MOUNTAIN, a considerable eminence of land, elevated above the surrounding country. It is commonly full of inequalities, cavities more or less exposed, and strata uncovered.

The Attraction of Mountains is a comparatively recent discovery, and affords 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 towards 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 upon the earth’s surface; so that the 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 proportionally 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 j” 1 yet no attempt to ascertain this matter by actual experiment was made till about the year 1738, when the French academicians, particularly MM. Bouguer and Condamine, who were sent to Peru to measure a degree under the equator, attempted to discover the attractive power of Chimboraço, a mountain in the province of Quito. According to their observations, which however were made under circumstances by no means favourable to an accurate solution of so nice and difficult a problem, the mountain of Chimboraço exerted an attraction equal to about 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 have since been taken to repeat it.

Through the munificence of his Britannic majesty, the Royal Society were enabled to undertake the execution of this delicate and importantexperiment; and the astronomer royal was chosen to conduct it. After various inquiries, the mountain of 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 easterly and westerly 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 contrariwise at the two stations, thereby doubling the effect. On the south 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 contrariwise; 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; so that the half would of course indicate the attractive power of the hill.

The various operations requisite for performing this experiment lasted about four months; and from them it appears that the sum of the two contrary attractions of the mountain of Schehallien, in the two temporary observations which were successively made half way up the hill (where the effect of its attraction would be greatest), was equal to H"∙6. From a rough computation, founded on the known law of gravitation, 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 this 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 of Schehallien has ever been a volcano, or is hollow, as it is extremely solid and dense, and seemingly composed of an entire rock. The inferences drawn from these experiments may be reduced to the following.

1. It appears that the moun tain of Schehallien exerts a sensible attraction; therefore, by the rules of philosophizing, we are warranted 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, was 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 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 1 By a very easy calculation, it is found that such a mountain would attract the plumb-line 1' 18" from the perpendicular.
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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 bad 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, Dr Maskelyne 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 vacuity in the midst of it.” But were that the case, the attraction of mountains, and even smaller inequalities in the earth’s surface, would, contrary to experiment, be very great, 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 attraction of the elevated parts may be small; or where, by their situation, they may compensate or counteract the effectsof each other.

Mountain of Forty Days, a mountain of Judaea, situated in the plain of Jericho, to the north of that city. According to the Abbé Mariti’s description, the summit of it is covered neither with shrubs, nor turf, nor earth; it consists of a solid mass of white marble, the surface of which has become yellow by the injuries of the air. « The path by which you ascend to it,” says this writer, “fills one with terror, as it rises with a winding course between two abysses, which the eye dares scarcely behold. This path is at first pretty broad, but it at length becomes so confined, that one can with difficulty place both feet upon it at the same time. When we had ascended a little higher, we found an Arab stretched out on the path, who made us pay a certain toll for our passage. Here the traveller requires courage. One of the parapets of the path being broken, we clung to the part which remained until we had reached a small grotto, situated very commodiously, as it gave us an opportunity of recovering our breath. When we had rested ourselves a little, we pursued our course, which became still more dangerous. Suspended almost from the rock, and having before our eyes all the horror of the precipice, we could advance only by dragging one foot after the other; so that had the smallest fragments given way under us, we should have been hurried to the bottom of this frightful abyss.

“This mountain is one of the highest in the province, and one of its most sacred places. It takes its name from the rigorous fast which Christ observed here after having triumphed over the vanities of the world and the power of hell. In remembrance of this miracle, a chapel was formerly constructed on the summit of the mountain. It may be seen from the plain, but we could not approach it, as the path was almost entirely destroyed. It, however, may be accessible on the other side of the mountain, which we did not visit. A great many scattered grottos are seen here; in one of which, according to Q,uaresmius, were deposited the bodies of several anchorets, which are still entire. I have heard the same thing asserted in the country, but I could never meet with any person who had seen them. Here we enjoyed the most beautiful prospect imaginable. This part of the Mountain of Forty Days overlooks the mountains of Arabia, the country of Gilead, the country of the Ammonites, the plains of Moab, the plain of Jericho, the river Jordan, and the whole extent of the Dead Sea. It was here that the devil said to the Son of God, ‘ All these kingdoms will I give thee, if thou wilt fall down and worship me.’”

Mountains of the Lions, also in Africa, divide Nigritia from Guinea, and extend as far as Ethiopia. They were styled by the ancients the Mountains of God, on account of their being greatly subject to thunder and lightning.

Mountains of the Moon, a chain of mountains in Africa, extending between Abyssinia and Monomotapa, and so called from their great height.

Marble Mountains. Of these there are great numbers in Egypt, from which, although immense quantities have been carried off for the multitudinous works erected by the ancient Egyptians, yet, in the opinion of Bruce, there is still a sufficient supply to build Rome, Athens, Corinth, Syracuse, Memphis, Alexandria, and half a dozen more of such cities.