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HISTORY OF PHYSICAL ASTRONOMY.
ellipticity is so small that the closest inspection without measure could notjudge which was intended for the greater and which for the smaller axis.The whole quantity in dispute is less than one-sixteenth of this ellipticity.Instead of being surprised that such a difference exists, we may well beastonished at the accuracy of modern measures of all kinds, which makeso small a quantity a subject of controversy.”*
As soon as astronomers were persuaded that the principle of gravitationextended to the constituent parts of the celestial bodies, it became aninteresting question to ascertain whether the attraction of masses at theearth’s surface exerted any appreciable influence. The first attempt toestablish this fact was made by Bouguer and his associates, whileengaged in measuring an arc of the meridian in South America . Astro-nomical observations having been made in the neighbourhood of the loftymountain Chimborazo , it was found that the attraction of the latter causedthe plumb line to deviate towards it to the extent of 7".5. This resultwas so far confirmatory of the Newtonian theory; but it w r as generallyadmitted that the experiment had not been made with sufficient care tomerit the confidence of astronomers. The question was not mooted againuntil 1773, when, Dr. Maskelyne having suggested the repetition of theexperiment to the Royal Society , it was resolved by that body thatimmediate steps should be taken to carry it into effect. The mountain ofSchehallien, in Perthshire, was selected as offering peculiar advantagesfor this purpose. It is a narrow ridge running nearly east and west, andrising to an elevation of about 2000 feet above the level of the surround-ing country. The mode of proceeding employed in the experiment wasthis:—two stations were selected, one on the north side and the other onthe south side of the mountain ; and the difference of their latitudes wasdetermined, first by means of astronomical observations at each station,and then by means of the meridional distance between the two stations,and certain assumed elements of the earth’s figure. Now it is clear thatthe attraction of the mountain w’ould compel the plumb line to deviatefrom its vertical position, and would thereby occasion a displacement ofthe zenith point opposite in direction at each station. Hence thedifference of latitudes as determined by astronomical observation ought toexceed the difference resulting from geodetic computation by the sum ofthe displacements of the zenith points ; and this latter quantity is mani-festly the measure of the sum of the attractions exerted by the mountainat the two stations. Now the zenith distances of a certain number of stars,having been determined at both stations, were found to indicate a differenceof latitude amounting to 54".G. The meridipnal distance between thestations, as computed by a process of triangulation, was 4304.4 feet, andthe difference of latitude hence deduced was 42".9. The resultingdifference, 11".7, was, therefore, the quantity due to the attraction of themountain at the two stations. Thus it was established by actual ex-periment that the principle of gravitation operates not only between thebodies of the solar system, but also between the smaller masses of matterof which they are composed.
An interesting question suggested by the experiment just alluded to isthat relative to the mean density of the earth. It is clear that, if we knewthe quantity of matter contained in the mountain, this datum, combinedwith the ratio of the attraction of the mountain to that of the earth, as