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the two stations, they had a difference twice as great as if they had only observed stars on one side of the zenith. They would, however, have preferred the method of observing, first on the south, and then on the north side of the mountain, if it had not been that Chimboraço is inaccessible from the north. They found, in this way, that the zenith, by the action of the mountain on the plummet, had been carried 7" towards the south; a quantity vastly less than they had anticipated, and insufficient, in reality, considering that their instruments were not so perfect but that inconsistencies of 19′′, and even 26′′, sometimes entered into their observations, to determine the question whether the mountain had or had not a sensible effect on the plumb-line. From that time, however, to the year 1773, no attempt was made to determine this curious and interesting fact in physical astronomy. In that year, the AstronomerRoyal at Greenwich proposed to the Royal Society of London to make an experiment of the same kind on some of the mountains of Great Britain. After a careful survey of the principal mountains both in England and Scotland, the mountain of Schehalien, * in the latter country, was judged to be more
In a note on the word Schehalien, our author says: Montagne appelée dans le pays, en langue Erse, Maidenpap, qui veut dire orage perpetuel." There could not be a
advantageously situated than any other. Dr Maskelyne himself undertook the operation; and with the same excellent zenith sector which he had carried to the island of St Helena when he went to observe the transit of Venus in 1761, he observed the zenith distances of stars, first on the south, and then on the north side of the mountain. Notwithstanding a most unfavourable summer, he made 337 observations, and determined the zenith distances of the same 40 stars at each of the two stations. The difference of the latitude of the two stations obtained from these observations, compared with that which was inferred from the measurement of their distance on the ground, gave decidedly 5".8 for the action of the mountain on the plummet of the sector. The great number of these
more unfortunate translation. The Gaelic etymologists do indeed differ as to the derivation and import of the word Schehalien. According to one derivation, it signifies Maidenpap; according to another, it is said to signify perpetual storm: And if the figure of the mountain be brought as an evidence of the former derivation, the weather that so often prevails around it may be brought in support of the latter. The learned Baron, however, putting these two interpretations into one, has been so unlucky as to give Maiden-pap and perpetual storm as synonymous expressions. From this inaccuracy, his residence for several years in London ought to have delivered him; for though he could not learn there what was Erse, he might have learned what was English.
observations, and their perfect agreement with one another, leaves no doubt at all, that mountains such as Schehalien, or of the height of 3000 feet, are able to draw the plumb-line 5′′ or 6" out of the perpendicular.
We must not here omit to observe, that the researches of the Baron de Zach have brought out a circumstance hitherto unobserved, vastly to the credit of Dr Maskelyne's accuracy. That astronomer, as he tells us himself, though he had made observations on 43 stars, did not calculate the effect from any more than the ten which he considered as the best determined, in order that he might satisfy more speedily the impatience of the Society to be made acquainted with the result of his experiments. It does not appear that he himself afterwards, or any other astronomer, ever undertook the remaining part of this task, which, however, the Baron de Zach has now performed with his usual skill and accuracy. He has calculated the results of all the 337 observations which Dr Maskelyne had made on the zenith distances of the 43 stars just mentioned. Three of these stars not having been seen from the stations both on the north and south side of the mountain, cannot be taken into account. From the 40 that remain, the conclusion deduced is, that the celestial arc between the zenith of the two observatories was 54".651. Now, from the
measurement on the ground, the same arc comes out 43′′.019; the difference, 11′′.632, being the sum of the attractions of the opposite sides of the mountain. The half of this, 5".816, is the effect on each side, precisely the same which Dr Maskelyne has deduced from the observations which he considered as the best. This verification of his work is in itself highly satisfactory, and very gratifying to those who enjoyed the friendship, and who respect the memory of that excellent astronomer.
Among the means of resolving the problem of the attraction of mountains, we must not omit one which was proposed by Boscovich. This was, to suspend a plummet from a high tower, situated on the sea-shore, where the rise of the tide was very great, and where the different positions of the plumb-line, at high and low water, might be directly observed. This method, however, though simple at the first view, is incumbered by so many difficulties, that we believe it has never been undertaken. A very ingenious improvement on it, proposed by the late Professor Robison, consisted in observing the alteration of the level of a fluid, caused by the access and recess of the great wave of the tide, which alteration was to be measured by the reflection of a fixed object from the surface of the fluid. The fluid might be the water in a deep well, close to the sea-shore. We do not think
that this notion is entirely inapplicable to practice; and we believe all must agree that it is very ingenious.
The Baron de Zach, when at Marseilles in the year 1810, finding himself, as has been said, in a situation most favourable to astronomical observations, and being also furnished with good instruments, though not such as the zenith sector employed by Dr Maskelyne, yet conceived that the position of Marseilles, with a chain of hills rising on the one side, and the Mediterranean stretching out on the other, afforded great conveniency for trying whether, with such instruments as he possessed, the attraction of a mountain of moderate size could be rendered sensible.
The scene of his observations was the bottom of a chain of calcareous mountains, which, at the distance of 6000 or 8000 toises from the city of Marseilles, extends from east to west. The highest part of the chain, called the hill of Mimet, has an elevation of about 400 toises above the level of the sea. On the side of it, and at the height of about 250 toises, are the ruins of an old convent, known by the name of Notre Dame des Anges, commanding a fine view of the Mediterranean at the distance of five or six miles, extending indefinitely toward the south. In the south-west, at the distance of about 8000 toises from the coast, an insulated rock, in the middle of the sea, rises just above the sur