As the barometer is thus used in predicting the changes of the weather, so is it also serviceable in measuring the heights of mountains, which mathematicians cannot so readily do: for, as the higher we ascend from the surface of the earth the air becomes lighter, so the quicksilver in the barometer will descend in proportion. It is found to sink at the rate of the tenth part of an inch for every ninety feet we ascend; so that in going up a mountain, if I find the quicksilver fallen an inch, I conclude that I am got upon an ascent of near nine hundred feet high. In this there has been found some variation; into a detail of which it is not the business of a natural historian to enter.

silver in the tube will, by its own weight, endeavour to descend into that within the basin; but the external air, pressing on the surface of the quicksilver in the basin without, and no air being in the tube at top, the quicksilver will continue in the tube, being pressed up, as was said, by the air, on the surface of the basin below. The height at which it is known to stand in the tube, is usually about twenty-nine inches when the air is heavy; but not above twenty-six when the air is very light. Thus, by this instrument, we can with some exactness determine the weight of the air; and, of consequence, tell beforehand the changes of the weather. Before fine dry weather, the air is charged with a variety of vapours, which float in it unseen, and render it extremely heavy, so that it presses up the quick-wind-gun has been invented, which is an instrusilver; or, in other words, the barometer rises. In moist, rainy weather, the vapours are washed down, or there is not heat sufficient for them to rise, so that the air is then sensibly lighter, and presses up the quicksilver with less force; or, in other words, the barometer is seen to fall. Our constitutions seem also to correspond with the changes of the weather-glass; they are braced, strong, and vigorous, with a large body of air upon them; they are languid, relaxed, and feeble when the air is light, and refuses to give our fibres their proper tone.

But although the barometer thus measures the weight of the air with exactness enough for the general purposes of life, yet it is often affected with a thousand irregularities that no exactness in the instrument can remedy, nor no theory account for. When high winds blow, the quicksilver generally is low it rises higher in cold weather than in warm; and is usually higher at morning and evening than at mid-day: it generally descends lower after rain than it was before it. There are also frequent changes in the air, without any sensible alteration in the barometer."

In order to determine the elasticity of air, the

ment variously made; but in all upon the principle of compressing a large quantity of air into a tube, in which there is an ivory ball, and then giving the compressed elastic air free power to act, and drive the ball as directed. The ball, thus driven, will pierce a thick board; and will be as fatal, at small distances, as if driven with gunpowder. I do not know whether ever the force of this instrument has been assisted by means of heat; certain I am, that this, which could be very easily contrived by means of phosphorus, or any other hot substance applied to the barrel, would give such a force as I doubt whether gunpowder itself could produce.

The air-pump is an instrument contrived to exhaust the air from round a vessel adapted to that purpose, called a receiver. This method of exhausting, is contrived in the simple instrument by a piston, like that of a syringe, going down into the vessel, and thus pushing out its air; which, by means of a valve, is prevented from returning into the vessel again. But this, like all other complicated instruments, will be better understood by a minute inspection, than an hour's description: it may suffice here to observe, that by depriving animals, and other substances, of all air, it shows us what the benefits and effects of air are in sustaining life, or promoting vegetation.

! + Professor Leslie has proposed a theory of the depression of the barometer, in which he supposes that the wind, describing a curve in passing over the surface of the globe, acquires a centrifugal force sufficient to diminish the pressure of the air on the earth's surface, and consequently to depress the barometer. The digester is an instrument of still more exMr. Daniell, in his Meteorological Essays,' endeavours to controvert this theory. A writer in 'Jamie- it does beyond the tropics; besides, the barometer son's Edinburgh Journal' remarks, that the curvilin-rises about two-thirds of a line twice during each day ear motion of the wind, describing a circle about the in the torrid zone. The range of the barometer inearth, in place of always lowering the barometer, creases gradually as the latitude advances towards ought frequently to augment the pressure of the at- the poles, till it amounts to two or three inches.mosphere, because when the wind is from the east, The following table will explain the gradual increase the diurnal motion round the earth's axis is lessened, alluded to: it is compiled from the best authorities. and its centrifugal force weakened; and so the air will be at more liberty to gravitate or press freely on the earth's surface, and consequently to raise the barometer. Westerly winds, on the contrary, by crossing with the diurnal motion, increase the centrifugal force and diminish the pressure. Hence the reason why the barometer is commonly lower with westerly winds than easterly.-ED.

The variations of the barometer between the tropics are very trifling, and it is worthy of observation, it does not descend more than half as much in that part of the globe, for every 200 feet of elevation, as

Latitude.
0° 0'

22° 23'

Places.

