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a body is its weight, compared with the weight of an equal bulk of pure water-water being taken as a standard.

Mr. M. Can you tell me, then, how the specific gravity of a solid heavier than water is ascertained?

8. George. Weigh it first in air, and then in water. Divide the weight in air by the loss in water, the quotient will be the specific gravity of the body. Thus, if a solid weigh twenty pounds in the air and eighteen pounds in water, its specific gravity is ten; that is, it is ten times heavier than water.

Ida. Is it of much use to find the specific gravity of bodies? 9. Mr. M. I will give you an example of its use, and let you judge for yourself of its importance. I have heard you express a doubt as to the value of the silver cup you obtained as a prize at the Union Seminary. As it becomes tarnished so easily, you fear it is not real silver. If it is alloyed, it will probably be lighter than standard silver, which has a specific gravity of 10.47; that is, silver is nearly ten and a half times heavier than water. Can either of you find the specific gravity of the cup which Ida has gone to bring for examination?

10. John. Now I have the cup I will carefully weigh it. It weighs five and at half ounces in the air. I will now suspend it by a thread in water, and find how much less it will weigh. It has lost ten and a half pennyweights. I find, by dividing the weight in air by the loss in Fig. 13, to find the specific water, that the specific gravity of the cup is 10.47, which shows it to be made of

A

gravity of a solid.

standard silver.

Ida. I am glad my suspicions were unfounded; and now I recollect they were first suggested by one of the disappointed competitors.

11. Mr. M. It is a pity we have no way to remove your new suspicions of the motive of your rival. I have here a chain, bought for gold, which by chemical tests shows copper in its composition. It weighs two ounces, or forty pennyweights, in air, and thirtyseven pennyweights in water, from which I find the copper to be about three eighths of the whole weight. Fig. 14, the There is a very convenient instrument, called the hydrometer,* for finding the specific gravity of liquids.

Hydrom

eter.

The hydrometer, figure 14, consists of a hollow ball, B, with a long, slender, graduated stem, A D; and the ball is so loaded by a weight, C, that the stem will stand upright in water. The lighter the fluid, the greater the depth to which the hydrometer will sink.

Who can give me an account of the manner in which the principle of specific gravity was first discovered?

12. Ida. I have purposely brought a book containing an account of the discovery, which, with your permission, I will read. The article is entitled

ARCHIMEDES AND THE CROWN.

"King Hiero of Syracuse, or his son Gelon, it seems, had given out a certain amount of gold to be made into a crown, and the workman to whom it had been intrusted had at last brought back a crown of corresponding weight. But a suspicion arose that it had been alloyed with silver, and Archimedes was applied to by the king either to disprove or to verify the allegation. The great problem, of course, was to ascertain the precise bulk of the crown in its existing form; for, gold being so much heavier than silver, it is obvious that if the weight had been in any degree made up by the substitution of silver, the bulk would be proportionately increased. Now it happened that Archimedes went to take a bath while this problem was exercising his mind, and, on approaching the bath-tub, he found it full to the very brim. It instantly occurred to him that a quantity of water of the same bulk with his own body must be displaced before his body could be immersed.

13. "Accordingly, he plunged in; and while the process of displacement was going on, and the water was running out, the idea suggested itself to him that, by putting a lump of gold of the exact weight of the crown into a vessel full of water, and then measuring the water which was displaced by it, and by afterward putting the crown itself into the same vessel after it had again been filled, and then measuring the water which this, too, should have displaced, the difference in their respective bulks, however minute, would be at once detected, and the fraud exposed. As soon as he had hit upon this method of detection,' we are told, 'he did not wait a moment, but jumped joyfully out of the bath, and, running toward his own house, called out with a loud voice that he had found what he had sought. For, as he ran, he called out in Greek, "Eureka, Eureka!" "I have found it, I have found it."

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14. "No wonder that this veteran geometer, rushing through the thronged and splendid streets of Syracuse, and making the welkin ring with his triumphant shouts-no wonder that he should have rendered the phrase, if not the guise, in which he announced his success, familiar to all the world, and that Eureka, Eurēka,' should thus have become the proverbial ejaculation of successful invention and discovery in all ages and in all languages, from that day to this! The solution of this problem is supposed to have led the old philosopher not merely into this ecstatical exhibition of himself, but into that line of hydrostatical investigation and experiment which afterward secured him such lasting renown. And thus the accidents of a defective crown and an overflowing bath-tub gave occasion to some of the most remarkable demonstrations of ancient science."

15. "That account," said Mr. M.," which I perceive you have taken from a lecture of the Hon. Robert C. Winthrop on

The scale should be so graduated that when the hydrometer is immersed in pure water at the standard temperature, it may sink to the point which is marked 1. Then, when the hydrometer is immersed in any other liquid, the figure on the scale to which it sinks will show the specific gravity of that liquid. When the quantity of liquid is too small to float the hydrometer, other methods are used.

Archimedes and Franklin, is indeed a history of one of the most important events in the records of science. In that same lecture is a very interesting account of the visit of the Roman orator Cicero to the grave of the philosopher."

1 JA-SON, the hero of the famous Argonautic Expedition, as fabled in Grecian Mistory, sailed in the ship Argo to Colchis, in Asia Minor, for the purpose of recovering a "golden fleece" deposited there.

