the three forms of matter of which the world is composed." "It is not improbable," replied Mr. Maynard, "that this distribution of authority by the ancients was their mysterious way of exhibiting the truth which seems so plain to us. Jupiter and his wife Juno had special direction of atmospherical phenomena, such as thunder and lightning, wind, clouds, snow, and rainbows. Homer says the portion which fell to Jupiter was the 'extensive heaven in air and clouds.' 10. "I recollect reading," said Frank, "that Jupiter was also called Zeus,1 and that in old times the expression, 'What is Zeus doing?' was equivalent to 'What kind of weather is it ?" " "I would also remind you," continued Mr. Maynard, "that Neptune was the god of water in general, but especially of the sea, rivers, and fountains. Pluto's abode was in the solid earth; and his name, which in Greek means riches or wealth, indicates his supremacy over the solid forms of matter. Thus we see that Frank's question was quite appropriate; and the three forms of matter were evidently represented in this mystical manner by the wisest men of former times." 11. "I think," said Frank, "that they call rich men solid men in our day, which is most appropriate, as Pluto was the same as wealth, and had charge of the solid part of matter." "I do not think the solid men will thank you for your etymological discovery," said Ida. "It is certainly more fanciful than philosophical," said Mr. Maynard, who then proceeded to assign the subject of HYDROSTATICS for the next lesson. 1 ZEUS, the Greek name for Jupiter, pronounced in one syllable, as zūs. LES. II.-HYDROSTATICS, OR LIQUIDS IN A STATE OF REST. 1. Mr. M. As, in our lesson on the statics of solids, the knowledge of a few principles and definitions enabled you to solve many problems of apparent difficulty, so in the statics of liquids, or hydrostatics, you may expect to do the same by the same means. The first thing necessary is a definition of the term fluid. 2. Ida. I looked in Webster's Dictionary for a definition, and found it to be "any substance whose parts easily move and change their relative position without separation, and which yields to the slightest pressure." 3. John. Is not every thing fluid that is not solid? Mr. M. Yes, every thing is either solid or fluid. Water and air are both fluids, but they are not both liquids. Will George define the term liquid? 4. George. I took pains to look in Webster's Dictionary also for the term. I found it to be "a fluid or flowing substance; a substance whose parts change their relative position on the slightest pressure, and which flows on an inclined plane." I can not understand from this the difference between a fluid and a liquid. 5. Mr. M. All liquids are fluids, but all fluids are not liquids. Those fluids which tend to expand when at liberty, as air and gases, retain their name, and are properly called fluids; but such as do not so expand are commonly called liquids, as water, oil, and mercury. Many phenomena show that both attractive and repulsive forces exist between the particles which compose the mass of a body. When the attractive force is predominant, the body is a solid. When the two forces balance, the body is a liquid; and when the repulsive force predominates, the matter is a gas. In the last-named case the particles tend apart, so that some external force is required to keep them together. It is very important to keep these distinctions in mind, if you would understand the appearances you will be called upon to explain. Will John now inform us what is the most noticeable property of water aft er its fluidity? 6. John. I think every person must have observed the level surface of water when it is at rest. I have often heard people talk of a water-level. Mr. M. The earth, you know, is spherical, or nearly so; and as three fourths of its surface are covered with water, it is evident that the water-level conforms to the shape of the earth, which has a convex surface. This deviation from a plane, or a straight line, is found to be eight inches in one mile. Do you know what it would be for two miles? 7. Frank. I suppose it must be sixteen inches, and so on distance. for A any 6000 4000 4000 B John. There must be some mistake here; for I once stood on the ice, and with a good spyglass I could see an object at the very water's edge, and only three miles distant. 8. Ella. If Frank is correct, in four Fig. 1, showing the varia- thousand miles, the straight line would vary from the earth's curvature only four tion of the curve from the straight line A B. thousand times eight inches, or about half a mile; when it is plain that the variation must be as much as the earth's radius, or four thousand miles instead of half a mile! a wonderful difference. Mr. M. I think Frank must see that he is mistaken. 