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.

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.

1 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 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

glass as well after the exhaustion as before? Was there any more pressure put upon your hand by the operation?

John. The weight was certainly no more, but the sustaining spring or elasticity of the air was removed from below, and I felt the pressure, just as a man feels the pressure of debts when he has no money with which to pay them.

5. Mr. M. To understand all about the weight and elasticity of air is to understand pneumatics. Have any of you ascertained the entire weight of the atmosphere, which seems so light?

Ella. Somebody has calculated that the weight of the atmosphere is equal to that of a solid sphere of lead sixty miles in diameter. I would much like to understand how such astonishing calculations are made.

6. Ida. It is very easy indeed. We know that the pressure of air is fifteen pounds on each square inch; and all we have to do is to find how many, square inches there are on the earth's surface, and multiply by fifteen to obtain an answer in pounds.

Ella. How do people know that the pressure is fifteen pounds on a square inch?

Ida. It is in all the Philosophies.

7. Ella. But how did the philosophers find it out? I would also like to know how it is known to be about fifty miles high.

Mr. M. I perceive that Frank has been examining that matter, and that while you have been talking he has been figuring. I presume he can read to us, from the book which he has in his hand, an account of the way in which this great discovery was first made.

8. Frank. "The common pump was invented by Ctesibius 224 years B.C., and soon after it came into general use throughout the civilized world. The philosophers of the time explained its action by saying that when the piston was raised in pumping, and the air thereby removed, a vacuum would be formed over the water, but that 'nature abhorred a vacuum,' and consequently filled it with water as the most convenient material. 9. "So the water kept rising at each stroke of the pump, as the air was removed. Some wells were very deep, and it was found that whenever the depth was over 33 feet, the pumps were unable to raise the water. Finally, some engineers asked Galileo why the water would not rise higher than 33 feet. He is said to have replied that nature's abhorrence of a vacuum ceased at the height of 33 feet.""

10. Mr. M. Though the great Galileo did not know the true theory of the common pump, he certainly must have given such an answer rather in joke than in earnest. But let us have the rest of Frank's account.

11. Frank. "It is supposed that Galileo suggested to his pupil Torricelli that the weight of the air on the water surrounding the pump might press the water up into the pump when the pressure within the pump was removed. Galileo died soon after, and the next year, 1643, Torricelli determined to find out all about it. He thought that if the weight of the air was the cause, he could try the experiment of sustaining, by the pressure of the atmosphere, a column of water 33 feet high in a tube closed at the upper end. This would have been a difficult experiment to perform; but, fortunately, he knew that the specific gravity of quicksilver was 13 times that of water. Of course a column of quicksilver 24 feet high would balance a column of water 13 times as high, or about 33 feet.

32

31 30 29

28

Fig. 23, Torricelli's experiment.

13. Ella. It would be Torricelli.

12. "Torricelli took a glass tube more than 2 feet long, and filled it with quicksilver, and, after closing the upper end, inverted it, placing the end below the surface of quicksilver in a cup before removing his thumb. As he expected, the quicksilver did not all run down into the cup, but stood at the height of 30 inches in the tube. Evidently the pressure of the atmosphere upon the quicksilver in the cup sustained the column in the tube; and as the tube was one inch in area, and the column of quicksilver weighed 15 pounds, not only was the pressure of the atmosphere on a square inch of surface ascertained, but the instrument called the barometer was invented-an instrument to show the pressure of the atmosphere at different times and in different places."

very easy to try the experiment of

Mr. M. Will George tell me, now, how the altitude of the atmosphere is found by this instrument, the barometer?

George. By carrying a barometer up a high mountain, and noticing how much the mercury falls-that is, how much the pressure diminishes for every hundred or five hundred feet we ascend, we can easily calculate at what height there will be no pressure, and consequently no atmosphere, which is at a height of about forty-five miles.

14. Mr. M. That is correct. The pressure diminishes in a geometrical ratio as we ascend. Thus, at about four miles above the earth the air is only half as dense as at the surface; at eight miles, one fourth; and at twelve miles, one eighth, etc. But of what practical use is the barometer?

Ida. After once knowing how the barometer stands at different heights from the sea, it enables us to measure the height of mountains.

Frank. And also to foretell storms; for, as the air is usually lighter before a storm, this lightness is indicated by the falling of the quicksilver. I was reading this morning, in "Darwin's Botanic Garden,"

"How up exhausted tubes bright currents flow
Of liquid amber from the lake below,

Weigh the long column of the incumbent skies,
And with the changeful moment fall and rise."

15. Mr. M. Dr. Arnott relates a striking instance, which occurred to himself, of the great practical value of the barometer. I will read his account:

"We were in a southern latitude. The sun had just set with placid appearance after a beautiful day, and the usual mirth of the evening watch was proceeding, when the captain's order came to prepare with all haste for a storm. The barometer had begun to fall with appalling rapidity. As yet, the old sailors had not perceived even a threatening in the sky, and they were surprised at the extent and hurry of the preparations; but the required measures were not complete, when a more awful hurricane burst upon them than the most experienced had ever braved. Nothing could withstand it: the sails, already furled and closely bound to the yards, were riven away in tatters; even the bare yards and masts were in great part disabled, and at one time the whole rigging had nearly fallen by the board. 16. "Such, for a few hours, was the mingled roar of the hurricane above, of the waves around, and of the incessant peals of thunder, that no human voice could be heard, and, amid the general consternation, even the trumpet sounded in vain. In that awful night, but for the little tube of mercury which had given the warning, neither the extraordinary strength of the noble ship, nor the skill and energies of the commander would have saved one man to tell the tale. On the following morning the wind was again at rest, but the ship lay upon the yet heaving waves an unsightly wreck."

17. Ella. As the density of air diminishes so rapidly in ascending, would it not increase in the same ratio in descending below the surface of the earth?

Mr. M. Certainly; and at a depth of 58 miles the air we breathe would be more dense than gold, or the heaviest known substance, unless at that depth the pressure of the air should be partially modified by the attraction of the earth above. We should not think the saying."light as air" very appropriate under such a pressure. Who can tell me on what principle smoke is "drawn into the mouth," as it is said, in the process of smoking a cigar?

18. John. On the principle that a vacuum is produced in the mouth by the action of the cheeks, and the smoke is forced through the cigar by atmospheric pressure.

Ella. If that is so, a cigar in the smoking process must have fire at one end and a vacuum at the other.

Mr. M. Though your remark is very hard upon smokers,