PHILOSOPHICAL SOCIETY OF WASHINGTON, 83 



and a minimum of 0".15. The first and last stations differed by 

 0".05. Tlie extreme range of all the stations was 13".89. 



An effort was then made to see how far these discrepancies 

 could be accounted for by the local attractions of irregular masses 

 above the surface of the earth in the vicinity. 



The formulae given by Clarke in the "Principal Triangulation 

 of the Ordnance Survey" were applied in connection with con- 

 toured maps, to calculate the deflections due to the superincum- 

 bent masses. 



The difference between the observed and calculated deflections 

 had an average value of 1''.47; i. e. about two-thirds of the 

 observed deflection could not be accounted for by the topography 

 and must be due to causes underground. 



General geological maps were also exhibited, and it was sug- 

 gested that a large and quite regular deviation at all the stations 

 for 150 miles in the vicinity of the Red River was due to the 

 difference in density of the secondary and tertiary formations 

 crossing the line, oiiliquely, in that neighborhood. 



A short statement of the history of investigations on this sub- 

 ject, and its bearing on the determination of the figure of the 

 earth, concluded the paper. 



Mr. HiLOARD spoke of the large deviations found near the 

 Pacific coast in the operations of the U. S. Coast Survey. 



108th Meeting. June 3, 1876. 



The President in the Chair. 



Thirty-five members and visitors present. 



Mr E. P. Lull gave an account of the country traversed by 

 the late expedition under his command for the investigation of 

 the route for 



the INTEROCEANIC canal through NICARAGUA. 



Mr. J. W. Powell read 



A BIOGRAPHICAL NOTICE OF MR. A. R. MARVINE. 

 {This notice is published in Appendix X of this Bulletin.) 



Mr. T. N. Gill, in behalf of a committee appointed for the 

 purpose, reported the following resolutions, which were unani- 

 mously adopted. 



