ON THE ELLIPTICITY OF THE EARTH. 477 



relies; it inspires confidence if he finds the results 

 accordant, and it affords the opportunity if the results 

 should not be so, of endeavouring to clear up the 

 discrepancy on the spot. This was the chief purpose 

 which I contemplated in providing myself with the 

 attached pendulums ; and I found the full advantage of 

 the provision when, at stations nearly in the same lati- 

 tude with each other, and where, consequently, the rates 

 of the pendulums should theoretically have been nearly 

 the same, they differed to an amount of several seconds 

 in the day, the difference being shown by all the four 

 pendulums to nearly an identical amount ; leading to 

 the unavoidable conclusion that the discrepancy was not 

 due to errors of observation, but was the effect of natural 

 causes operating at the particular stations. In like 

 manner Mr. Baily, in calculating the results of Captain 

 Foster's experiments, appears to have derived from the 

 same circumstance the solution of the doubts, which he 

 is known to have entertained previously, of the true 

 causes of such discrepancies. In the report, p. 78, he 

 accompanies the table of comparative results by the re- 

 mark that the table " clearly shows that the vibrations 

 of a pendulum are powerfully affected, in many places, 

 by the local attraction of the substratum on which it is 

 swung, or by some other direct influence at present un- 

 known to us, and the effect of which far exceeds the 

 errors of observation. For it will be seen, from an in- 

 spection of the table, that all the pendulums tell (sub- 

 stantially) the same story at the same place."* 



* It was at one time imagined that the angle which the plane in 

 which the pendulum vibrated made with the astronomical or mag- 



