SWEET morn of life! all hail, ye hours of ease!
When blooms the cheek with roseate, varying dies;
When modest grace exerts each power to please,
And streaming lustre radiates in the eyes.
Thy past hours innocent, thy present gay,
Thy future, halcyon hope depicts without allay.
Day spring of life! oh, stay thy fleeting hours!
Thou fairy reign of ev'ry pleasant thought!
Fancy, to cheer thy path, strews all her flowers,
And in her loom thy plan of years is wrought.
By thee for goodness is each heart caress'd,
The world, untried, is judg'd by that within thy breast.
Sweet state of Youth! O harmony of soul!
Now cheerful dawns the day, noon brightly beams,
And evening comes serene, nor cares controul,
And night approaches with soft infant dreams
Circling, the moon beholds th' accustom'd round,
Life's smiling charities awake, and joys abound.
Season of hope, and peace, and virtues, stay!
And for our bliss let inexperience rest!
For what can prudent foresight's beam display?
Why the barb'd arrow pointed at our breast!
Teach to suspect the heart we guileless trust,
And, ere we are betray'd, to think a friend unjust.
Thou candid age! with ardent friendship fraught,
That fearless confidence to none denies :

Better sometimes deceiv'd-and, artless, taught
By thy own griefs the wisdom of the wise.

For sad experience, with sorrowing breath,

Sheds, weeping sheds, the pristine roses in hope's wreath.

Season belov'd! Ah, doom'd to pass away!

With all thy freshness, all thy flatt'ring joys,
With blooming beauties envied; powerful sway,
With laughing hours, the future ne'er annoys.
Ah! be thou spent as virtue bids to spend !
Then-though we wish thy stay-no sighs thy reign

.shall end.

FOR THE POLYANTHOS.

A COURSE OF

LECTURES ON NATURAL PHILOSOPHY,
BY S. LATHROP, JUN, A. M.

LECTURE THE SIXTH.
Pneumatics

Part Second.

THE air of the atmosphere, is a mixture, or possibly a combination of three different gases-oxygen, nitrogen, or azotic gas and a small portion of carbonic acid gas, and of water. The former of these, seems to be the only ingredient, on which the effect of the air, as a chemical agent, depends. Hence combustible bodies burn in atmospheric air, only in consequence of the oxygen gas which it contains; and when this is exhausted, air is no longer capable of supporting combustion.

Atmospheric air ministers to the support of human life only in consequence of the oxygen gas which it contains. Air having been received into the lungs, and again expired, is found to have lost considerable of its oxygenous part, viz. 10 to 12 per cent. It proves fatal, to animals, however, long before the purer part is wholly exhausted; and hence it appears, that a considerable portion of oxygen gas is even necessary to fit the air for supporting respiration. That air is necessary to the support of combustion and, on the same prin ciple, of animal life, (for it is well known, that the lungs of an animal can never perform their functions, where a candle will not burn,) nay be demonstrated by this experiment. (Candle under an exhausted receiver.)

*

In the first part of the Lecture on Pneumatics, I considered the weight and pressure of the atmosphere. A column of air, reaching from the earth to the highest part of the atmosphere, which possesses the power of refracting light, is estimated at a weight of 15 pounds for every square inch on which it rests. The pneumatic pistol will give you an idea of the weight of the atmosphere on a flat surface, when the air from under it is extracted, and a Boylean vacuum produc

VOL. II.

30

ed. (Experiment with pneumatic cannon.) This experiment. was accompanied with considerable noise, occasioned by the quantity of air, and the velocity with which it entered the vacuum. Mr. Papin has calculated that this velocity is sufficient to carry it through 1300 feet in a second of time.

The syphon affords a very probable solution of the nature of intermitting springs and fountains. Many instances of these occur. At Gravesend there is a pond out of which the water ebbs all the time the tide is coming into the adjacent river, and flows while the tide is going out. This phenomenon probably arises from a subterranean reservoir, equal in capacity to the quantity of water that rises and falls in the pond. Between this reservoir and the pond, there may be a natural syphon by which they communicate with each other, and act as already explained; and a second natural syphon may in the same manner convey it away from the pond when it is filled to a certain height.*

The syphon may be used for many entertaining, as well as useful purposes. It is sometimes made the instrument of much amusement in pneumatic experiments, particularly when fixed in a vessel called Tantalus's cup, which, being filled with water, on being presented to the lips, cheats the person who attempts to drink'; for the liquid flows through the longer leg of the concealed syphon, and falls on his feet instead of running into his mouth. This cup derives its name from the fable of Tantalus, thus related by Homér.

