544 Geological Society : — 



viscosity, p the density of the gas, and m the mass of a mole- 

 cule, then the nnmber of molecules which in unit time cross 

 a section of the tube is 



p Yircft 



m Sfi, 



Hence we have, when the pressure is steady, 



p Yira* u — v i 



m bfi u + v e 

 or 



P7ra 4 1m — v itn 



S/Jb p u + v € 



Thus the potential gradient will vary inversely as the pres- 

 sure of the gas, and the ratio of the pressure at the positive 

 electrode to that at the negative will depend upon the square 

 of the reciprocal of the pressure. 



Taking the case of a tube 1 millim. in diameter, 10 centim. 

 in length, filled with hydrogen at a pressure of one ten- 

 thousandth of an atmosphere, and conveying a current of one 

 ten-thousandth of an ampere, I find from the preceding- 

 formula that, supposing u much greater than v 9 the pressure 

 at the positive electrode would exceed that at the negative by 

 about twenty per cent. 



XLIX. Proceedings of Learned Societies. 

 GEOLOGICAL SOCIETY. 



June 19th, 1895.— Dr. Henry Woodward, F.K.S., President, 



in the Chair. 



[Continued from p. 395.] 



rPHE following communications were read : — 

 -*- 3. ' The Chalky Clay of the Fenland and its Borders : its Consti- 

 tution, Origin, Distribution, and Age.' By Sir Henry H. Howorth, 

 K.C.I.E., M.P., E.B.S., E.G.S. 



The distribution of the Clay (so often termed Chalky Boulder 

 Clay) is noticed, and it is stated that it is surrounded on all sides by 

 country occupied by different deposits, being mainly separated from 

 the sea on the east and north-east by sand}" and pebbly materials, 

 while on every other side it is clearly and sharply defined. The 

 paucity of foreign stones is noted as compared with natives, and the 

 similarity of the matrix of the Chalky Clay to the material of the 

 older deposits of the neighbourhood. The author maintains that the 

 contents of the Clay indicate movement of material from west to east 



