THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



JULY 1886. 



I. On the Variations of Gravity at certain Stations of the 

 Indian Arc of the Meridian in Relation to their Bearing 

 upon the Constitution of the Earth's Crust By Rev. 0. 

 ¥imM&,M.A. y F.G.S* 



IT is well known that, during the geodetic operations 

 for the measurement of the Indian arc of the Meridian, 

 it was found that the attraction of the Himalayas upon the 

 plumb-line was less than it ought to have been, according to 

 what Archdeacon Pratt calculated that their mass should have 

 produced. This was attributed by Sir G. B. Airy to a 

 downward protuberance of rock, having the same density as 

 the mountains, into a substratum of greater density, so that 

 their position would be approximately one of hydrostatic 

 equilibrium ; and he showed that such an arrangement, by 

 which their weight would be sustained, would have the effect 

 upon the plumb-line of the kind, which had to be accounted 

 for, greatly reducing the attraction of the mountains upon 

 the plumb-line at a distant station, but much less so at a station 

 near them f. 



Pratt himself, however, preferred to attribute the anomaly 

 to a supposed deficient contraction of the crust of the Earth 

 beneath the mountains during the secular cooling of the 

 globe, causing their relative elevation; and to this he attributed 

 the diminution of density. He retained the average density 



* Communicated by the Author. 



f Trans. Royal Soc. vol. cxlv. p. 101 (1855), quoted in ' Physics of 

 the Earth's Crust ' (Macmillan, 1881), p. 145. 



Phil. Mag. S. 5. Vol. 22. No. 134. July 1886. B 



