493 



In conclusion, it deserves to be mentioned that there appears to 

 exist a similar series of arsenetted ureas. Triethylarsine, when left 

 for some weeks in contact with sulphocyanide of phenyl, deposits 

 small crystals of a body which I believe to be the arsenic-compound 

 carresponding to the phosphorus-urea described in this paper. This 

 body requires a more minute examination. 



II. "On the Deflection of the Plumb-line in India caused by 

 the Attraction of the Himalaya Mountains and the elevated 

 regions beyond, and its modification by the compensating 

 effect of a Deficiency of Matter below the Mountain Mass." 

 By the Venerable Archdeacon PRATT. Communicated by 

 Mr. STOKES, Sec. R.S. Received October 25, 1858. 

 (Abstract.) 



The author begins by referring to his former paper, published in 

 the 'Transactions' for 1855, in which he calculated the deflections 

 caused by the mountain mass on the north of Hindostan, at three 

 principal stations of the Great Arc, in the plane of the meridian, viz. 

 Kaliana (lat. 29 30' 48"), Kalianpur (24 7' 11"), and Damargida 

 (18 3' 15"). He made them 27"'853, 11"'968, and 6"'909 (or 

 more correctly, as revised in the present Paper, 27 "'978, 12"'047, 

 and 6"'790) ; and showed that the comparison of these two portions 

 of the arc which, if it be elliptical, and if the amplitudes are accu- 

 rately known, ought to give the exact ellipticity of the arc in ques- 

 tion gives an ellipticity of ^-^g-, instead of the mean -g-J^. 



2. He next states that the Astronomer Royal, in a subsequent 

 communication (in 1855), suggests that there is most probably a de- 

 ficiency of matter immediately below the mountains which will cause 

 a negative deflection, and so compensate for the mountain attraction. 

 Three objections are urged against this hypothesis, as stated by 

 Mr. Airy. It requires (1) that the solid crust should be only about 

 ten miles thick ; (2) that the crust should be lighter than the lava 

 on which it rests ; (3) that wherever there is a protuberance up- 

 wards in mountain masses and table-lands, there must be a corre- 

 sponding projection of the crust downwards into the fluid, which it 

 is difficult to conceive, as the same reason which is used to show it 

 would prove also that, where there are hollows above as in deep 



VOL. ix. 2 L 



