after the cause, which has hitherto been inscrutable, it will best become us to rest satisfied with admiration.

At the city of Modena, in Italy, and about four miles round it, wherever it is dug, when the workmen arrive at the depth of sixty-three feet they come to a bed of chalk, which they bore with an augre five feet deep; they then withdraw from the pit before the augre is removed, and upon its extraction the water bursts up through the aperture with great violence, and quickly fills this new-made well, which continues full, and is affected neither by rains nor droughts. But that which is most remarkable in this operation is the layers of earth as we descend. At the depth of fourteen feet are found the ruins of an ancient city, paved streets, houses, floors, and different pieces of Mosaic. Under this is found a solid earth, that would induce one to think had never been removed; however, under it is found a soft oozy earth, made up of vegetables; and at twenty-six feet depth, large trees entire, such as walnut-trees, with the walnuts still sticking on the stem, and their leaves and branches in exact preservation. At twenty-eight feet deep a soft chalk is found, mixed with a vast quantity of shells; and this bed is eleven feet thick. Under this, vegetables are found again, with leaves and branches of trees as before; and thus alternately chalk and vegetable earth to the depth of sixty-three feet. These are the layers wherever the workmen attempt to bore; while in many of them they also find pieces of charcoal, bones, and bits of iron. From this description, therefore, it appears that this country has been alternately overflowed and deserted by the sea one age after another; nor were these overflowings and retirings of trifling depth or of short continuance. When the sea burst in, it must have been a long time in overwhelming the branches of the fallen forest with its sediment; and still longer in framing a regular bed of shells eleven feet over them. It must have therefore taken an age at least to make any one of these layers; and we may conclude, that it must have been many ages employed in the production of them all. The land also, upon being deserted, must have had time to grow compact, to gather fresh fertility, and to be drained of its waters before it could be disposed to vegetation, or before its trees could have shot forth again to maturity.

We have instances nearer home of the same kind given us in the Philosophical Transactions; one of them by Mr. Derham. An inundation of the sea at Dagenham, in Essex, laying bare a part of the adjacent pasture for above two hundred feet wide, and in some places twenty deep, it discovered a number of trees that had lain there for many ages before; these trees, by lying long under ground, were become black and hard, and their fibres so tough, that one might as easily break a wire as any of them; they lay so thick in the place where they were found, that in many parts he could step from one to another; he conceived, also, that not only all the adjacent marshes for several hundred acres were covered underneath with such timber, but also the marshes along the mouth of the Thames for several miles. The meeting with these trees at such depths he ascribes to the sediment of the river and the tides, which, constantly washing over them, have always left some part of their substance behind, so as, by repeated alluvions, to work a bed of vegetable earth over them to the height at which he found it.

The levels of Hatfield-Chace, in Yorkshire, a tract of above eighteen thousand acres, which was yearly overflown, was reduced to arable and pasture land by one Sir Cornelius Vermusden, a Dutchman. At the bottom of this wide extent are found millions of the roots and bodies of trees, such as this island either formerly did or does at present produce. The roots of all stand in their proper postures; and by them, as thick as ever they could grow, the respective trunks of each, some

above thirty yards long. The oaks, some of which have been sold for fifteen pounds a-piece, are as black as ebony, very lasting, and close grained. The ash-trees are as soft as earth, and are commonly cut in pieces by the workmen's spades, and as soon as flung up into the open air turn to dust. But all the rest, even the willows themselves, which are softer than the ash, preserve their substance and texture to this very day. Some of the first appear to have vegetated even after they were fallen, and to have from their branches struck up trees as large as the parent trunk. It is observable that many of these trees have been burnt-some quite through, some on one side; some have been found chopped and squared, others riven with great wooden wedges-all sufficiently manifesting that the country which was deluged had formerly been inhabited. Near a great root of one tree were found eight coins of the Roman emperors; and in some places the marks of the ridge and furrow were plainly perceivable, which testified that the ground had formerly been patient of cultivation.

