Puslapio vaizdai

Fig. 35.


4. Mr. M. It is a wheel and axle made
to revolve by the weight of several per-
sons stepping constantly on the circum-
ference of a long wheel. The tread-
wheel 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:

We've nothing in the world to do
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 Fig. 36. of the most surprising instances of the power gained by the use of the wedge?

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.

"Then the master,

With gesture of command

Waved his hand;

And at the word,

Loud and sudden there was heard,

All around them, and below,
The sound of hammers, blow on blow,
Knocking away the shores and spurs.
And see! she stirs !

She starts-she moves-she seems to feel
The thrill of life along her keel,

And, spurning with her foot the ground,
With one exulting, joyous bound,
She leaps into the ocean's arms.'

Fig. 33.

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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 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.

Fig. 37.

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 spiral' 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?

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 consists of two parts. In the figure which I here show you, ɑ, a, is called the screw; c, c, the nut; and b, the lever.

13. John. I see, if the screw is turned far enough one way, it will be raised from the nut a distance equal to that between the threads, while if it is turned

Fig. 39.

the other way it will be lowered the same distance. I would like to know how the power gained by its use is calculated.

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?



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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.*

15. Mr. M. I am glad you understand the beautiful simplicity of the formula I gave you in the last conversation.


Fig. 40.

*The distance described by the power must be ten feet in diameter, or 31.416 feet in Divide this by one circumference, which, multiplied by the power 10, will give 314.16. inch, or one twelfth of a foot, and we obtain 3769.92 pounds for the pressure exerted by a power of ten pounds.

John. I have heard of an endless screw, but I do not exactly understand its arrangement.

16. Mr. M. When the screw is applied to a toothed wheel, it is called a perpetual or endless screw, as it constantly moves in one direction, and keeps the wheel moving round. I presume, by a little study, you could calculate its power.* Frank. The screw must be a power of very extensive plication. It even propels large vessels round the world.

17. Ida. I suppose it was this application of the power that uncle John had in mind whe he said England and America were held together by screws.

Mr. M. Politically as well as mechanically, this is a most important and powerful application of the screw. Do you understand the mode of its application to the propulsion of ships?

18. Frank. I have seen a vessel in a dry dock, and, as the water was withdrawn, had a good opportunity to see how the enormous screw, or rather part of a screw, was fixed. A piece of iron, called a shaft, came through the timbers, and the blades composing the screw were attached to the end of it; and any one could see that by turning, it would push against the water, and by the reaction would propel or push forward the vessel.

Fig. 41. 19. Mr. M. The screw of the steam-ship Great Britain was fifteen feet in diameter, and it was turned by a power reckoned equal to a thousand horses. Have you heard of any other remarkable application of this power?

John. I have read of light-houses constructed on piles screwed down firmly into the sand.

George. There is also a kind of pump, called Archimedes' Screw, which I would like very much to understand.

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* If the winch, or handle, be 20 inches long, and the screw 2 inch. es in diameter, there is evidently a power of 20 gained; then, if the wheel have 30 teeth, and the screw at each revolution throws off 1 tooth, this is a power of 3 gained, which multiplied by 20, the other power, gives a power of 600. Again, say that the cylinder which supports the weight is only half the diameter of the wheel, that is an additional power of 2 to 1, by which multiply the former power, and the result is 1200 as the power gained by this machine


20. Mr. M. This pump will be explained when we come to the subject of Hydraulics. We have now treated of the three primary and the three modified or compound mechanical powers. I think, from the readiness of your answers, that you understand enough of them to enable you to make any ordinary calculations pertaining to machinery. Our next conversation will be miscellaneous mechanical matters.

1 WAYS, the timbers on which a vessel is launched.

tending from stem to stern at the bottom, and supporting the whole frame. 4 SPI'-RAL, winding like a screw.

2 PRO-SA-1C, pertaining to prose, hence less interesting than a poetical description 5 PRO-PUL'-SION, the act of driving forward. would indicate. 6 HY-DRAUL'-Ics, that branch of Natural Philosophy which treats of fluids considered as in motion. See Fifth Reader.

3 KEEL, the principal timber in a ship, ex



1. Mr. M. In our last conversation we discussed the law of equilibrium of the mechanical powers, as it is in theory; but when we make a practical application of these powers, a deduction2 must be made for friction, or the rubbing of surfaces against each other. Will each of you name some instance of the utility3 of friction.

2. John. The bands that turn the wheels of mills and factories are kept from slipping by friction.

George. The cars are drawn over the railroad by the friction of the large, or driving wheels of the locomotive, and when they slip round, as they sometimes do in starting, I nave seen the engineer throw sand on the rails to increase the friction.

3. Frank. When the engineer wishes to stop the cars, he blows the whistle as a signal, and the brakeman turns a wheel, which brings a rubber against the car-wheels, and they are soon stopped by friction.

Ida. If there is any utility in dancing, I can give an instance of the utility of friction in the chalk which is sometimes put so grotesquely on the floors of dancing-halls.

4. Ella. The friction caused by ashes thrown on icy side walks is certainly useful.

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