CHAPTER VIII.

THE THEORIES OF THE ATMOSPHERE, AND VOID SPACES FOR PLANETARY MOTION REFUTED BY DECISIVE FACTS;-THE ACTUAL STATE OF THE OCEAN AN IRRESISTIBLE PROOF AGAINST THE THEORIES OF

THE ATMOSPHERE, GRAVITY, AND EARTHLY MOTION.

WHILE arranging the elementary materials of the Copernican System, it occurred to the framers of it, that air, such as we breathe upon the surface of the earth, if co-extensive with the planetary orbits, would most effectually prevent the machine from going; that their imaginary worlds flying through it, with the incredible velocities ascribed to them, would experience such a mighty resistance, as would inevitably sweep every thing from their surfaces; or rather shiver them to pieces, and disperse them as dust. They therefore found it necessary to assert, that, what is termed the atmosphere, extended no more than about forty-five miles from the surface of the globe; and that the earth carries it about, comparatively, as a man carries his coat. That beyond this airy coat is what Sir Isaac Newton called the ætherial regions-a perfect vacuum, or what amounts to nearly the same thing, through which the earth, without experiencing any sensible resistance, moves with its atmosphere one hundred and forty one times more rapidly than a cannon ball! and of course, that whether drops of water, or air balloons, be floating

about in it, they are carried forward with the same velocity as the point of a solid rock!

To invent such plausible reasons as would give currency to this extraordinary hypothesis was a point of paramount importance; and they accordingly produced the delusive experiments of the barometer to show that the air gravitated, and that it decreased in density according to the increase of distance from the surface of the earth, by a certain mathematical ratio: so that according to Newton's calculation a globe containing an inch of such air as we breathe on the surface of the earth, if rarified to what he pretends to demonstrate it to be at the distance of four thousand miles above our heads, it would fill all the planetary regions of the solar system, as far as Saturn at least! Here is a notable instance of this celebrated philosopher's expansive imagination: it proves what fine ideas the brain, with the aid of mathematics, can spin out, when exercised in the freaks of fancy. Upon similar principles of calculation M. Amontons, in a paper which he laid before the French Royal Academy, observed that air might be compressed so as to be rendered heavier than gold, or platina; and imagined that the centre of the earth contained a sphere of about six thousand, four hundred and fifty-one fathoms of air compressed to a density superior to that of any known substance! He further imagined, that the earthquakes which occasionally convulse the globe are caused by air so compressed, being occasionally expanded by the heat of subterraneous fires! Such speculations as these are reckoned wonderfully sublime and profound; and if a man attempt to reason against them, our intolerant philosophers immediately cry out, that he is insane and that he ought to be cloathed in a strait jacket! I shall however risk the imputation and consider the subject

apart from their theoretical expansions and compressions, as its effects appear in the operations of nature.

Philosophers almost unvaryingly confound the elasticity, or spring of the air, with its weight, and accordingly conclude, that a base of an inch square supports a column of air of fifteen pounds weight; and by the same rule a middle-sized man is constantly pressed by about fifteen tons of air! Now if that be true, how is it that a man exists when, by mounting aloft in a balloon, until the barometer falls to ten or twelve inches, the pressure upon him, according to that rule, is suddenly reduced two-thirds, or about ten tons? When Mr. Robertson ascended at Hamburgh, a few years ago, it does not appear that he bursted or even experienced any inconvenience whatever, from that cause, although the quicksilver in his barometer sunk as low as twelve inches and a half. Mr. Baldwin, too, when he ascended from Chester, in the year 1785, expressly mentions, that he experienced no inconvenience whatever; nor did Mr. Brydone when on the summit of Etna; the French on the Andes; or Dr. Heberden on the peak of Teneriffe. Others I admit are said to have experienced some difficulty of breathing when placed in elevated situations; but what does that prove? Not an increase of rarity, but the reverse. Dr. Fletcher, formerly an English envoy at the court of Russia, states, that when you there pass out of a warm room into a cold one, you will "sensibly feel your breath to wax stark and even stifling with the cold as you draw it in and out." The same sensation is mentioned by the French philosophers as felt by them at Tornea; in breathing they said their breasts seemed to be rent. The experience of every one proves that breathing is more difficult in frosty weather, when the density of the air is increased by cold, than in warm weather when it is rarified by heat.

I consider air to be a simple, homogeneous fluid, created quite distinct from water or any other substance ;* and formed into a body as expressed in the sixth and seventh verses of the first chapter of Genesis, for the purpose of continuing the motions of inanimate bodies in the heaven, and to preserve and promote the existence and growth of animal and vegetable bodies on the earth; to give articulation to sounds; to enable us to sail upon the ocean and for other beneficial purposes. I consider that its pressure, apart from motion, is equal in all directions; that it gravitates no more downwards than it does upwards and that all the changes that are observed in the state of its pressure, are caused by the increase, or decrease, of motion, heat, or the weight of earthy or watery substances which float in it near the surface of the globe. Mr. Bruce when at Yambo, Jidda, and Loheia, on the coast of the Red Sea, found the quicksilver in the barometer three to five inches lower than it

* My opinion of the atmosphere is supported by that of William Jones, F. R. S. in his phisiological disquisitions, which I have perused since writing the above; he states as follows.

"The various parts which enter into this compound fluid of the atmosphere, have perplexed the subject to such a degree with those who have undertaken to study the nature of the air, that some have supposed the nature of the air to be nothing but water rarified, others nothing but salt of some kind in another form. Thus we might dispute about wine, beer and spirits, till we had lost sight of the element of water; but here we are in less danger, because water is a grosser fluid, and more obvious in its simple form. When all other parts are removed which enter into the composition of the atmosphere, there certainly remains a fluid, which is the vehicle and substratum of them all: in so much that if there were neither earth, nor salt, nor oil, nor sulphur, nor water, still there would be that air, which gives motion to the lungs and is the spring of animal life. This simple fluid is the first object of enquiry to those who consider the nature of the air; and the properties of air, which arise from the mixture of other things with it, are to be regarded rather as accidents than properties."

is generally found in England; and in particular at Jidda, which is nearly on a level with the surface of the Red Sea, it stood at the same height as it does upon the top of the mountain of Snowdon in Wales, which is estimated at one thousand two hundred yards above the level of the sea. Now I am of opinion that so great a difference in the pressure of the air, in those two situations, must be entirely owing to the difference of the climates: the atmosphere of the former being dry, and that of Wales, on the contrary, humid: the air in the latter part being loaded with watery particles, of course gives it a greater pressure downwards. Boerhaave, I think, is of opinion, that the gravity of the air depends entirely upon the water and other substances floating in it.

At the point to which Mr. Robertson ascended it appears, that about three-fifths of the pressure upon the barometer was by some cause taken off: a Newtonian will ask, how could so great a change have happened, if the air was there increased in density? I think, it is easily accounted for. At that elevation both the humidity and warmth were greatly diminished. The elasticity of air is much increased by the operation of heat upon the water mixed with it, as is plainly evident in the cylinder of a fire engine. Mr. Robertson says he could obtain little or no signs of electricity;-little or no heat to warm and increase the spring of the air;* it was, therefore, comparatively torpid, but it does not follow that it was less dense. If not less dense, the same philosophers will ask, why then does not the balloon continue to rise above a certain elevation? I reply, for the same reasons that vapours cannot rise above a certain height.

* "It does not appear that there is a single experiment to evince any elasticity in air, independent of fire." Jones's Physiological Disq.

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