The learned naturalist who has given this description has pretty plainly evinced that this forest in particular must have been thus levelled by the Romans, and that the falling of the trees must have contributed to the accumulation of the waters. "The Romans," says he, "when the Britons fled, always pursued them into the fortresses of low woods and miry forests: in these the wild natives found shelter; and, when opportunity offered, issued out, and fell upon their invaders without mercy. In this manner the Romans were at length so harassed, that orders were issued out for cutting down all the woods and forests in Britain. In order to effect this, and destroy the enemy the easier, they set fire to the woods, composed of pines, which spreading, the conflagration destroyed not only the forest, but infinite numbers of the wretched inhabitants who had taken shelter therein. When the pine-trees had thus done what mischief they could, the Romans then brought their army nearer, and, with whole legions of the cap tive Britons, cut down most of the trees that were yet left standing-leaving only here and there some trees untouched as monuments of their fury These, unneedful of their labour, being destitute of the underwood and of their neighbouring trees, were easily overthrown by the winds, and remained on the places where they happened to fall. The forest, thus fallen, must necessarily have stopped up the currents both from land and sea, and turned into great lakes what were before but temporary streams. The working of the waters here, the consumption and decay of rotten boughs and branches, and the vast increase of water-moss which flourishes upon marshy grounds, soon formed a covering over the trunks of the fallen trees, and raised the earth several feet above its former level. The earth thus every day swelling, by a continual increase from the sediment of the waters, and by the lightness of the vegetable substances of which it was composed, soon overtopt the waters by which this intumescence was at first effected; so that it entirely got rid of its inundations, or only demanded a slight assistance from man for that purpose." This may be the origin of all bogs, which are formed by the putrefaction of vegetable substances, mixed with the mud and slime deposited by waters, and at length acquiring a sufficient consistency.

From this we see what powerful effects the sea is capable of producing upon its shores, either by overflowing some or deserting others by altering the direction of these, and rendering those craggy and precipitate which before were shelving. But the influence it has upon these is nothing to that which it has upon that great body of earth which forms its bottom. It is at the bottom of the sea that the greatest wonders are performed, and the most rapid changes are produced; it is there that the motion of the tides and the currents have

their whole force, and agitate the substances of which their bed is composed. But all these are almost wholly hid from human curiosity; the miracles of the deep are performed in secret; and we have but little information from its abysses, except what we receive by inspection at very shallow depths, or by the plummet, or from divers, who are known to descend from twenty to thirty fathoms.

The eye can reach but a very short way into the depths of the sea; and that only when its surface is glassy and serene. In many seas it perceives nothing but a bright sandy plain at the bottom, extending for several hundred miles, without an intervening object. But in others, particularly in the Red Sea, it is very different: the whole bottom of this extensive bed of waters is, literally speaking, a forest of sub-marine plants, and corals formed by insects for their habitation, sometimes branching out to a great extent. Here are seen the madrepores, the sponges, mosses, sea-mushrooms, and various other marine productions, covering every part of the bottom; so that some have even supposed the sea to have taken its name from the colour of its plants below. However, these plants are by no means peculiar to this sea, as they are found in great quantities in the Persian Gulf, along the coasts of Africa, and those of Provence and Catalonia.

ance.

The bottom of many parts of the sea near America presents a very different though a very beautiful appearThis is covered with vegetables, which make it look as green as a meadow, and beneath are seen thousands of turtles and other sea animals feeding thereon. In order to extend our knowledge of the sea to greater depths, recourse has been had to the plummet-which is generally made of a lump of lead of about forty pounds weight fastened to a cord. This, however, only answers in moderate depths; for when a deep sea is to be sounded, the matter of which the cord is composed, being lighter than the water, floats upon it, and, when let down to a considerable depth, its length so increases its surface that it is often sufficient to prevent the lead from sinking; so that this may be the reason that some parts of the sea are said to have no bottom.

In general, we learn from the plummet that the bottom of the sea is tolerably even where it has been examined; and that the farther from the shore the sea is in general the deeper. Notwithstanding, in the midst of a great and unfathomable ocean we often find an island raising its head, and singly braving its fury. Such islands may be considered as the mountains of the deep; and, could we for a moment imagine the waters of the ocean removed or dried away, we should probably find the inequalities of its bed resembling those that are found on land; here extensive plains there valleys; and in many places mountains of amazing height. M. Buache has actually given us a map of that part of its bottom which lies between Africa and America, taken from the several soundings of mariners; in it we find that same uneven surface that we do upon land, the same eminences, and the same depressions. In such an imaginary prospect, how ever, there would be this difference-that as the tops of land-mountains appear the most barren and rocky, the tops of sea-mountains would be found the most verdant and fruitful.

The plummet, which thus gives us some idea of the inequalities of the bottom, leaves us totally in the dark as to every other particular; recourse, therefore, has been had to divers; these, either being bred up in this dangerous way of life, and accustomed to remain some time under water without breathing, or assisted by means of a diving-bell, have been able to return some confused and uncertain accounts of the places below. In the great diving-bell improved by Dr. Halley, which was large enough to contain five men, and was supplied with fresh air by buckets that alternately rose and fell, they de;

