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 application. 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 when 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 I have seen a vessel in a dry dock, and, as the water was withdrawn, had a good opportunity to see how the enormous screw, or rather part of a screw, was fixed.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. 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?

Fig. 41.

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.

1200

*If the winch, or handle, be 20 inches long, and the screw 2 inches 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 30 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.

tending from stem to stern at the bottom, and supporting the whole frame.

1 WAYS, the timbers on which a vessel is launched. 2 PRO-SA-I¤, pertaining to prose, hence less 4 SPI'-RAL, winding like a screw. 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

LESSON IX.

MISCELLANEOUS MECHANICAL MATTERS.

1. Mr. M. In our last conversation we discussed the law of equilibrium1 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 have 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 some times put so grotesquely on the floors of dancing-halls.

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

Mr. M. It would be difficult for you to name any of the ordinary occupations of life without giving an instance of the utility of friction. It holds the nails and screws in our houses, enables us to walk, and even to hold knives, pencils, and books in our hands. It is increased by roughness, and it has been found that there is more friction between pieces of metal of the same kind, than between similar pieces of different metals.

5. Frank. I wonder if that was the reason why Juno's chariot wheels were of brass, and the axle of iron or steel. Homer, who is good authority on such matters, says,

"Hebe to the chariot roll'd

The brazen wheels, and joined them to the smooth

Steel axle."

6. Mr. M. If I may add to Master Frank's classical allusions, I will mention that the gates of the infernal regions, according to Homer, were of iron, and the threshold of brass; though, if I recollect correctly, Virgil says "they are open night and day."

7. John. I know that all machinists say that surfaces of brass and steel move upon each other easier than when both are alike.

Frank. That is just what the engineer of a steam-ship said when I was on board, and asked why he was using what he called anti-attrition metal, made from copper, antimony, and tin.

8. Ida. I see now why a jeweled watch is better than a common one. The friction is less.

Ella. When we apply oil to our sewing machines, I suppose it must be to diminish friction.

9. Mr. M. You have an excellent habit of observation, which saves me much time and trouble in giving illustrations and experiments. Useful as friction is, we sometimes try to avoid it, as in putting wheels under loads to be transported, and casters or rollers on tables and other articles of furniture. 10. John. I would like to ask a question. May not the pulley be regarded as a modification of the lever?

Mr. M. The wheel we call the pulley may be so considered, but, taken as a whole, the cord and wheel may be called the

pulley, though the term cord would be more proper. Now, as we are approaching the conclusion of the department of Philosophy called Mechanics, I would suggest that each one of you propose a question involving some of the principles which have formed the topics of our conversations.

11. Frank. I am really glad to have such an opportunity to get a solution" of my own difficulties and those of my classmates. I would like to ask if it is possible to construct a machine which, when put in motion, will never stop till it is worn out.

12. Mr. M. In other words, a perpetual motion. Thousands of dollars have been uselessly spent in vain attempts to accomplish it. I will reply to your very proper question by reading a brief extract from Professor Loomis's Natural Philosophy. He says,

66

By perpetual motion in mechanics we understand a machine which moves without ceasing, and requires no new application of force from without. A machine which renews itself (as, for example, a watch which runs for 24 hours, and then winds itself up, so as to be ready to run another 24 hours, without any assistance from beyond itself) would be such a perpetual motion as has been long sought for by visionary inventors. A machine of this kind is impossible, because no combination of machinery produces any positive increase of power."

B

13. A great many machines have been proposed for pro

ducing perpetual motion. Here is a drawing of one of them-a large wheel, carrying twelve equal arms, each movable on a hinge, and having at its extremity a heavy ball. But all machines for perpetual motion have failed, unless sustained by expansion and contraction from change of temperature, or electrical action in some way; and when the motion is thus sustained, it is no more perpetual motion than the paper-mill at Niagara Falls.

Fig. 42.

14. Frank. The reply satisfies me fully, and I shall report it to a good neighbor of ours who is constantly engaged in efforts to produce a perpetual motion.

John. I would like to understand what is meant by a horse-power and a unit of work.

15. Mr. M. What is called a unit of work is the labor expended in raising one pound of matter one foot in height, in opposition to gravity. The eminent engineer Watt estimated that a horse could perform 33,000 units of work in a minute; in other words, that a horse could raise 33,000 pounds to the height of one foot in a minute of time, or one pound to the height of 33,000 feet. To see if you understand my reply, I will propose a question. How many horses' power will be required to raise 500 pounds of coal per minute from a pit 330 feet deep?

16. John. The amount of work consists of the power multiplied by the distance; therefore 500 pounds raised 330 feet will be 165,000 units of work. A horse can perform 33,000 of these units in a minute; therefore I divide the whole number of units by 33,000, and get for an answer 5 horses' power. Mr. M. John has answered admirably. What question has George?

George. I wish to know if the large hind wheels of a carriage tend to push forward the small fore wheels.

17. Mr. M. They do not; hence the wheels of railroad cars are made of the same size. In carriages it is convenient to have the fore-wheels smaller, on account of turning the carriage more easily, and often for facility in getting into and out of them. Besides, the line of traction, or draft, should extend to a point lower than the horse's breast, otherwise the collar by which he draws will rise up and choke him, which would be very inconvenient for all concerned.

Ida. My question is one which I never could understand. Why can ships sail in opposite directions when driven by the same wind?

18. Mr. M. I will try to make the matter plain to you. On the opposite page is a drawing in which you will see the direction of several ships, and the position in which the wind strikes against the sails. The wind, which is here represented as blowing from the north, strikes directly against the ship at m, and she is scudding, or sailing before the wind, in the same direction the wind blows. In all the other ves