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FOR a long period the course of time was marked by the flowing of sand in the glass. The timekeeper was in consequence called the hour-glass. It may still be seen in many a cottage, and sometimes on the table of the teacher or lecturer.

The half-hour glass is employed on board ship; and the three-minute glass is well known by the name of the egg-glass, to mark the time for the boiling of eggs.

The shape is not a matter of very great importance, but the form generally used is that of two pear-shaped vessels joined at their smaller ends.

The hour glass is made of two bulbs of glass which are blown of equal size, and the sand is

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placed in one of them before the other is joined to it. The best sand is that called silver sand. size of the opening left between the two bulbs determines the rate of the flow of the sand. In this glass all the sand runs out of the one bulb into the other in exactly one hour.

Sometimes the bulbs are made smaller, so that it takes only half an hour, or a quarter of an hour for the sand to run out of the one into the other.

Hour-glasses are mostly inclosed in a wooden frame to keep them from breaking. They are, perhaps, scarcely so good to measure time by as clocks and watches, but they ought not to be despised, as they are exceedingly cheap, simple, and measure time much more correctly than is generally imagined; and they were known and used long before clocks and watches were invented.

In our next lesson we shall learn something more about the sand-glass as a measure of time.

QUESTIONS:-1. Long ago, how was time marked? 2. What was the time-keeper called? 3. Where is it still to be seen? 4. Have you ever seen one? 5. Where is the half-hour glass used? 6. What is the use of the three-minute glass? 7. Have you one. 8. Tell me the shape of any sand-glass. 9. How is an hour-glass made? 10. What is it which fixes the rate at which the sand flows? 11. What is done with these glasses to keep them from breaking? 12. What are better for keeping the time than sand-glasses? 13. Whether are watches or sand-glasses the dearer?

WORD LESSON :—

bulbs

de-spised' ex-act'-ly

in-closed'

con'-se-quence de-ter'-mines ex-ceed'-ing-ly in-vent'-ed

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LESSON LI.

The Hour-glass.—Part II.

cone, bulb, round base and | ex-pla-na'-tion, making plain. per'-fect-ly, completely.

pointed top. emp'-ty, containing nothing, pres'-sure, force or weight. void. quan'-ti-ty, portion, amount. ex-pec-ta'-tion, what is looked quick'-ly, speedily, very soon. for. suc-ces'-sion, following one by ex-per'-i-ment, trial.

one.

It is a fact worthy of notice that the flow of the inclosed sand is perfectly equal whatever be the quantity contained in the glass at any period of its flowing. In other words, it runs as fast, and no faster, when the upper cone or bulb is nearly full as it does when it is nearly empty; nor is there from first to last any difference in the rate of its course.

Now, this is contrary to our expectation. We should have thought that when the upper glass was full of sand, the lower grains in it would have upon them a greater pressure from the mass above, and, therefore, would be more quickly forced through the narrow opening into the under cone than when it was only a quarter full and near the close of the hour.

But that it is otherwise may be proved from the following simple experiment.

Take a quantity of silver-sand, dry it on a hot stone-plate, and sift it through a fine sieve. Take also a tube of any length you please, closed at one end; in this make a small opening, say an eighth of an inch, and, placing the finger over this, fill the

tube with the sand. Now, hold the tube steadily, and, removing the finger, let the sand flow into a glass measure marked on the side, for any given time, say a quarter of a minute. Note the quantity of sand which has gone through into the marked measure. Next, let the tube be only half or quarter full of sand, and begin to measure again for the like time, and we will find that exactly the same quantity of sand has fallen through; and, again, if the sand in the tube should be pressed down, the flow of it will not in the least degree be hastened.

This seems somewhat strange, but the explanation is easy. Sand, if allowed to fall quietly on any surface, does not take the form of a square, a cube, or flat and round body like a penny; but forms itself into a cone-like heap resembling a spinningtop turned upside down, and which heap preserves always a certain fixed angle.

If we look into the lower cone of the hour-glass when the sand is flowing, we will observe the sand as it falls taking this particular form. The same shape may be noticed when a load of fine, dry sand has been thrown from a cart or a barrow.

Sand thus falling at a fixed angle, it will be seen that when poured into a tube it must fill it with a succession of cone-like heaps, and that all the weight which the bottom of the tube bears is only that of the heap which first falls, and that the succeeding heaps are thus prevented from having any perpendicular pressure on the bottom.

QUESTIONS:-1. State the fact mentioned here, which is worthy of our notice. 2. Whether does the flow run quicker when the upper glass is nearly full or when it is nearly empty? 3. Is this what one at first might expect? 4. Tell how it may be proved the rate of the flow of sand is the same all through. 5. What does this seem at first sight? 6. Can it be easily explained? 7. Well, explain it. 8. If we look into the lower cone or glass when the sand is flowing, what shape do we notice the sand taking when it falls? 9. When or where may we see the same shape with fine sand? 10. Explain how it is that the whole weight or pressure of the sand does not rest on the bottom of the cone.

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ex-pec-ta'-tion per-pen-dic'-u-lar some'-what vi'-o-lent-ly

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THE breaking waves dashed high

On a stern and rock-bound coast; And the woods, against a stormy sky,

Their giant branches tossed;

And the heavy night hung dark,

The hills and waters o'er,

When a band of exiles moored their bark

On the wild New England shore.

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