9. Frank. I am very sure it was so stated in the Philosophy I studied, but I see it can not be right. Mr. M. The distance the straight line varies from the curve may be found, for short distances, by multiplying the square of the distance in miles by eight inches. Now can Frank tell the deviation for two miles? 10. Frank. The square of two is four; and four, multiplied by eight, gives thirty-two inches, which must be the deviation for two miles. Mr. M. You have now given a correct reply. If John had been six feet in height, he could have seen just three miles on the ice of a lake, as you will see by reversing the process I gave you. Will John show how to do it? 11. John. Six feet are seventy-two inches, which, divided by eight, gives nine for a quotient, and the square root of nine is three, which is miles. Mr. M. As you may have occasion to put such calculations into practice, I would request you to notice that the difference between the true and apparent level varies as the square of the distance for any distance that can occur in leveling. 12. Ida. I think the engineers of the Erie Canal must have had occasion to put that rule into practice when they gave the levels to the workmen who constructed it. Mr. M. I am glad so important a matter can be so interesting to you. Are you aware that water will rise to the same level when in different vessels which have a communicating pipe between them? John. I have often seen such a result. Is not that the principle on which water is distributed in cities? 13. Mr. M. In most of our large cities, water is conveyed into the upper stories of houses by this very principle. Water will rise to the level of its source, whether the pipes are of cast iron or porous strata of the earth. In this way water is obtained in many places by boring wells two thousand feet or more in depth. The water which fell as rain on some distant mountain, and which was slowly making its subterranean way hundreds of feet below the surface, rises where an opening is made to supply the necessities of man on the otherwise arid plain. 14. George. Are not these called Artesian wells? I have read of several recently bored in the Sahara Desert. Mr. M. The inhabitants of the oases where these wells have been bored were wild with delight and wonder as they saw the water rush forth from the dry sands; and they have given them such names as "the well of bliss," "the well of gratitude," etc. 15. John. I do not wonder the wandering tribes of the B desert believed that the French, who bored the wells, had wrought a miracle. To them it was a miracle; but to us, only water rising to its level, as we see every day in a tea-kettle. Fig. 2. 16. Ida. I have just read a verse from Eliza Cook's poems which I will repeat: "Traverse the desert, and then ye can tell And then ye may reckon what water is worth." 17. Mr. M. It is thought that these wells will work a great social revolution in those regions. The various tribes, instead of wandering, like their ancestors, from one place to another, will settle around these fertilizing springs, and begin to cultivate the earth even in those sandy deserts. Artesian wells have been bored in Charleston, S. C., St. Louis, Mo., Columbus, O., La Fayette, Ind., Louisville, Ky., and many other places in this country. In Alabama they are of incalculable value, and are very numerous on plantations and in villages. 18. The annexed cut of a vertical section of the earth's crust shows the principle of the Artesian well. The stratum A, and the one below it, are impervious to water, but between them is a fissure or seam along which the water penetrates from the lake on the hills. Wells are bored in the valley through which the water rises with great force as soon as the boring enters the fissure between the strata. The water may be carried up in pipes to the very level of the lake. LESSON III.-HYDROSTATICS-Continued. 1. "I WILL introduce the subject for this lesson," said Mr. M., "by showing you one of the ways in which an ignorant contriver tried to obtain a constant flow of water-a kind of perpetual Bmotion-by means of a vessel like this. "He reasoned thus: A pound of water in A must more than balance an ounce in B, and must therefore be constantly pushing the ounce forward into A again, thus causing a constant flow of water in continuous current. Fig. 5, an ounce of wa- What think you of his success ?" ter balances a pound. 2. Ella. I think he found the water to rise no higher in B than in A. Mr. M. You think correctly. You must see that as the downward pressure in B is equal to that in A, the pressure of water is by no means as the mass, but as the vertical height of the fluid. George. I have been reading about this hydrostatic paradox-how any quantity of water, however small, may balance any quantity, however great. I think I see how it is, as the tube may be very small, and the vessel with which it communicates very large, and the water will Fig. 6, the water in a balances the whole mass in b. |