ppear to many to be defects and blemishes in the earth; but they are truly of the utmost use and necessity to the well-being both of man and other animals. They serve as screens to keep off the cold and nipping blasts of the northern and eastern winds; they serve for the production of a great number of vegetables and minerals, which are not found in any other soil; the long ridges and chains of lofty and topping mountains, being generally found to run from east to west, serve to stop the evagination of the vapours towards the poles, without which they would all run from the hot countries, and leave them destitute of rain. Mr Ray adds, that they condense these vapours, like alembic heads, into clouds; and so, by a kind of external distillation, give origin to springs and rivers; and by amassing, cooling, and contipitating them, turn them into rain, and by that means render the fervid region of the torrid zone habitable. He farther adds, that many creatures cannot live but in particular situations; and even the tops of the highest and the coldest mountains are the only places where some creatures, as well birds as quadrupeds, will live.

M. Buffon remarks, that the highest mountains of the world, as well as the largest, are situated in the torrid zone; and the nearer we approach the equator, the greater are the inequalities on the earth's surface.

"A short enumeration of mountains and islands (says he) will be sufficient to establish this point.—In America, the Cordilleras, which are the highest mountains in the world, lie precisely under the equator; and they extend on both sides a considerable way beyond the tropic circles. The highest mountains of the Moon, of Monomotapa, and the great and little Atlas in Africa, lie either under or very near the equator. In Asia, mount Caucasus, the chain of which, under different names, runs into China, and through this whole extent, lies nearer the equator than the poles. In Europe, the Pyrenees, the Alps, and the mountains of Greece, which form one chain, are still less distant from the equator than the pole.

"These chains of mountains of which we have given an enumeration, are higher and of greater extent, both in length and breadth, than those of more northern countries. With regard to their direction, the Alps form a continued chain which runs across the whole continent from Spain to China. They commence on the sea-coast of Galicia, join the Pyrenees, traverse France by Vivares and Auvergne, run through Italy, and stretch into Germany above Dalmatia, until they reach Macedonia; from thence they join the mountains of Armenia, the Caucasus, the Taurus, the Imaus, and at last terminate on the coast of Tartary. Mount Atlas in the same manner traverses the whole continent of Africa, from the kingdom of Fez to the Straits of the Red Sea. The mountains of the moon have likewise the same direction; but the mountains of America have an opposite direction. The vast chains of Cordilleras, and other mountains, run more from south to north than from east to west."

This assertion concerning the magnitude and height of mountains, is no doubt necessary for the support of his theory of the earth*; but it is by no means agreeable to fact. The mountains of the moon, though much nearer the equator than the Alps, are not, by all accounts, equal to them in size; one of the peaks of the Alps, named Mont Blanc, being reckoned the highest point of land in Europe, Asia, or Africa. According to some late computations, this peak is more than 800 feet higher than mount Etna would be with Vesuvius set on its top; so that we may reckon it little inferior to the highest mountains even in America. In Mr Forster's account of the Southern Thule also, he tells us of an exceeding high mountain seen on that island, and which was thought to be little less than two miles perpendicular; and yet this island lies in a very considerable south latitude: so that the height of mountains seems by no means to be in proportion to the vicinity of the equator or torrid zone.

The most remarkable mountain in the world for shape, is that called the needle mountain, or the inaccessible mountain, in Dauphiny.—This is a vast hill, placed as it were bottom upwards, or set on its summit on the earth, with its broad base elevated in the air; it is but about 1000 paces in circumference at bottom, and above 2000 at the top. On the centre of the plain at the top there stands another small and very narrow, but very high hill. It obtained its name from from the supposed impossibility of ascending it, on account of its projection outwards. Some hardy persons, however, once ventured to climb it; and found at the top a number of the chamois, animals by no means qualified for climbing, and which doubtless had never either ascended or descended the mountain, and which must be supposed to have bred there for many ages; though it is very difficult to account for their coming there.

The difficulty and danger of ascending to the tops of mountains, proceeds not from the thinness of the air as has been commonly reported; but the reason is, that they rise with such a rugged and precipitate ascent, that they are utterly inaccessible. In some places they appear like a great wall of 600 or 700 feet high; in others, there stick out enormous rocks, that hang upon the brow of the steep, and every moment threaten destruction to the traveller below.

