1855.] Pendulum-experiments at Harton Colliery. 19 



turbance would have been about 27 seconds. But tlie disturbance 

 was really found to be only 12 seconds. Consequently the proportion 

 of the density of the mountain to the earth's density was that of 

 12 to 27, or 4 to 9 nearly. And from this, and the ascertained 

 density of the naountain, it followed that the mean specific gravity 

 of the earth would be about five times that of water. The only 

 objection to this admirable experiment is, that the form of the 

 country near the mountain is very irregular, and it is difficult to say 

 how much of the 12 seconds is or is not really due to Schehallien. 



The second class is what may be called a cabinet experiment, 

 possessing the advantage of being extremely manageable, and the 

 disadvantage of being exceedingly delicate, and liable to derange- 

 ment by forces so trifling that they could with difficulty be avoided. 

 Two small balls upon a light horizontal rod were suspended by a 

 wire, or two wires, forming a torsion balance, and two large leaden 

 balls were brought near to attract the small balls from the quiescent 

 position. We could make a calculation of how far the great balls 

 would attract the little ones, if they were as dense as the general 

 mass of the earth ; and comparing this with the distance to which 

 the leaden balls really do attract them, we find the proportion of 

 the density of the earth to the density of lead. The peculiar 

 difficulty and doubt of the results in this experiment depend on the 

 liability to disturbances from other causes than the attraction of the 

 leaden balls, especially the currents of air produced by the 

 approach of bodies of a different temperature ; and after all the 

 cautions of Cavendish, Reich, and Baily, in their successive 

 attempts, it seems not impossible that the phaenomena observed may 

 have been produced in part by the temperature of the great balls as 

 well as their attraction. 



These considerations induced Mr. Airy, in 1826, to contem- 

 plate a third class of experiments, namely, the determination of the 

 difference of gravity at the top and the bottom of a deep mine, by 

 pendulum experiments. Supposing the difference of gravity found, 

 its application to the determination of density (in the simplest case) 

 was thus explained. Conceive a spheroid concentric with the ex- 

 ternal spheroid of the earth to pass through the lower station in the 

 mine. It is easily shown that the attraction of the shell included 

 between these produces no effect whatever at the lower station, but 

 produces the same effect at the upper station as if all its matter 

 were collected at the earth's centre. Therefore, at the lower station 

 we have the attraction of the interior mass only : at the upper 

 station we have the attraction of the interior mass (though at a 

 greater distance from the attracted pendulum) and also the attrac- 

 tion of the shell. It is plain that by making the proportion of these 

 theoretical attractions equal to the proportion actually observed by 

 means of the pendulum, we have the requisite elements for finding 

 the proportion of the shell's attraction to the internal mass's attrac- 

 tion, and therefore the proportion of the matter in the shell to the 



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