240 



In the various excavations that have been made in the prosecu- 

 tion of this inquiry, many objects of art of historical interest have 

 been discovered ; but as these do not come within the province of 

 the Royal Society, the author proposes to give an account of them 

 in a memoir to be laid before another learned body. 



The following communications were read : 



II. "On the Computation of the Effect of the Attraction of 

 Mountain-masses, as disturbing the apparent astronomical 

 latitude of stations in Geodetic Surveys." By GEORGE 

 B. AIRY, Esq., F.R.S., Astronomer Royal. Received 

 January 25, 1855. 



The author commences with remarking that his surprise had been 

 excited by the result obtained by Archdeacon Pratt*, namely, that 

 the computed attraction of the elevated country north-east of India 

 considerably exceeds the disturbance which it was sought to explain. 

 But on consideration the author perceived that this result might have 

 been anticipated, on the extensively received supposition that the 

 interior of the earth is a dense fluid or semi-fluid (which for conve- 

 nience he calls lava), and that the exterior crust floats upon it. For, 

 he remarks, this crust cannot be supposed at any part to be very high 

 upwards (as in mountains), at least to any great horizontal extent, 

 unless there is a corresponding projection downwards into the lava. 

 Upon making a numerical calculation, even with the crust 100 miles 

 thick, it was shown that there would be such a tendency of the 

 table-land to crack and sink in the middle as no cohesion of rocks 

 can resist. He conceives that the state of the ground may be pro- 

 perly illustrated by a raft of timber floating on water : if one piece 

 of timber projects higher into air than the others, we are certain 

 that it also projects lower into water than the others. Assuming 

 this as established, then it is evident that the horizontal attraction 

 of a mountain-mass on a point at a considerable distance is nearly 

 evanescent, because the increase of attraction of the part which is 



* Proceedings of the Royal Society, December 7, 1854. 



