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often to be met with: so much experience and skill in the nicest observations of science, can but seldom be combined with the hardiness of rural, we might almost say, of savage life. It were therefore to let slip a most favourable occasion for promoting the interests of science, not to take this opportunity of inquiring farther into the attraction of mountains. The instruments are already on the spot, as well as the hardy, experienced, and skilful observer who is to use them; so that the same thing can never be undertaken at so little expence to the public, and in a manner so truly economical, and so highly advantageous to science.
As an additional reason for including the inquiry into the attraction of mountains, in the plan of the trigonometrical survey, we must be permitted farther to state, that there are several circumstances in the experiments at Schehalien, which should render the repetition of them extremely desirable.
Though nothing could easily be added to the accuracy of the astronomical part, of which we have just now seen the strongest and most impartial evidence, yet equal praise cannot be bestowed on the trigonometrical survey, by which the magnitude and figure of the mountain were determined. The theodolite employed was but an imperfect instrument; it gave the angles to minutes
only it was furnished with telescopes of a very moderate magnifying power; and, though the work of Ramsden, was in all respects inferior to the instruments now employed for the like purposes. Mr Burrowes, into whose hands this part of the work was committed, was new in the employment; and, though skilled in mathematics and astronomy, had no experience in the sort of work he was employed now to conduct. As to all, therefore, that relates to the density of the earth, and the conclusions grounded on the figure and magnitude of the mountain, it must not be supposed that the same precision is to be found as in the determinations purely astronomical.
We are enabled to state this with the more confidence, that circumstances have led us to study the detail of this survey with a more minute attention, than has probably ever been done by any one except Dr Hutton, who has so ably conducted the computations grounded on it. In this examination we have remarked, that when the solid content of the mountain is reduced into columns of equal attraction, according to Dr Hutton's method, owing to some imperfection in the survey, the lengths of those columns cannot always be accurately ascertained; and, particularly when they come nearly to the level of the observations, that it is often uncertain whether they rise above that level,
or fall short of it, and, of consequence, whether a certain quantity is to be applied as an augmentation or a diminution of the whole attraction.
There were even faults in the plan, no less than in the execution of the experiments. The observatories were placed too high on the sides of the mountain; they were about half way up; so that between a sixth and a seventh of the total effect of the attraction was lost. The sections were vertical, and carried at random, some entirely, but many of them only partially across the mountain, instead of being conducted horizontally round it, and connected together by two vertical sections at right angles to one another.
In the distance to which the survey extended, no principle seems to have been adopted as a guide, except a very insecure one, that at the distance of a mile and a half, or two miles, the action of a mountain of ordinary size could not sensibly affect the direction of gravity. The knowledge obtained from the experiments at Schehalien afford a much better, and more secure principle for fixing the limits within which the attraction of a great mass of matter may be supposed to produce a sensible effect.
Add to this, that at the time of these experiments no attention, or next to none, was bestowed on the structure of the mountain, and the distribution of the materials which compose it. This
omission, accordingly, gave no inconsiderable degree of vagueness to the conclusions deduced concerning the density of the earth.
It is true, that two gentlemen, zealous to contribute to the accuracy of this interesting inquiry, endeavoured, not long ago, by a mineral survey of Schehalien, to remedy this defect, and to ascertain, with some degree of precision, the specific gravity of the rocks which compose that mountain. They succeeded, perhaps, as far as the nature of the thing will now admit; but certainly much less, than if a mountain of simpler structure had been the subject of examination, or if the mineral survey had been undertaken along with the trigonometrical, when the instruments of observation were on the spot, and all the stations distinctly recognised.
These circumstances, though they go no farther than to render the limits within which the accuracy of the results are contained, more distant than they would otherwise have been, are certainly to be held as good grounds for wishing to have the same experiments repeated, with an attention to all the improvements that have been made since the time when they were instituted. The opportunity, then, that now presents itself, we hope, will not be overlooked, when the instruments, as has been said, are prepared, and when observers are at hand, zealous to engage in the work, instructed in
all the resources of their art, and accustomed to overcome all the difficulties of their situation. Such an enterprise would form a very noble conclusion of the present survey; and would distinguish it from all others yet made, as much for the variety and importance of the objects contained in the plan of it, as for the perfection of the execution. It is already infinitely to the credit of the country, and those entrusted with the government of it, that, during the long and expensive war in which the nation has been involved, this great work of science has been carried on as in the midst of profound peace. We may therefore hope, that the termination of an arduous contest, and the restoration of tranquillity to the world, will permit this national work to be completed with an extent and accuracy worthy of the spirit with which it has been begun and carried on.