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an undershot wheel, placed in the current. The force of the current turns the wheel, which moves the machinery connected with it.

23. John. There is an old wheel of this kind at the foot of the cascade which we are just coming to, and that will give us a practical illustration.

George. Where the streams are small, rapid, and have sufficient fall, I have frequently seen mills driven by what is called an overshot wheel. The water falls upon the wheel, and by its weight, principally, turns it.

Frank. There is also a water-wheel, called the breast-wheel, which receives the water against the side of it instead of the top or the bottom. In this case the water acts partly by its momentum and partly by its weight.

24. Mr. M. Of these wheels the overshot is the most powerful; but both that and the breast-wheel require a considerable fall of water, while the undershot wheel, which is the least powerful, requires merely a strong current.

Ida. I have seen the experiments with what is called "Barker's mill," which is moved by the pressure of a column of water.*

25. Ella. Here is the cascade itself; and there, at the foot of it, is the old wheel which John spoke of! I understand now what an undershot wheel is.

Ida. And don't you see-where the water tumbles over that ledge-another little wheel? That is a breast-wheel. How swiftly it spins round!

Ella. And there is still another and larger wheel lower down! That is an overshot wheel. It does seem as though some one has had all these wheels put up for the illustrating this lesson in hydraulics!

purpose of

* Fig. 20, Barker's mill. In what is called "Barker's mill," the machinery is moved by hydrostatic pressure. It consists of a hollow clyindrical pipe, A B, of considerable height, terminating above in a funnel-shaped cavity, resting below on a pointed steel pivot, and communicating below with a cross tube, or arms, E F, closed at the extremities, but having openings on the opposite sides, near each end of the cross tube. A pipe, G, above, regulates the supply of water, and keeps the vertical pipe full.

If the openings at E and F be closed, it is evident that the hydrostatic pressure in the cross tube will be the same on all parts of its surface, and will be proportioned to the height of the pipe A B; but let the water flow at the orifices, and there will be more pressure on one side of the cross tube than on the other, and the machine will revolve in the direction of the greater pressure.

The movement of Barker's mill was long attributed, but erroneously, to the reaction of the jets (pressing upon the elastic air) against the extremities of the cross tubes. On the principle of hydrostatic pressure, as here illustrated, the turbine wheel has lately been invented. It is the most powerful and economical of all water-engines. See Fig. 21, next page.

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26. All seemed to be of the same opinion; while Mr. Maynard only smiled, and said he hoped they would examine all the wheels, and be able to give a full description of them at another time. All were soon busy in clambering up the rocks, and noticing the working of the wheels; and after some time spent in this way, all returned to the foot of the cascade, when John described the new turbine wheel which his father had lately had put up in his mill, and which is moved by the action of the pressure of a column of water.*

"This wheel," said Mr. Maynard, "can be made to utilize from three fourths to four fifths of the theoretical power of the water, while the undershot wheel will not often give to machinery more than one quarter of the water power. The breast-wheel, when well constructed, will utilize a little more than one half of the moving power of the water, and the overshot wheel about two thirds."

Before the little party left this pleasant spot, Frank happened to remark that the numerous little water-falls in this cascade forcibly reminded him of Southey's poem about the "Cataract of Lodore," "for the water," said he, 66 comes running, and jumping, and dancing, and leaping down in almost every imaginable variety of form and motion." Thereupon Frank, being invited to recite the poem, gave the following:

THE CASCADES OF ROCKY GLEN.

[The Cataract of Lodore.]

"How does the water come down at Lodore?"
My little boy asked me thus, once on a time;
And moreover he tasked me

To tell him in rhyme.

Anon at the word,

There first came one daughter,
And then came another,

*Fig. 21, the turbine wheel, consists of a fixed upright cylinder I J, which admits the water, placed upon another and larger fixed cylinder, represented here by the inner curves, the latter encompassed by the moving wheel A B, in the form of a rim. Through N passes a shaft, by which motion is imparted to machinery. From the tall central cylinder the water passes, under great pressure, into the curved compartments of the larger fixed cylinder, where it receives such a direction as to strike the divisions of the revolving rim to the best advantage. B As this wheel acts upon the principle of hydrostatic pressure, its power is proportionate to the height of the column of water in the central cylinder I J. When a column of water can be obtained of considerable height, the turbine wheel is an engine of great power. It is extensively used in the cotton factories of Lowell, Mass.