In this manner almost all the tops of the highest mountains are bare and pointed. And this naturally proceeds from their being so continually assaulted by thunders and tempests. All the earthy substances with which they might have been once covered, have for ages been washed away from their summits; and nothing is left remaining but immense rocks, which no temple has hitherto been able to destroy.

Nevertheless, time is every day and every hour making depredations; and huge fragments are seen tumbling down the precipice, either loosened from the summit by the frost or rains, or struck down by lightning. Nothing can exhibit a more terrible picture than one of these enormous rocks, commonly larger than an house, falling from its height with a noise louder than thunder, and rolling down the side of the mountain. Dr Plot tells us of one in particular, which being loosened from its bed, tumbled down the precipice, and was partly shattered into a thousand pieces. Notwithstanding, one of the largest fragments of the same, still preserving its motion, travelled over the plain below, crossed a rivulet in the midst, and at last stopped on the other side of the bank! These fragments, as was said, are often struck off by lightning, and sometimes undermined by rains; but the most usual manner in which they are disunited from the mountain, is by frost: the rains infusing between the interstices of the mountain, continue there until there comes a frost; and then, when converted into ice, the water swells with an irresistible force, and produces the same effect as gun-powder, splitting the most solid rocks, and thus shattering the summits of the mountain.

But not rocks alone, but whole mountains, are, by various causes, disunited from each other. We see, in many parts of the Alps, amazing clefts, the sides of which so exactly correspond with the opposite, that no doubt can be entertained of their having been once joined together. At Cajeta, in Italy, a mountain was split in this manner by an earthquake; and there is a passage opened through it, that appears as if elaborately done by the industry of man. In the Andes these breaches are frequently seen. That at Thermopylae, in Greece, has been long famous. The mountain of the Troglodytes, in Arabia, has thus a passage through it; and that in Savoy, which nature began, and which Victor Amadeus completed, is an instance of the same kind.

We have accounts of some of these disruptions, immediately after their happening. "In the month of June, in the year 1714, a part of the mountain of Diableret, in the district of Valais, in France, suddenly fell down, between two and three o'clock in the afternoon, the weather being very calm and serene. It was of a conical figure, and destroyed 55 cottages in the fall. Fifteen persons, together with about 100 beasts, were also crushed beneath its ruins, which covered an extent of a good league square. The dust it occasioned instantly covered all the neighbourhood in darkness. The heaps of rubbish were more than 300 feet high. They stopped the current of a river that ran along the plain, which now is formed into several new and deep lakes. There appeared, through the whole of this rubbish, none of those substances that seemed to indicate that this disruption had been made by means of subterraneous fires. Most probably, the base of this rocky mountain was rotted and decayed; and thus fell, without any extraneous violence." In the same manner, in the year 1618, the town of Pleurs, in France, was buried beneath a rocky mountain, at the foot of which it was situated.

These accidents, and many more that might be enumerated of the same kind, have been produced by various causes: by earthquakes, as in the mountain at Cajeta; or by being decayed at the bottom, as at Diableret. But the most general way is, by the foundation of one part of the mountain being hollowed by waters, and thus wanting a support, breaking from the other. Thus it generally has been found in the great chasms in the Alps; and thus it almost always is known in those disruptions of hills which are known by the name of land-slips. These are nothing more than the sliding down of an higher piece of ground, disrooted from its situation by subterraneous inundations, and settling itself upon the plain below.

There is not an appearance in all nature that so much astonished our ancestors as these land-slips. In fact, to behold a large upland, with its houses, its corn, and cattle, at once loosened from its place, and floating as it were upon the subjacent water; to behold it quitting its ancient situation, and travelling forward like a ship, in quest of new adventures; this is certainly one of the most extraordinary appearances that can be imagined; and, to a people ignorant of the powers of nature, might well be considered as a prodigy. Accordingly, we find all our old historians mentioning it as an omen of approaching calamities. In this more enlightened age, however, its cause is very well known; and, instead of exciting ominous apprehensions in the populace, it only gives rise to some very ridiculous lawsuits among them, about whose the property shall be; whether the land which has thus slipped, shall belong to the original possessor, or to him upon whose grounds it has encroached and settled. What has been the determination of the judges is not so well known; but the circumstances of the slips themselves have been minutely enough and exactly described.

