George. Will not the explanation of the common pump belong to this lesson ?

9. Mr. M. Its principle has already been illustrated in the account which was given of the discovery of the weight of the atmosphere. It is evident that we have only to exhaust the air in a pipe, the open end of which is placed in water, and the water will be pressed up to fill the vacuum. Here are illustrations of two different kinds of pumps, one of which is the forcing-pump, which illustrates the principle of the fireengine.*

Frank. Darwin very prettily explains the action of the common pump in the following lines:

"Thus does the sliding piston bear

The viewless columns of incumbent air;

Press'd by the incumbent air, the floods below,
Through opening valves, in foaming torrents flow,
Foot after foot with lessened impulse move,

And, rising, seek the vacancy above."

forcing it up through D. There is now a pressure of air on the water in B equivalent to the pressure exerted (on the principle of the hydrostatic paradox) by the column of water in F, and this pressure is exerted to throw the water up through E and cause the play of the fountain. Thus the contents of B are actually transferred to C, and the air that was in C passes into B. When C thus becomes filled with water and B with air, the fountain must stop.

*The common pump, represented by Fig. 29, consists of three parts, the suction-pipe, the barrel, and the piston. The suction-pipe, f e, is of sufficient length to reach down to the water in the well. The barrel, C B, is a perfect cylindrical cavity, in which the piston G moves, air-tight, up and down by the rod d. It is commonly moved by a lever, but in the figure a rod and handle, D E, are represented. On one side is the spout F. At the top of the suction-pipe at H there is a valve, b, opening upward, and also one at a. When the piston is raised from the bottom of the barrel, a vacuum is produced in the barrel, the valve b opens, and if the pipe, fe, be full of water, the water rushes into the barrel, being pressed up by the atmosphere resting on the water in the well. On depressing the piston the water rushes up through the valve a, and after a few movements the water is poured out at the spout F.

Fig. 29.
E

Fig. 30.

In the forcing-pump, Fig. 30, the piston, g, has no valve. On the box at H is a valve, b, opening upward, and when the piston is elevated the water rises into the barrel, BC. During the downward movement of the piston the valve b shuts, and the water passes by a channel around m, through the lateral pipe M O M N into the air-chamber, K K. The entrance to this air-chamber is closed by a valve at a, and from the chamber proceeds a tube, H G, open at both ends. After a few movements of the piston the lower end of this tube becomes covered with water, the air is compressed into the space G H, and thereby the water is thrown out in a continuous jet, S.

10. Mr. M. Do you know of any instrument besides the forcing-pump and the fire-engine that acts upon the principle of condensed air?

John. I have seen an air-gun charged with air instead of powder.

Mr. M. That is a good illustration of the principle that the density and elasticity of air are directly as the force of compression. Here is a drawing of the air-gun. Air is con

Fig. 31, the Air-gun,

densed into the ball A, which is attached to the gun. A bullet is then put into the

barrel, and by a peculiar lock a portion of the condensed air is let in behind the bullet, which is thrown out with almost the force of gunpowder.

George. Can another bullet be thrown out without refilling the ball?

11. Mr. M. Yes, a dozen or more; but each one with less force than the one before it, as the air in the ball gradually loses its density, and consequently its elasticity. Sometimes the air-chamber is in the stock of the gun, which makes it more convenient.

John. I have made little fountains by inserting through the cork of a bottle half filled with water a common pipe-stem, and after blowing through the tube the water would spout up to a considerable height. Was not this owing to condensed air pressing on the surface of the water?

Mr. M. It was caused by condensed air; but you had to be careful to let the tube reach below Fig. 32, Bottle the surface of the water?

Fountain.

Ella. Is not the kite an atmospheric machine? 12. Mr. M. Yes; and although it is a playing machine, it is elevated on strictly scientific principles. It is really pulled up an inclined plane of air by the tension and weight of the string.*

Ida. And so the kite slid down the plane when the boy clapped his hands and let the string slip through his silly fingers!

Frank. This reminds me of a very pretty piece of poetry about "the lost kite."

Mr. M. I am glad to find you so imbued with the poetry of science; and as this lesson has not been very poetical, you may repeat the piece, if you please.

13.

14.

15.

16.

17.

THE LOST KITE.

My kite! my kite! I've lost my kite!
Oh! when I saw the steady flight
With which she gained her lofty height,
How could I know that letting go

That naughty string would bring so low
My pretty, buoyant, darling kite,

To pass forever out of sight?

A purple cloud was sailing by,

With silver fringes o'er the sky;

And then I thought, it seemed so nigh,
I'd make my kite go up and light
Upon its edge so soft and bright,
To see how noble, high, and proud
She'd look while riding on a cloud!

As near her shining mark she drew,

I clapped my hands; the line slipped through
My silly fingers; and she flew

Away! away! in airy play,

Right over where the water lay!

She veered and fluttered, swung, and gave

A plunge, then vanished in the wave!

I never more shall want to look

On that false cloud or babbling brook;

Nor e'er to feel the breeze that took

My dearest joy, to thus destroy

The pastime of your happy boy.

My kite! my kite! how sad to think
She flew so high, so soon to sink!