scended fifty fathoms. In this huge machine, which was let down from the mast of the ship, the doctor himself went down to the bottom, where, when the sea was clear, and especially when the sun shone, he could see perfectly well to write or read, and much more to take up anything that was underneath: at other times, when the water was troubled and thick, it was as dark as night below, so that he was obliged to keep a candle lighted at the bottom. But there was one thing very remarkable that the water, which from above was usually seen of a green colour, when looked at from below appeared to him of a very different one, casting a redness upon one hand like that of damask roses-a proof of the sea's taking its colour not from anything floating in it, but from the different reflections of the rays of light. Upon the whole, the accounts we have received from the bottom by this contrivance are but few. We learn from it, and from divers in general, that while the surface of the sea may be deformed by tempests, it is usually calm and temperate below; that some divers who have gone down when the weather was calm, and came up when it was tempestuous, were surprised at their not perceiv ing the change at the bottom. This, however, must not be supposed to obtain with regard to the tides and the currents, as they are seen constantly shifting their bottom-taking their bed with great violence from one place and depositing it upon another. We are informed, also, by divers, that the sea grows colder in proportion as they descend to the bottom; that as far as the sun's rays pierce it is influenced by their warmth; but lower, the cold becomes almost intolerable. A person of quality, who had been himself a diver, as Mr. Boyle informs us, declared, that though he seldom descended above three or four fathoms, yet he found it so much colder than near the top that he could not well endure it; and that being let down in a great diving-bell, although the water could not immediately touch him, he found the air extremely cold upon his first arrival at the bottom.

From divers, also, we learn that the sea in many places is filled with rocks at bottom; and that among their clifts and upon their sides various substances sprout forward, which are either really vegetables or the nests of insects, increased to some magnitude. Some of these assume the shape of beautiful flowers; and, though soft when taken up, soon harden, and are kept in the cabinets of the curious.

But of all those divers who have brought us inform ation from the bottom of the deep, the famous Nicola Pesce, whose performances are told us by Kircher, is the most celebrated. I will not so much as pretend to vouch for the veracity of Kircher's account, which he assures us he had from the archives of the kings of Sicily; but it may serve to enliven a heavy chapter. "In the times of Frederic, king of Sicily, there lived a celebrated diver, whose name was Nicolas, and who, from his amazing skill in swimming, and his persever ance under water, was surnamed the "Fish." This man had from his infancy been used to the sea, and earned his scanty subsistence by diving for corals and oysters, which he sold to the villages on shore. His long acquaintance with the sea at last brought it be almost his natural element. He frequently was known to spend five days in the midst of the waves, without any other provisions than the fish which he caught there, and ate raw. He often swam over from Sicily to Calabria, a tempestuous and dangerous passage, carrying letters from the king, He was frequently known to swim among the gulfs of the Lipari Islands, no way apprehensive of danger.

"Some mariners out at sea one day observed something at some distance from them, which they regarded as a sea-monster; but upon its approach it was known to be Nicolas Pesce, whom they took into their ship, When they asked him whither he was going in so stormy

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. The digester is an instrument of still more extraordinary 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 pot, with a lid to screw close on, so that, when down, no air can get in or return; into this pot meat and bones are put, with a small quantity of water, and then the lid screwed close; a lighted lamp is put underneath, and, what is very extraordinary yet equally true, in six or eight minutes the whole mass, bones and all, are dissolved into a jelly; so great is the force and elasticity of the air contained within, struggling to escape, and breaking in pieces all the substances with which it is mixed. Care, however, must be taken not to heat this instrument too violently; for then the inclosed air would become irresistible, and burst the whole with perhaps a fatal explosion.

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 forcingpump, the fire-engine, the steam-engine, the syphon, and many others, belong not to the naturalist, but to 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 in 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 science to which they more properly appertain.

CHAP. XIX.

AN ESSAY TOWARDS A NATURAL HISTORY OF THE AIR.

A late eminent philosopher has considered ou atmosphere as one large chemical vessel, in which an infinite number of various operations are constantly performing. In it all the bodies of the earth are continually sending up a part of their substance by evaporation, to mix in this great alembic and to float awhile in common. Here minerals, from their lowest depths, ascend in noxious or in warm vapours to make a part of the general mass; seas, rivers, and subterraneous springs furnish their copious supplies; plants receive and return their share; and animals-that, 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 he 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 everyway resembling air by heat, which by cold becomes water again. Everything we see gives off its part to the air, and has a little floating atmosphere of its own round it. The rose is encompassed with a sphere of its odorous particles; while the nightshade infects the air with scents of a more ungrate ful 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 emana. tions 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!

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 chymists, 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 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 encrusted; 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 seem crumbling into dust. Gold alone seems to be exempted from this state of dissolution-it is never found to contract rust, though exposed never so long. The reason of this seems to be, that seasalt-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 volatalize and rise with heat, there is but a small proportion of it in the atmosphere. In the laboratories 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.

Bodies of a softer nature are obviously destroyed by the air. 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 retain both their strength and gloss. The same happens in