ASTRONOMY OF EIGHTEENTH CENTURY. 257 



sity of the earth was calculated by Dr. Hutton, and in 1775 he pub- 

 lished in the " Philosophical Transactions" an account of the methods 

 he had followed, with his conclusion that the mean density of the earth 

 was to that of Schehallien as 9 to 5. And as the density of this moun- 

 tain had been calculated to be 2-5, the resulting density of the earth 

 was 4*48. That is as much as to say, that the earth would weigh four 

 and a half times as much as a globe of water of the same size. Since 

 that period other surveys of Schehallien have been made, and a higher 

 density has been assigned to the mountain, the result of which is to 

 raise the mean density of the earth to nearly 5, which agrees well with 

 determinations made by other methods, one of which we shall now de- 

 scribe, as the experiments are extremely interesting. 



The Rev. John Mich ell, about 1790, devised a method of directly 

 observing the attraction of gravitation, and of measuring the mean 

 density of the earth. The apparatus consisted of a light wooden rod 

 suspended at its centre by a very fine long wire, so that the rod, bearing 

 a leaden ball at each end, remained horizontal. When the rod had 

 come to rest, the wire would be without torsion, and in this condition 

 of affairs a large spherical mass of lead was brought near each of the 

 balls, so that the attraction might conspire to turn the rod horizontally, 

 thereby to a certain extent twisting the wire. The mechanical force 

 corresponding with each degree of torsion could be easily ascertained, 

 and thence the deviation of the rod became the measure of the attrac- 

 tive action exercised by the masses of metal placed near the leaden 

 balls. This attractive force could therefore be compared with that of 

 the earth, and thus the mass and consequently the density of the latter 

 could be determined. The Hon. Henry Cavendish, in J798, commu- 

 nicated a paper to the Royal Society, in which he describes the ex- 

 periments he had made with Michell's apparatus. The mean result of 

 his experiments gives the earth the density 5 -45. Cavendish, however, 

 observed certain anomalies in the indications afforded by the appara- 

 tus, and he therefore did not put forward his result as entitled to full 

 confidence. He appears to have entertained the design of examining 

 into the cause of the anomalies. Cavendish's experiments have been 

 repeated during the present century with great care and with many in- 

 genious improvements in the details of the apparatus and the modes 

 of observing the deflections. The cause of the anomalies, which were 

 again observed by the more recent experimenters, was found to be 

 connected with the radiation of heat, as indeed Cavendish himself had 

 supposed. The value for the earth's density, as determined in 1837 

 in this way by Reich, a professor at Freiberg in Saxony, is 5 -44 ; and, 

 as determined by Mr. Baily in 1842, it is 5-66. 



Since Galileo there has been no name in the annals of astronomical 

 discovery more illustrious than that of WILLIAM HERSCHEL (1738 

 1822). He was born in Hanover in 1738, and was the son of a musi- 

 cian in somewhat narrow circumstances. At the age of fourteen he 



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