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P

3

To second and third the request their brother, And to hear how the water comes d. wn at Lodorc, With its rush and its roar,

As many a time
They had seen it before.

So I told them in rhyme,
For of rhymes I had store.

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From its sources, which well
In the tarn on the fell;
From its fountains

In the mountains;

Its rills and its gills;

Through moss and through brake,
It runs and it creeps
For a while, till it sleeps
In its own little lake.
And thence, at departing,
Awakening and starting,
It runs through the reeds,
And away it proceeds,
Through meadow and glade,
In sun and in shade,

And through the wood-shelter,
Among crags in its flurry,
Helter-skelter,

Hurry-skurry.

Here it comes sparkling,
And there it lies darkling:
Now smoking and frothing,
In tumult and wrath in,
Till, in this rapid race
On which it is bent,
It reaches the place
Of its steep descent.

The cataract strong
Then plunges along,
Striking and raging,
As if a war waging
Its caverns and rocks among;
Rising and leaping,
Sinking and creeping,

Swelling and sweeping,
Showering and springing,
Flying and flinging,
Writhing and ringing,

Eddying and whisking,

Turning and twisting,
Spouting and frisking
Around and around,
With endless rebound;
Smiting and fighting,
A sight to delight in ;
Confounding, astounding,

Dizzying and deafening the
ear with its sound:
Collecting, projecting,
Receding and speeding,
And shocking and rocking,
And darting and parting,
And threading and spreading,
And whizzing and hissing,
And dripping and skipping,
And hitting and splitting,
And shining and twining,
And rattling and battling,
And shaking and quaking,
And pouring and roaring,
And waving and raving,
And tossing and crossing,
And foaming and roaming,
And dinning and spinning,
And guggling and struggling,
And heaving and cleaving,
And moaning and groaning;
And glittering and frittering,
And gathering and feathering,
And whitening and brightening,
And quivering and shivering,
And hurrying and skurrying,
And thundering and floundering;

Dividing and gliding and sliding,
And falling and brawling and sprawling,
And driving and riving and striving,
And sprinkling and twinkling and wrinkling,
And sounding and bounding and rounding;
And grumbling and rumbling and tumbling,
Delaying and straying and playing and spraying,
Advancing and prancing and glancing and dancing,
And thumping and plumping and bumping and jumping,
And dashing and flashing and splashing and clashing;
And so never ending, but always descending,
Sounds and motions forever are blending,
All at once and all o'er, with a mighty uproar-
And this way the water comes down at Lodore.

[graphic]

After the recital of this very eccentric poem, as Ida called it, and some remarks from Mr. Maynard upon its character, and the proper reading of it, the little party set out on their

return homeward, both pleased and profited by their day's

excursion.

Coor, a lobe-footed water-fowl. 2 HERN, contracted from heron.

13 BICK'-ER, to move unsteadily; to play backward and forward.

4 THORP, a small village; a hamlet.

LESSON VI.-PNEUMATICS.

1. "My young friends," said Mr. Maynard, as he approached a large table covered with numerous tubes, glasses, pumps, jars, etc., "if you have carefully read the books you possess, and such as I loaned you from the library, you can make this one of the most delightful lessons in our whole course. Pneumatics, you know, is the science of the pressure and motion of elastic fluids. Air and steam are good examples of elastic fluids; the former representing such as are permanently gaseous, and the latter such as are condensible into a liquid state. Do you recollect what we gave in a former lesson as one of the characteristics of elastic fluids ?"

2. Frank. I think it was their immediate tendency to expand when at liberty to do so.

Mr. M. You will see by some experiments with the airpump that air is highly elastic. I would first state that we are living in an ocean of atmosphere about fifty miles in height, and quite surrounding the globe. The air-pump is similar to a water-pump in construction, but made so accurately as not to leak air. I will now pump some of the air

Fig. 22, atmospheric press

ure.

from this glass, which you see is open at both ends; but, in order to remove the air, or rather to obtain a vacuum inside, it will be necessary to stop the air from entering from the top. John, will you place your hand on the top for a stopper? I will now remove the air from under John's hand.

3. "Oh!" exclaimed John, " it sucks my hand down." "What sucks your hand down ?" said Mr. M.

"Really," replied John, "I think I was mistaken, for I now understand that it is the weight of the air on my hand pressing it down. I learned long ago that the weight of the atmosphere is about fifteen pounds on a square inch, but I never before had so clear an idea of it."

4. Mr. M. Why could you not move your hand from the

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