In the lands of Slatberg, in the kingdom of Ireland, there flood a declivity gradually ascending for near half a mile. In the year 1713, and on the 10th of March, the inhabitants perceived a crack on its side, somewhat like a furrow made with a plough, which they imputed to the effects of lightning, as there had been thunder the night before. However, on the evening of the same day, they were surprized to hear an hideous confused noise issuing all round from the side of the hill; and their curiosity being raised, they resorted to the place. There, to their amazement, they found the earth for near five acres all in gentle motion, and sliding down the hill upon the subjacent plain. This motion continued the remaining part of the day, and the whole night: nor did the noise cease during the whole time; proceeding, probably, from the attrition of the ground beneath. The day following, however, this strange journey down the hill ceased entirely; and above an acre of the meadow below was found covered with what before composed a part of the declivity.

However, these slips, when a whole mountain's side seems to descend, happen but very rarely. There are some of another kind, however, much more common; and, as they are always sudden, much more dangerous. These are snow-slips, well known, and greatly dreaded by travellers. It often happens, that when snow has long been accumulated on the tops and on the sides of mountains, it is borne down the precipice either by means of tempests or its own melting. At first, when loosened, the volume in motion is but small; but it gathers as it continues to roll, and, by the time it has reached the habitable parts of the mountain, it is generally grown of enormous bulk. Wherever it rolls, it levels all things in its way; or buries them in unavoidable destruction. Instead of rolling, it sometimes is found to slide along from the top; yet even thus it is generally as fatal as before. Nevertheless, we have had an instance, a few years ago, of a small family in Germany that lived for above a fortnight beneath one of these snow-slips. Although they were buried during that whole time in utter darkness, and under a bed of some hundred feet deep, yet they were luckily taken out alive, the weight of the snow being supported by a beam that kept up the roof; and nourishment being supplied them by the milk of a sheep that was buried under the same ruin.

Attraction of Mountains. This is a late discovery, and a very considerable confirmation of Sir Isaac Newton's theory of universal gravity. According to the Newtonian system, an attractive power is not only exerted between those large masses of matter which constitute the sun and planets; but likewise between all comparatively smaller bodies, and even between the smallest particles of which they are composed. Agreeably to this hypothesis, a heavy body, which ought to gravitate or tend toward the centre of the earth, in a direction perpendicular to its surface, supposing the said surface to be perfectly even and spherical, ought likewise, though in a less degree, to be attracted and tend towards a mountain placed on the earth's surface: so that a plumb-line, for instance, of a quadrant, hanging in the neighbourhood of such a mountain, ought to be drawn from a perpendicular situation, in consequence of the attractive power of the quantity of matter of which it is composed, acting in a direction different from that exerted by the whole mass of matter in the earth, and with a proportionably inferior degree of force.

Though Sir Isaac Newton had long ago hinted at an experiment of this kind; and had remarked, that "a mountain of an hemispherical figure, three miles high and six broad, would not, by its attraction, draw the plumb-line two minutes out of the perpendicular (A):" yet no attempt to ascertain this matter, by actual experiment, was made till about the year 1738; when the French academicians, particularly Meflins Bouguer and Condamine, who were sent to Peru to measure a degree under the equator, attempted to discover the attractive power of Chimborazo, a mountain in the province of Quito. According to their observations, which were however made under circumstances by no means favourable to an accurate solution of so nice and difficult a problem, the mountain Chimborazo exerted an attraction equal to eight seconds. Though this experiment was not perhaps sufficient to prove satisfactorily even the reality of an attraction, much less the precise quantity of it; yet it does not appear that any steps had been since taken to repeat it.

