MOUNTAIN

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;" 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 Chimborazo, 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 Chimborazo 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 important experiment; and the astronomer royal was chosen to conduct it. After various inquiries,

the mountain of Schehallien, situated nearly in the centre of Mountain, 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 11.6. 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 mountain 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

Mountain 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, 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 effects of each other.