do you not see that one pulley moves up just as the weight does, and that it doubles the power?
Ida. I have long known that a movable pulley has this effect, but I never understood it before.
38. Mr. M. Not every movable pulley, but only those that have one arranged as you see this. I
have here two movable pulleys with one rope passing over them. What will one pound at P balance at W?
Ida. Four pounds.
39. Mr. M. That is correct. I have again three movable pulleys, as in the figure: what will one pound at P balance at W?
Ida. I was about to say six, but I see it would be eight. The figures on the strings show the tension13 or weight at each place.
Ella. Does the power in the last example really move eight times as far as the weight is raised?
40. Mr. M. Exactly eight times as far whenever it is but one eighth as heavy. John. Do workmen call them pulleys?
Mr. M. The ropes used about a pulley are called tackle, and the pulleys blocks; therefore, when a sailor or workman collects together all things necessary for the application of this machine, he speaks of the block and tackle.
What is the next simple mechanical power?
Ella. The Inclined Plane.
Mr. M. And what is an inclined plane?
Frank. The book says it is a plain surface inclined11 to the horizon.15
41. Mr. M. That is true; but let us take a familiar example. To lift barrels of flour into a cart from the ground would be rather hard work; but to roll them up an inclined plank or plane would be comparatively easy. Suppose the cart to be
three feet high, and the plane six feet long, how much power will be required to sustain a barrel of flour, weighing two hundred pounds, on the plane?
42. George. One hundred pounds,
if the principle by which the lever is calculated applies; for, to roll it up, the power will have to follow the barrel the
length of the plane; but the barrel will only be raised the height of the cart in a vertical direction. Now if the weight. be raised but half the space described by the power, it must be twice as great.
43. Mr. M. Very well explained indeed; and I must call you George Archimedes.
Frank. These mechanical powers are more like each other than one would have supposed.
44. Mr. M. The principle you so much admire, and so easily comprehend, is commonly known as the "law of virtual velocities," and I hope you will point out the first exception you find to this law. I expected to finish the subject of mechanics at this conversation, but find it necessary to continue the same subject in our next lesson.
45. "You have seen in this and the previous conversations," continued Mr. Maynard, as the class were about to separate, "that very common matters abound in philosophy; and, indeed, every thing that we can notice by any of our senses is capable of affording us some instruction. I will here remind you-what I have often said before-that it is of great importance to you in early life to cultivate such an inquiring state of mind as will not only incline you to notice every thing around you, but also to examine into the causes of things. By so doing you will find matters of interest in the most common things of life, and every thing will preach to you philosophy."
46. The system of education pursued by Mr. Maynard, and his great success in it, were such as to discredit 16 the doctrine that pupils must entirely master one subject before entering upon another. Proceeding upon the idea that the human mind is not a unit in its operations, and that its harmonious development demands a great variety of subjects for the exercise of its varied powers, the range that he gave to his pupils in the domains of knowledge was a very extensive one. Like the honey-bee, they were led to pass from flower to flower in Na
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-CHĂN'-IC-AL, pertaining to machines 11 Ax'-Is or PIV'-OT, the point of suspension and the principles of mechanics. Mechanics is that science which treats of the 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-LĬS'TA, and SCOR'-PI-O, were warlike engines for throwing stones, darts, and javelins to a distance.
on which the lever turns.
AR-CHI-ME'-DES, a famous mathematician and mechanician of antiquity, born at Syracuse, in Sicily. Referring to the powers of the lever, he is said to have remarked, "Give me where I may stand, and I will move the world." By the invention of machines he for a long time defended Syracuse on its being besieged by the Romans under Marcellus.
4 AP-PRE-CIATE, set a proper value upon.
13 TEN'-SION, the straining, tightness or
6 IN-FLEX'-I-BLE, that can not be bent.
8 FORM'-U-LA, a general statement of a
9 MAN'-U-AL, a small hand-book containing
HO-RI'-ZON, the level circle which touches the earth at the place of the spectator, and is bounded by the line in which the earth and skies seem to meet.
16 DIS-CRED'-IT, show the error of; deprive of credit.
17 PÄR-TERRES' (pär-tārs), flower-gardens. 18 PER-CEP-TION, the notice which the mind takes of external objects.
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. 3. Frank. So is the capstan, used on large vessels for raising anchors. It is a perpendicular windlass, with holes in its head, in which capstan-bars are put, and many men can work at
Mr. M. That is very handsomely explained. What modifications of the wheel and axle can you name?
John. The windlass seems to be, in reality, the same thing.
George. Is not the tread-mill, sometimes used to raise a heavy mass of iron for the purpose of driving piles, or long logs for wharves, a kind of wheel and axle?