Through the munificence of his Britannic majesty, the royal society were enabled to undertake the execution of this delicate and important experiment; the astronomer royal was chosen to conduct it. After various inquiries, the mountain Schehallien, situated nearly in the centre of Scotland, was pitched upon as the most proper for the purpose that could be found in this island. The observations were made by taking the meridian zenith distances of different fixed stars, near the zenith, by means of a zenith sector of ten feet radius; first on the south, and afterwards on the north side of the hill, the greatest length of which extended in an east and west direction.

It is evident, that if the mass of matter in the hill exerted any sensible attraction, it would cause the plumb-line of the sector, through which an observer viewed a star in the meridian, to deviate from its perpendicular situation, and would attract it contrary ways at the two stations, thereby doubling the effect. On the fourth side the plummet would be drawn to the northward, by the attractive power of the hill placed to the northward of it; and on the north side, a contrary and equal deflection of the plumb-line would take place, in consequence of the attraction of the hill, now to the southward of it. The apparent zenith distances of the stars would be affected contrarywise; those being increased at the one station which were diminished at the other: and the correspondent quantities of the deflection of the plumb-line would give the observer the sum of the contrary attractions of the hill, acting on the plummet at the two stations; the half of which will of course indicate the attractive power of the hill.

The various operations requisite for this experiment lasted about four months; and from them it appears, that the sum of the two contrary attractions of the mountain Schehallien, in the two temporary observations which were successively fixed half-way up the hill (where the effect of its attraction would be greatest) was equal to $11''$. From a rough computation, founded on the known law of gravitation, and

By a very easy calculation it is found that such a mountain would attract the plumb-line $1'18''$ from the perpendicular. and on an assumption that the density of the hill is equal to the mean density of the earth; it appears that the attraction of the hill should amount to about the double of this quantity. From thence it was inferred, that the density of the hill is only about half the mean density of the earth. It does not appear, however, that the mountain Schehallien has ever been a volcano, or is hollow; as it is extremely solid and dense, and seemingly composed of an entire rock.

The inference drawn from these experiments may be reduced to the following:

1. It appears, that the mountain Schehallien exerts a sensible attraction; therefore, from the rules of philosophy, we are to conclude, that every mountain, and indeed every particle of the earth, is endowed with the same property, in proportion to its quantity of matter.

2. The law of the variation of this force, in the inverse ratio of the squares of the distances, as laid down by Sir Isaac Newton, is also confirmed by this experiment. For if the force of attraction of the hill had been only to that of the earth as the matter in the hill to that of the earth, and had not been greatly increased by the near approach to its centre, the attraction thereof must have been wholly insensible. But now, by only supposing the mean density of the earth to be double to that of the hill, which seems very probable from other considerations, the attraction of the hill will be reconciled to the general law of the variation of attraction in the inverse duplicate ratio of the distances, as deduced by Sir Isaac Newton from the comparison of the motion of the heavenly bodies with the force of gravity at the surface of the earth; and the analogy of nature will be preserved.

3. We may now, therefore, be allowed to admit this law, and to acknowledge, that the mean density of the earth is at least double of that at the surface; and consequently that the density of the internal parts of the earth is much greater than near the surface. Hence also, the whole quantity of matter in the earth will be at least as great again as if it had been all composed of matter of the same density with that at the surface; or will be about four or five times as great as if it were all composed of water.—This conclusion, Mr Meldelyne adds, is totally contrary to the hypothesis of some naturalists, who suppose the earth to be only a great hollow shell of matter; supporting itself from the property of an arch, with an immense vacancy in the midst of it. But, were that the case, the attraction of mountains, and even smaller inequalities in the earth's surface, would be very great, contrary to experiment, and would affect the measures of the degrees of the meridian much more than we find they do; and the variation of gravity, in different latitudes, in going from the equator to the poles, as found by pendulums, would not be near so regular as it has been found by experiment to be.

4. As mountains are, by these experiments, found capable of producing sensible deflections of the plumb-lines of astronomical instruments; it becomes a matter of great importance, in the mensuration of degrees in the meridian, either to choose places where the irregular attractions of the elevated parts may be small; or where, by their situation, they may compensate or counteract the effects of each other.

For measuring the heights of mountains, see the article BAROMETER, and the same in the APPENDIX.