"Be this," the mother said, and smiled,

"A lesson to thee, simple child!

And when, by fancies vain and wild

As that which cost the kite that's lost,

Thy busy brain again is crossed,

Of shining vapor then beware,

Nor trust thy joys to fickle air!

18.

"I have a darling treasure, too,

That sometimes would, by slipping through

My guardian hands, the way pursue

From which, more tight than thou thy kite,

I hold my jewel, new and bright,

Lest he should stray without a guide,

To drown my hopes in sorrow's tide!"

19. Mr. M. There are many natural phenomena dependent on the atmosphere which we shall learn in the department of Physical Geography. But, before we conclude this lesson on

and ST; and since these are in the direction of the two sides of a parallelogram, it will not obey either, but ascend through O B, its diagonal. (See Fourth Reader, p. 298.) It is thus virtually pulled up the inclined plane O B.

pneumatics, I desire to read you a riddle, hoping each of you will be Edipus enough to guess the answer.

A RIDDLE.

"Mortal', wouldst thou know my name',
Scan the powers I proudly claim'.
O'er this globe's capacious round
With fairy sprightliness I bound';
O'er sea and land my power extends,
To every herb my care descends.
Did I withhold my vital breath',
Nature's forms would sink in death'.
When confined, or swiftly driven
By angry spirits in the heaven',
My wrath in thunders I make known',
And Discord claims me as her own'.
'Tis love of freedom makes me wild-
When uncontroll'd, my nature's mild;
And oft the nymph', in dewy grot',
Seeks solace from my plaintive note';
O'er lovers' graves I waft a sigh,
And breathe the sound of sympathy.

And know, ye sons of Albion's isle',
That when the Hero of the Nile,
Midst crowds with mournful pomp array'd,
In the cold lap of Earth was laid',
I sympathized with Britain's tear,
And waved the banner o'er his bier'.
'Tis I who from the trembling lyre
Breathe tones of love and soft desire';
'Tis I, the spirit of the shell,

Who fill with notes the listening dell';
And, when the war-trump sounds alarm',
"Tis I who summon men to arm'.
Made captive by the arts of man',
My various services began';

To grind his corn', to drain his lands',
I soon was tasked', to spare his hands'.
Should he to foreign climes proceed',
He yokes me like the neighing steed',
And, by my quick but easy motion',
He traverses the stormy ocean'.

His children, too, my presence court',
To give them toys', and make them sport':
Without my aid, their kites would lie
As useless weights, that ne'er could fly`;
Their humming-tops would soundless spin,
Unless I breathed a spell within':
The modest maid, without my power,
Would wither like her kindred flower'.
Unless my cup of sweets she sips',
Where are the rubies of her lips'?
Unless my glowing rouge she seeks',
Where are the roses of her cheeks'?'
What art again can strew her tresses
With half the grace my skill possesses'?
Ev'n goddesses are represented

In draperies which I invented.
Sometimes', 'tis true', I am so frail
As ruffian-like to raise your veil',

And thus to curious man reveal

The charms you modestly conceal'.
Revenge the deed'. Announce my Name',
For now you know the powers I claim'."

23. "It must be air," exclaimed the whole class with one

voice.

1

"But what is meant by the allusion to the funeral of the 'hero of the Nile ?"" said İda.

Mr. M. I quote an extract from the description of the ceremonies of Nelson's interment in St. Paul's: "Never shall I forget the thrilling effect which was produced on the assembled multitude by the solemn movement of the banners in the dome as the bier slowly advanced along the aisle in the Cathedral; a movement which was accidentally occasioned by a current of air from the western entrance, although, to the eye of fancy, it seemed as if some attendant spirit had directed the colors, under which the hero had bled and conquered, to offer this supernatural testimony of respect and sorrow."

LESSON VIII.—THE STEAM-ENGINE.

1. "You recollect," said Mr. Maynard, "that it was stated in a former lesson that water does not require to be so hot to boil when the pressure of the atmosphere is partially removed, as it does when it is subject to that pressure."

John. I recollect it, and I would ask if it does not require a higher temperature when the pressure is increased.

2. Mr. M. Under ordinary circumstances of the pressure of one atmosphere, of 15 pounds on the square inch, water boils at the temperature of 212°; but increase the pressure to 45 pounds on each square inch, or three atmospheres, and it will not boil below 275°.

Frank. There is a kind of kettle, made very strong, with an air-tight cover which is fastened down, and as the steam which first rises presses down by its elasticity, the water becomes hot enough to cook bones.

3. Mr. M. That is called Papin's Digester, and in it the water really never boils, but it becomes sufficiently hot to cause the separation of the gelatinous matter from bones. As this is used for soup, I suppose you call it cooking bones. Ella. I should not like to have one of those dangerous kettles in our kitchen, for fear it would burst.

Mr. M. It has an orifice closed with a plug, which is held down by a weight until a certain pressure is exerted by the steam, when it rises up and lets the steam escape. It then falls back again until the pressure becomes sufficient to raise it. 4. Ida. Is that what is called the safety valve?

Mr. M. It is; and safety valves are used in all boilers where the steam is liable to be very much compressed.