xxv.] ACCORDANCE OF THEORIES. 567 



operations are of an entirely different character from those 

 of Cavendish and Baily's experiments. Sir Henry James' 

 similar determination from the attraction of Arthur's Seat 

 gave 5-14. 



A third distinct method consists in determining the force 

 of gravity at points elevated above the surface of the earth 

 on mountain ranges, or sunk below it in mines. Carlini 

 experimented with a pendulum at the hospice of Mont 

 Cenis, 6,375 feet above the sea, and by comparing the 

 attractive forces of the earth and the Alps, found the 

 density to be still smaller, namely, 4*39, or as corrected 

 by Giulio, 4'95O. Lastly, the Astronomer Eoyal has on 

 two occasions adopted the opposite method of observing 

 a pendulum at the bottom of a deep mine, so as to com- 

 pare the density of the strata penetrated with the density 

 of the whole earth. On the second occasion he carried his 

 method into effect at the Harton Colliery, 1,260 feet deep ; 

 all that could be done by skill in measurement and careful 

 consideration of all the causes of errror, was accomplished 

 in this elaborate series of observations l (p. 291). No doubt 

 Sir George Airy was much perplexed when he found that 

 his new result considerably exceeded that obtained by any 

 other method, being no less than 6-566, or 6-623 as finally 

 corrected. In this case we learn an impressive lesson 

 concerning the value of repeated determinations by distinct 

 methods in disabusing our minds of the reliance which we 

 are only too apt to place in results which show a certain 

 degree of coincidence. 



In 1844 Herschel remarked in his memoir of Francis 

 Baily, 2 " that the mean specific gravity of this our planet is, 

 in all human probability, quite as well determined as that 

 of an ordinary hand-specimen in a mineralogical cabinet, 

 a marvellous result, which should teach us to despair of 

 nothing which lies within the compass of number, weight 

 and measure." But at the same time he pointed out that 

 Baily's final result, of which the probable error was only 

 0*0032, was the highest of all determinations then known, 

 and Airy's investigation has since given a much higher 

 result, quite beyond the limits of probable error of any of 



1 Philosophical Transactions (1856), vol. cxlvi. p. 342. 



2 Monthly Notices of the Eoyal Astronomical Society, for 8th Nov. 

 1844, No. X. vol. vi p. 89. 