"There Tantalus along the Stygian bounds,

Pours out deep groans-with groans all hell resounds.
E'en in the circling flood refreshment craves,
And pines with thirst amidst a sea of waves,
And when the water to his lips applies,
Back from his lips the treacherous water flies,
Above, beneath, around his hapless head,
Trees of all kinds delicious fruitage spread.

* For an interesting and ingenious paper on the subject of the natural syphons by which Boston is supplied with water, the reader is referred to the second volume of Transactions of the American Academy of Arts and Sciences. The article is from the pen of the Rev. John Lathrop, D.D.

The world is indebted to the great Gallileo for the discovery and demonstration of the air's gravity. He found by experiment that water might be raised to a certain height, and no further. To account for this phenomenon, he substituted the pressure of the atmosphere as a cause, instead of the common doctrine of nature's abhorrence of a vacuum. For there was no doubt that a vacuum could be obtained as high as a tube could be raised and a piston drawn; but it was found that water would rise in it no higher than the point where the weight of its column could be counterbalanced by an equal weight of atmosphere, acting on a similar base. Torrecellius, Borellius, and others, especially the English philosophers, availed themselves of this hint, and to their improvement of it on hydrostatical and pneumatic principles, we are indebted for that useful and elegant instrument, the Barometer. The machinery at first used for experiments on the subject of the air's pressure being sufficient to account for the rise and support of fluids in vacuo, was unwieldy, and troubleAn immense wooden shaft, nearly forty feet in length, was bored in the manner of a common pump, but left closed at one end. Still higher than the shaft, stages were erected to work the piston, in order to exhaust the air from the pipe, and procure a vacuum. After this object was obtained, it was necessary to invert the upper end in the water of a large reservoir or tank; this was performed by the aid of the sail of a windmill. Ingenious men could not long submit to this laborious and clumsy operation. It soon occurred to Torrecellius, that if a column of water 33 feet high were a counterpoise to a whole column of the atmosphere, then a column of mercury of about two feet and a half, would also be a counterpoise to it; since quicksilver is nearly fourteen times. heavier than water, and so, the 14th part of the height, or about two feet and a half, would be as heavy as the column of water. This reasoning was verified; for having filled a glass tube with quicksilver, and inverted it in a bason containing a quantity of the same fluid, the mercury presently descended until its height above that in the bason, was about

some.

two feet and a half, just as he expected. And this, is what is from him, called the Torrecellian experiment.

But it may be questioned here, why a column of air, separated from the body of the atmosphere, and enclosed in a glass receiver, only 30 inches should have the power to support the mercury, as much as a column reaching to the utmost height of the atmosphere. `This circumstance is explained by knowing the fact, that the elasticity of the air increases or diminishes with an alteration of its density, and in exact proportion to it. It is no matter, whether the air be compressed, and, retained in any space, by the weight of the atmosphere, or any other cause, as, in either case it will endeavor to expand with the same force. And therefore, if such air as is near the earth, be enclosed in a glass vessel, so as to have no communication with the external air, the pressure of such enclosed air, will be equal to the whole weight of a similar column of the whole external atmosphere.

- The variation in the density of the atmosphere was also made evident by the rise and fall of the mercury within the tube. A series of accurate observations, enabled the inventor and the improvers of the Barometer to mark those variations, and from them to indicate the changes which are about to take place in the state of the weather. The weight of the atmosphere being least in stormy weather, the mercury sinks to about 28 inches-but in fine, clear and serene weather, the atmosphere will sustain the column of mercury about 30 inches high; and between these extremes all the changes that happen in the weight of the air, near the surface of the earth, are included. The phenomena of the atmosphere afford fruitful subjects for hypothesis and conjecture. To what is this different density of the air, as shewn by the barometer, and evinced by our own sensations, attributable? We now know that water may be formed from two kinds of air, and may again be separated into two constituent parts; in fact that water and air are, by various natural processes, convertible into each other. Why then may not these processes go on in the atmosphere; why may not the atmosphere dif