Peru,

BANGE OF THE BAROMETER,
Greatest. Annual

0.20

Calcutta,

0.77

33° 55'

Cape Town,

0.89

40° 55'

Naples,

1.00

51° 8'

Dover,

2.47

1.80

53° 13'

Middlewick,

traordinary effects than any of the former; and sufficiently discovers the amazing force of air, when its elasticity is augmented by fire. A common tea-kettle, if the spout were closed up, and the lid put firmly down, would serve to become a digester, if strong enough. But the instrument used for this purpose, is a strong metal

science to which they more properly appertain.6

CHAP. XIX.

pot, with a lid to screw close on, so that, when AN ESSAY TOWARD A NATURAL HISTORY OF THE AIR. down, no air can get in or return: into this pot meat and bones are put, with a small quantity A LATE eminent philosopher has considered our of water, and then the lid screwed close: a light- atmosphere as one large chemical vessel, in which ed lamp is put underneath, and, what is very an infinite number of various operations are conextraordinary (yet equally true), in six or eight stantly performing. In it all the bodies of the minutes the whole mass, bones and all, are dis-earth are continually sending up a part of their solved into a jelly; so great is the force and substance by evaporation, to mix in this great elasticity of the air contained within, struggling alembic, and to float a while in common. Here to escape, and breaking in pieces all the sub-minerals, from their lowest depths, ascend in stances with which it is mixed. Care, however, noxious, or in warm vapours, to make a part of must be taken not to heat this instrument too the general mass; seas, rivers, and subterrane violently; for then, the enclosed air would be- ous springs, furnish their copious supplies; plants come irresistible, and burst the whole, with per- receive and return their share; and animals that, haps a fatal explosion. by living upon, consume this general store, are found to give it back in greater quantities when they die. The air, therefore, that we breathe, and upon which we subsist, bears very little resemblance to that pure elementary body which was described in the last chapter; and which is rather a substance that may be conceived, than experienced to exist. Air, such as we find it, is one of the most compounded bodies in all nature. Water may be reduced to a fluid every way re

water again. Every thing we see gives off its parts to the air, and has a little floating atmosphere of its own round it. The rose is encompassed with a sphere of its own odorous particles; while the night-shade infects the air with scents of a more ungrateful nature. The perfume of musk flies off in such abundance, that the quantity remaining becomes sensibly lighter by the loss. A thousand substances that escape all our senses, we know to be there; the powerful emanations of the loadstone, the effluvia of electricity, the rays of light, and the insinuations of fire. Such are the various substances through which we move, and which we are constantly taking in at every pore, and returning again with imperceptible discharge !

There are numberless other useful instruments made to depend on the weight, the elasticity, or the fluidity of the air, which do not come within the plan of the present work; the design of which is not to give an account of the inventions that have been made for determining the nature and properties of air, but a mere narrative of its effects. The description of the pump, the forcing-pump, the fire-engine, the steam-engine, the syphon, and many others, belong not to the na-sembling air, by heat; which, by cold, becomes turalist, but the experimental philosopher: the one gives a history of Nature, as he finds she presents herself to him; and he draws the obvious picture: the other pursues her with close investigation, tortures her by experiment to give up her secrets, and measures her latent qualities with laborious precision. Much more, therefore, might be said of the mechanical effects of air, and of the conjectures that have been made respecting the form of its parts; how some have supposed them to resemble little hoops, coiled up in a spring; others, like fleeces of wool; others, that the parts are endued with a repulsive quality, by which, when squeezed together, they endeavour to fly off, and recede from each other. We might have given the disputes relative to the height to which this body of air extends above us, and concerning which there is no agreement. We might have inquired how much of the air we breathe is elementary, and not reducible to any other substance; and of what density it would become, if it were supposed to be continued down to the centre of the earth. At that place we might, with the help of figures, and a bold imagination, have shown it twenty thousand times heavier than its bulk of gold. We might also prove it millions of times purer than upon earth, when raised to the surface of the atmosphere. But these speculations do not belong to natural history; and they have hitherto produced no great advantages in that branch of

This great solution, or mixture of all earthly bodies, is continually operating upon itself; which, perhaps, may be the cause of its unceasing motion; but it operates still more visibly upon such grosser substances as are exposed to its influence; for scarce any substance is found capable of resisting the corroding qualities of the air. The air, say the chemists, is a chaos, furnished with all kinds of salts and menstruums; and, therefore, it is capable of dissolving all kinds of bodies. It is well known, that copper and iron are quickly covered, and eaten with 6 See Supplementary Note A to next chapter. 1 Boyle, vol. ii. p. 593.

