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LXXXVII.-HOW A FLY WALKS ON THE
1. “Will you explain to us, father, the means by which flies are enabled to ascend a pane of glass and walk with ease along the ceiling of the room! You know you told as the other day you would do so."
2. “Well, Harriet, I will try; though I am not sure that I shall be able to make you understand me."
3. “O, never fear that!” exclaimed Harriet and her two little brothers at the same time; “we can surely understand how a fly walks — it must be very simple."
4. “Undoubtedly very simple; but it requires some previous knowledge of philosophy.”
5. “O, if the walking of a fly is at all connected with philosophy, I assure you that I shall want to know nothing about it, for I hate philosophy, it is such dry stuff.”
6. “Never mind my sister, father,” said William. “ James and I want very much to understand, and Harriet need not stay to hear the explanation, if she does not wish to."
1. “Well, come, my boys, to the library. I have just arranged my solar microscope, to show you the foot and
og of a fly, and some other curious things. I have likevise my air pump ready, which will help to explain what you want to know."
8. Harriet looked a little disappointed, and wished that she had not pronounced so decidedly against philosophy, for she was very fond of seeing, and only disliked the labor of studying.
9. Her father, observing the moody expression of her countenance, said, “I wish, William, that you would try to persuade your sister to go with us to the library.” William had no difficult task to perform, and in a minute they were all seated in the library, eager to hear what could be said about the little pedestrian.
10. “The fly, my children,” began their father, “every time he moves his foot, performs a philosophical experiment, similar, in every respect, to that which I now show you, by moving the handle of the air pump. You perceive that this glass vessel, which is put on this brass plate, now adheres so firmly to it that I am unable to force it away.”
11. “How wonderful !” exclaimed Harriet. “It is as fast to the plate as the friend* of Hercules, that I read about the other day, was to the stone on which he sat, in the drear dominions of Pluto."
12. “How is this done, father? it looks like some conjurer's trick. I see nothing pressing upon the glass, to cause it to stick so fast."
13. “ Though you can not see it, I assure you there is something pressing very hard upon and all around it, and that something is the air.”
14. “ You astonish me. Has the air weight? I never heard of that before."
15. “But you have heard of hurricanes sweeping away forests and houses, and rendering the country over which they passed a wilderness; haven't you? They are almost as much to be dreaded as earthquakes, and a hurricane is only air put in motion.”
16. "I have been very stupid not to find out that air has weight; but how is it that we do not feel it, father? I should think," continued Harriet, “that if it was so heavy it would pin us to the earth, as Prometheus was
* Prometheus; fastened to a rock by order of Jupiter, and afterwards released by Hercules, who was the most celebrated of the mythviogical gods of the Greeks, and noted for his great strength. Pluto was another of their gods. He was king of the shades, and had control of the dead.
fastened to the rock, and then we should be in a pretty condition, I think. How will you answer that, father?” I have had a more puzzling question to answer,
I assure you. The air is a very subtile fluid, and finds its way into every crevice; and one of its properties is, that it presses equally in all directions, up and down and sideways, and we perceive its weight only when we remove the air from one side of a body, so as to cause the whole weight to be upon the other. In this glass vessel I withdrew the air that was in the inside of it, and which pressed it upwards with a force exactly equal to that with which the air above pressed downwards, and then the whole weight of the atmosphere pressing in one direction kept it firmly attached to the brass plate."
18. “That is a very beautiful arrangement,” cried William ; "I shall never breathe the air again without thinking of its wonderful properties."
19. “I will take off this vessel and put this one on, which is open at both ends; now put your hand, Harriet, on the upper end, and I will cause a slight vacuum to take place, so that you may feel the pressure."
20. “Stop, father, you will crush my hand if you move that handle another time. Do look at my hand, William; the gripe of a giant is nothing to that.”
21. William tried the experiment for himself. “How heavy is the atmosphere, father? Is it very great ?"
22. “It presses upon the surface of all bodies near the level of the ocean with a force equal to fourteen pounds on every square inch. I will now make another experiment to show its pressure.
23. “I will place on this same glass vessel, which you see is open at both ends, this piece of glass, which so closely fits it as to exclude the air. I now withdraw the air from under it."
24. “What a crash, father!” exclaimed William and Harriet at the same instant, as the glass was shivered to pieces by the weight of the air.
25. “I think you can now understand that if a fly has the power to withdraw the air from under its feet as it moves along, the pressure of the atmosphere is sufficient to hold it fast to any surface, however smooth, and however much inclined to the horizon.”
26. “If the fly can do that, he is more of a philosopher than I took him for," said William. “ But I am impatient to see how he accomplishes the feat.”
27. “Here is the leg of a common fly, that I have placed in the solar microscope; I will bring it to the proper focus. It is now so much magnified that we can examine the various parts of it with ease.”
28. “What a strange-looking thing it is, and so large; my arm is nothing to it. How I should like to see an elephant put into a microscope.”
29. “What an idea, Harriet! why an elephant would appear as large as one of the Alps!” exclaimed William.
30. “Microscopes are used only to examine bodies that are too small for the naked eye to see. You will observe a line of light running up the middle of the leg. This line of light shows that the leg is hollow. On the foot you can distinctly observe a flap or membrane, to which are attached two points, one in front and the other behind. These the fly can move at pleasure, and can extend or contract the flap just as it pleases. When the fly wishes to move without the trouble of raising himself in the air, he stretches out these points, tightens the flap, draws the air from under it, through the tube in the legs, and moves along the polished surface of the glass with as much ease and security as you do on the garden walk."
31. “How delightful! How beautiful! How ingenious !” they all exclaimed at once. “I shall never see a fly again without feeling an interest in him.”