295 



offers, the box is towed by a line at the stern of a vessel which is 

 trading up and down the coast of Chili. It is almost too soon to 

 expect any decisive results at present, but in a few months I hope 

 to be enabled to send both the original copper, and that which has 

 been exposed to the action of the sea. 



III. " On the Causes of the great Variation among the different 

 Measures of the Earth's Mean Density." By Captain 

 W. S. JACOB, Director of the Observatory at Madras. 

 Communicated by the Rev. BADEN POWELL, F.R.S. 

 Received October 25, 1856. 



The result of the Pendulum experiments in the Harton Colliery, 

 undertaken by the Astronomer Royal in 1854, and detailed in his 

 paper presented to the Royal Society in January 1856, appears at 

 first sight rather startling, as adding to the already somewhat dis- 

 crepant measures or estimates of the earth's mean density one more 

 discordant than ever ; so that we have now values ranging from 4' 7 

 to nearly 6*6 ; a range, which, in the absence of any sufficient ground 

 for selecting any one as true to the exclusion of the rest, would seem 

 to deprive us of all confidence in their correctness as measures, and 

 leave them rather to be classed as estimates of a very rough de- 

 scription. 



But it will be my endeavour to show, that, while none of the 

 methods employed are capable of giving strictly accurate results, the 

 Cavendish experiment is the one which may be relied on as giving a 

 good approximation to the truth, within limits of error (when con- 

 ducted with proper precaution) far less than those to which either of 

 the other methods are liable. 



The three principal methods which have been tried are, 1st, the 

 Schehallien or Huttonian, which consists in comparing the total 

 attraction of the earth with that of a mountain mass, by measuring 

 astronomically the inclination of the normals at a given distance in 

 the meridian-plane on each side of the mass -, and then inferring the 

 attraction of the mass from the difference of this inclination from 

 what it would be on an exact spheroid ; 2nd, the Cavendish experi- 