2 See Supplementary Note A, p. 156.

rust; and that in the climates near the equator, no art can.keep them clean. In those dreary countries, the instruments, knives and keys, that are kept in the pocket, are nevertheless quickly incrusted; and the great guns, with every precaution, after some years, become useless. Stones, as being less hard, may be readily supposed to be more easily soluble. The marble of which the noble monuments of Italian antiquity are composed, although in one of the finest climates in the world, show the impressions which have been made upon them by the air. In many places they seem worm-eaten by time; and, in others, they appear crumbling into dust. Gold alone seems to be exempted from this general state of dissolution; it is never found to contract rust, though exposed never so long: the reason of this seems to be, that sea-salt, which is the only menstruum capable of acting upon, and dissolving gold, is but very little mixed with the air; for salt being a very fixed body, and not apt to volatilize, and rise with heat, there is but a small proportion of it in the atmosphere. In the elaboratories and shops, however, where salt is much used, and the air is impregnated with it, gold is found to rust as well as other metals.

its favourite climate, not less than its proper soil The lower ranks of animals also seem formed for their respective climates, in which only they can live. Man alone seems the child of every climate, and capable of existing in all. However, this peculiar privilege does not exempt him from the influences of the air; he is as much subject to its malignity as the meanest insect or vegetable. With regard to plants, air is so absolutely necessary for their life and preservation, that they will not vegetate in an exhausted receiver. All plants have within them a quantity of air, which supports and agitates their juices. They are continually imbibing fresh nutriment from the air, to increase this store, and to supply the wants which they sustain from evaporation. When, therefore, the external air is drawn from them, they are no longer able to subsist. Even that quantity of air which they before were possessed of, escapes through their pores, into the exhausted receiver; and as this continues to be pumped away, they become languid, grow flaccid, and die. However, the plant or flower thus ceasing to vegetate, is kept, by being secured from the external air, a much longer time sweet than it would have continued had it been openly exposed.

That air which is so necessary to the life of vegetables, is still more so to that of animals; there are none found, how seemingly torpid soever, that do not require their needful supply. Fishes themselves will not live in water from whence the air is exhausted; and it is generally supposed that they die in frozen ponds, from the want of this necessary to animal existence. Many have been the animals that idle curiosity has tortured in the prison of a receiver, merely to observe the manner of their dying. We shall,. from a thousand instances, produce that of the

Bodies of a softer nature are obviously destroyed by the air.3 Mr. Boyle says, that silks brought to Jamaica, will, if there exposed to the air, rot even while they preserve their colour; but if kept therefrom, they both retain their strength and gloss. The same happens in Brazil, where their clothes, which are black, soon turn of an iron colour; though in the shops, they preserve their proper hue. In these tropical climates also, such are the putrescent qualities of the air, that white sugar will sometimes be full of maggots. Drugs and plasters lose their virtue, and become verminous. In some places they are obliged to expose their sweetmeats by day in the sun, other-viper, as it is known to be one of the most vivawise the night-air would quickly cause them to putrefy. On the contrary, in the cold arctic regions, animal substances, during the winter are never known to putrefy; and meat may be kept for months without any salt whatsoever." At first, upon the air's being drawn away, it This experiment happily succeeded with the eight Englishmen that were accidentally left upon the inhospitable coasts of Greenland, at a place where seven Dutchmen had perished but a few years before; for killing some rein-deer for their subsistence, and having no salt to preserve the flesh, to their great surprise they soon found it did not want any, as it remained sweet during their eight months' continuance upon that shore.

These powers with which air is endued over unorganized substances, are exerted in a still stronger manner over plants, animals of an inferior nature, and lastly, over man himself. Most of the beauty and the luxuriance of vegetation, is well known to be derived from the benign influence of the air; and every plant seems to have

3 Buffon, vol. iii. p. 62. 4 Ibid. vol. iii. p. 68.

cious reptiles in the world; and as we shall feel but little compassion for its tortures. Mr. Boyle took a new-caught viper, and shutting it up into a small receiver, began to pump away the air.5

began to swell; some time after he had done pumping, it began to gape, and open its jaws; being thus compelled to open its jaws, it once more resumed its former lankness; it then began to move up and down within, as if to seek for air, and after a while foamed a little, leaving the foam sticking to the inside of the glass; soon after, the body and neck grew prodigiously tumid, and a blister appeared upon its back; an hour and a-half after the receiver was exhausted, the distended viper moved, and gave manifest signs of life; the jaws remained quite distended; as it were from beneath the epiglottis, came the black tongue, and reached beyond it; but the animal seemed, by its posture, not to have any life; the mouth also was grown blackish within; and in

5 Boyle's Physico-Mechan. Exper. passim.

this situation it continued for twenty-three hours. | the apparent strength of his constitution. Most But upon the air being re-admitted, the viper's mouth was presently closed, and soon after opened again; and for some time those motions continued, which argued the remains of life." Such is the fate of the most insignificant or minute reptile that can be thus included. Mites, fleas, and even the little eels that are found swimming in vinegar, die for want of air. Not only these, but the eggs of these animals will not produce in vacuo, but require air to bring them to perfection.

when the vomit attacks them, are seized with a delirium, that, were they not tied down, they would tear themselves to pieces, and thus expire in the midst of this furious paroxysm. This dis

unwholesome by their supply; and animal putre- | which none are ever found to recover. Some, faction seems to furnish a quantity of vapour, at least as noxious as any of the former. All these united, generally make up the mass of respiration, and are, when mixed together, harmless; but any one of them, for a long time singly pre-order, in milder climates, takes the name of the dominant, becomes at length fatal.

