because ignorance and the silly idea of its attraction fills you with so many vain terrors.

I have often told you, that the earth and the planets move round the sun nearly on one level, almost as level as the table, and that their deviations, comparatively speaking, are not greater than the thickness of our stout dining-table.

Now these comets come, as it were, from the ceiling and the floor, sometimes obliquely, but more commonly perpendicularly. They seem like stray or outside planets of other systems, which in moving through space cross the gaseous vortex of our system, and getting entangled by it, are carried round the centre, as our sun; moving by their own right-line force, and deflected by the motions of the vortex, and making between the two their own strange orbits.

Their tails, as they are improperly called, are nothing more than the light of the sun condensed and refracted by their dense atmospheres, like the projection of light from a bottle of water held near a candle. For when viewed through a telescope, a comet appears to be nearly all atmosphere, and the light or projection is always exactly opposite to the sun, going before the comet as it recedes from the sun, therefore no tail or train. Yet, my dears, there are people, aye, and persons called philosophers too, who tell us, that this simple stream of light affects our atmosphere, changes the weather, and creates diseases; besides a hundred other fancies, as that we are all to be drowned by a comet's tail, that such a thing as this stream of light occasioned the deluge, &c. &c.!

The attraction-people then tell us about comets coming into contact with the earth, about the danger of some one running away with the moon, and I know not what; in my opinion, however, the laws of action and re-action are sufficient to keep such great bodies asunder, and, in point of fact, when

I

one of these passed very near Jupiter, neither appeared to affect the other, and Jupiter's moons were not carried away!

Apropos of our moon; I have sometimes thought it may have been a comet, which coming near the earth got entangled in its motions; and, perhaps, said one of you one day to me, with some wit and little truth, its atmosphere filled our seas! The moon, however, is a very singular body, 2000 miles in diameter, having no atmosphere, no water, and no clouds, and appearing, through our best telescopes, like a rough or dried cinder. It is not very obliging to us curious terrestrials, for it shews but one side to us, so that if there are inhabitants on the other side they never saw the earth. Being enlightened by the sun, its light and dark half is variously inclined towards us in its monthly orbit round the earth, and hence its changeable phases, which you can easily understand by means of a globe and a candle.

As the earth and moon maintain a constant action and reaction on each other, and the waters adjust their balance, so these move in accordance with the moon, and hence the tides. When the action of the sun concurs, we then have higher tides, as those called spring tides, after the new moon and the full moon. On this account, and its convenient light, we are under great obligations to this noble satellite.

Having thus wandered among the stars till I have descended to the moon, which is but 240,000 miles distant, I will now descend lower, consider our own atmosphere, and finally the earth itself and some of its peculiarities.

**

LETTER LIII.

The Clouds and Atmosphere.

MY ESTEEMED Children.

As life is short and art is long, I fear you will consider me as involving you in an endless labyrinth of study, but, thus far, you ought not to be educated in ignorance, and you may pursue all these subjects further as your taste suggests or your opportunities permit. To open your eyes on the world, to view the sun, moon, planets, and stars, to behold the successions of day and night, and of winter and summer, and to be the patients of the winds and weather, and know nothing about them, is to be on the level of brutes.

In my last letter I brought you from the regions of the moon to the confines of the earth. Here, at heights which vary from halfa mile to three miles, you meet with floating vapours or clouds in multitudes of forms, though susceptible of classifica tion into three,-the stratus, or level, the cirrus, or feathery, and the cumulus, or pondrous and round. These are exhalations from the earth, and they settle themselves at certain heights, accordingly as they are rare or dense, or as the air, wind, and other circumstances modify them. They ascend no higher and do not leave the earth, because, owing to the re-action of the earth against the gas which fills all space, to which its own motions contribute, the air becomes denser as it approaches the earth. In this respect it has been compared to fleeces of wool laid one upon the other, the lowest of which, by the action or pressure of the upper fleeces, are rendered more compact than the upper ones. The chemical properties appear to be the same through all space, for meteors inflame and explode at the height of 120

miles, and the luminous projections of comets prove the presence of gas by its reflection; but the mechanical properties of weight, elasticity, or re-action, density, and condensation of chemical power, are increased as we descend towards the earth.

This terrestrial re-action or condensation of the gas of space is sufficiently great at the height of 44 miles to reflect rays of light, and hence the twilight after the sun is set, on the principle on which the tops of hills are illumined after the valleys are in shadow.

The atmosphere may be looked upon not only as the general receptacle of the aqueous vapours of the sea and rivers, but likewise of all mineral exhalations; of the streams which are constantly arising from the perspiration of whatever enjoys animal or vegetable life, and from the instantaneous decomposition of those substances when deprived of life; of the smaller seeds of terrestrial and aquatic plants; of the eggs of an infinity of species of imperceptible animalcules; of the gases separated by combustion from all sorts of fuel; and of a variety of other substances, which are elevated, and for a time kept suspended by natural and accidental causes, from which sources are derived those various impurities which have been discovered inall atmospherical or rain water.

To understand what is passing in the atmosphere, it is necessary to bear in mind, that heat is always the motion of very small atoms. The light of the sun is atoms in motion, and these striking on the surfaces of bodies, continuing their action, and parting with their motion, it is received by the atoms of the bodies, and they are thereby raised into the atmosphere. This is particularly the case with all fluids. But as they rise in the atmosphere, the motions which elevated them from the surface being more and more diffused, they become stationary at a certain height, and form masses or clouds. The air, too, becomes rarer as we ascend, because the

mass above which presses downward is less and less, and from this rarity also the denser vapours will not rise above a certain height.

As, however, the atoms in circular motion, which form air, seek to expand on every side, they display every way a certain force, called Elasticity, and as this elasticity necessarily accords with the pressure of the surrounding parts, so the weight of the air from above, and the elasticity of the air, always are equal to each other, and at the surface of the earth they are equal to sixteen pounds to the square inch. This, therefore, is the weight or pressure of air in all bodies, and it would break them in pieces if all bodies in their pores did not contain air of equal elasticity, so that the pressure of air is only perceivable when there is no elastic air within to resist the action of that without.

On this principle cupping-glasses are formed. The air is expelled by heat, the cup is then quickly placed on the flesh, and the pressure of nearly sixteen pounds to the square inch drives the surrounding flesh into the cup. In the same way barometers are made. A tube exhausted of air has its open end plunged into a basin of quicksilver, and the pressure of the air on the quicksilver drives it up the vacuum as high as twenty-eight or thirty-one inches, i. e. about fifteen or sixteen pounds to the square inch, as the air is less or more elastic. Pumps, too, are formed on the same principle. The piston-rod makes a vacuum in the hollow tube placed in the water, and the pressure of the air on the water is so great, that it is capable of driving the water thirty-three feet high, equal to sixteen pounds to the square inch.