1.

2.

3.

4.

LESSON V.-HYDRAULICS-THE EXCURSION.

SONG OF THE BROOK.

I COME from haunts of coot1 and hern ;2
I make a sudden sally,

And sparkle out among the fern,
To bicker3 down a valley.

By thirty hills I hurry down,
Or slip between the ridges;
By twenty thorps, a little town,
And half a hundred bridges;

I chatter over stony ways
In little sharps and trebles,
I bubble into eddying bays,

I babble on the pebbles.

And out again I curve and flow,

To join the brimming river;

For men may come, and men may go,

But I go on forever.-TENNYSON.

5. "The day is so pleasant, and the subject of our lesson so inviting," said Mr. Maynard, "I propose a walk by "The River,' where we can better witness some experiments appropriate to our studies. You know that, in plain English, the lesson to-day is about water in motion."

6. "I shall be delighted," said Ida, as they were crossing the lawn, "to study this lesson in the pleasant valley; for I had feared it would be all about mills and resistances of fluids-important enough for millwrights and engineers, but of little interest to Ella and myself. Now I shall ramble where

'Joy smiles in the fountain, health flows in the rills,
And the ribbons of silver unwind from the hills."""

7. Ella. I really fear that Ida and I will learn but little philosophy in this lovely valley, "where streamlets flow and wild flowers blow." Ida, let us study the poetry of the sub

ject first.

"How beautiful the water is!

To me 'tis wondrous fair

No spot can ever lonely be

If water sparkle there;

It hath a thousand tongues of mirth,

Of grandeur, or delight,

And every heart is gladder made

Where water greets the sight."

8. Mr. M. I am glad you will all enjoy this topic, and that

the girls can talk about "ribbons of silver," while the boys are discussing the merits of undershot and overshot wheels; but I shall be disappointed if you do not find that the very poetry of" water in motion" is full of philosophy, and that the philosophy is very poetical. You can all moralize on the subject, also, as you see

"The rivers, how they run
Through woods and meads, in shade and sun,
Sometimes swift, sometimes slow,
Wave succeeding wave, they go
A various journey to the deep,

Like human life, to endless sleep."

I think we will continue along "The River" as far as Rocky Glen, where is a fine well of water, with an old-fashioned sweep; and then, if the girls are not too much fatigued, we will follow the glen, and go up to the Cascades, where, as George will recollect, are the remains of an old mill.

9. George. I have been there frequently, and a wild but beautiful spot it is, too.

Ida. I have heard so much about the Cascades, I know I shall be delighted to see them. I am sure Ella and I can easily walk as far as that and back again.

Mr. M. As we shall have this running stream constantly "babbling" to us, with its "thousand tongues of mirth," as Ella said, let me ask how it is that it has this speaking power'? John. By its motion, I suppose.

10. Mr. M. Then tell me, if you please, what makes the water move at all'?

John. The bed of the river is an inclined plane, and the particles of water roll down by the force of gravity, just as a marble from a desk.

Mr. M. Very well; this force of gravity is such that, in large rivers, a fall of three inches in a mile is said to give a velocity of three miles an hour.

George. Would it not be the same in small streams?

11. Mr. M. By no means. The friction of the water against the banks and bottom tends to retard the motion. In pipes the friction is so great that, in a tube one inch in diameter and two hundred feet long, only one fourth as much water will be discharged as would escape from a simple aperture of the same size.

Frank. I see the river is much wider in some places than in others. Is not the current the most rapid in the narrowest parts?

12. Mr. M. It is. I have here an instrument called a streammeasurer. It consists of a vertical tube with a trumpet

shaped extremity, bent at a right angle. When plunged in motionless water the level in the tube corresponds with that outside, but the impulse of a stream causes the water to rise in the tube until its vertical pressure counterpoises the force. Let us try it first in the wide, and then in the narrow places. You see quite a difference in the velocity.

13. George. I have just thrown some pieces of bark, one near the middle, and the other near the shore. See how much faster the piece near the middle goes down stream.

Ida. Before I came to Glenwild I lived in sight of a navigable river, and I used to wonder why Figure 15, the the boats, in descending, kept near the middle, and Stream-mea- those ascending went nearer the shore. I understand it now. Boats going down had more assistance from the current, and those coming up had less resist

surer.

ance.

14. Mr. M. There is also a greater velocity at the surface than near the bottom from the same cause.

John. I think I see why wide rivers are higher in the middle than near the banks. The water, running more swiftly, tends to draw along that on each side of it, which it can not do without lowering the surface on each side.

ing water.

15. Mr. M. You must not confound the velocity at the surface of a river, and at different depths, with that of water running from apertures in a reservoir. If in this vessel, Fig. 16, orifices be made at different depths, the velocities of discharge will be as the square roots of the depths. That is, if D is one foot below the surface, and A four feet, a quart will run from A, while only a pint will be discharged from an orifice of the same size at D.

16. Frank. As water will run into a Fig. 16, the velocity of spout- submerged empty vessel with the same velocity that it will flow from a full one, I can see why a leak in a ship near the keel is so dangerous. Mr. M. We have been talking about water in motion; let us now talk about hydraulic machinery. First, can each of you describe some method you have seen, or heard of, for raising water from wells?

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