The effects of heat in producing a noxious quality in the air are well known. Those torrid regions under the Line are always unwholesome. At Senegal, I am told, the natives consider forty as a very advanced time of life, and generally die of old age at fifty. At Carthagena, in America, where the heat of the hottest day ever known in Europe is continual, where, during their winter season, these dreadful heats are united with a continual succession of thunder, rain, and tempests, arising from their intenseness, the wan and livid complexions of the inhabitants might make strangers suspect that they were just recovered from some dreadful distemper: the actions of the natives are conformable to their colour; in all their motions there is somewhat relaxed and languid: the heat of the climate even affects their speech, which is soft and slow, and their words generally broken. Travellers from Europe retain their strength and ruddy colour in that climate, possibly for three or four months; but afterwards suffer such decays in both, that they are no longer to be distinguished from the inhabitants by their complexion. However, this languid and spiritless existence is frequently drawled on sometimes even to eighty. Young persons are generally most affected by the heat of the climate, which spares the more aged; but all, upon their arrival on the coasts, are subject to the same train of fatal disorders. Few nations have experienced the mortality of these coasts so much as our own: in our unsuccessful attack upon Carthagena, more than three parts of our army were destroyed by the climate alone; and those that returned from that fatal expedition, found their former vigour irretrievably gone. In our more fortunate expedition, which gave us the Havannah, we had little reason to boast of our success; instead of a third, not a fifth part of the army were left survivors of their victory, the climate being an enemy that even heroes cannot conquer.

The distempers that thus proceed from the cruel malignity of those climates, are many: that, for instance, called the Chapotonadas, carries off a multitude of people; and extremely thins the crews of European ships, whom gain tempts into those inhospitable regions. The nature of this distemper is but little known, being caused in some persons by cold, in others by indigestion. But its effects are far from being obscure; it is generally fatal in three or four days; upon its seizing the patient, it brings on what is there called the black vomit, which is the sad symptom after

s Ulloa, vol. i. p. 42.

bilious fever, and is attended with milder symptoms, but very dangerous in all.

There are many other disorders incident to the human body, that seem the offspring of heat; but to mention no other, that very lassitude which prevails in all the tropical climates, may be considered as a disease. The inhabitants of India," says a modern philosopher, sustain an unceasing languor, from the heats of their climate, and are torpid in the midst of profusion. For this reason, the great Disposer of nature has clothed their country with trees of an amazing height, whose shade might defend them from the beams of the sun; and whose continual freshness might, in some measure, temperate their fierceness. From these shades, therefore, the air receives refreshing moisture, and animals a cooling protection. The whole race of savage animals retire in the midst of the day, to the very centre of the forests, not so much to avoid their enemy man, as to find a defence against the raging heats of the season. This advantage which arises from shade in torrid climates, may probably afford a solution for that extraordinary circumstance related by Boyle, which he imputes to a different cause. In the island of Ternate, belonging to the Dutch, a place that had been long celebrated for its beauty and healthfulness, the clove-trees grew in such plenty, that they in some measure lessened their own value: for this reason, the Dutch resolved to cut down the forests, and thus to raise the price of the commodity: but they had soon reason to repent of their avarice; for such a change ensued, by cutting down the trees, that the whole island from being healthy and delightful, having lost its charming shades, became extremely sickly, and has actually continued so to this day. Boerhaave considered heat so prejudicial to health, that he was never seen to go near a fire.

An opposite set of calamities are the consequence, in climates where the air is condensed by cold. In such places, all that train of distempers which are known to arise from obstructed perspiration, are very common; 10 eruptions, boils, scurvy, and a loathsome leprosy, that covers the whole body with a scurf, and white putrid ulcers. These disorders also are infectious; and, while they thus banish the patient from society, they generally accompany him to the grave. The men of those climates seldom attain to the age of fifty; but the women, who do not lead such laborious lives, are found to live longer.

The autumnal complaints which attend a wet summer, indicate the dangers of a moist air. The long continuance of an east wind also, shows the 9 Linnæi Amænitates, vol. v. p. 444.

10 Crantz's History of Greenland, vol. i. p. 235