810 Geological Society: — 



of chloride molecules with the glowing platinum is capable 

 of liberating electrons from the platinum atoms. 



Precisely how the light- emission depends upon the libera- 

 tion and return of the electron, whether it takes place on the 

 return only*, or also at other stages of the path of the 

 metallic atoms in their free state, awaits further experiment. 

 I am, Gentlemen, 



Yours faithfully, 

 University College, London, J], £[. DA C. ANDRADE. 



Oct. 6, 1912. 



LXXXIIL Notices respecting JSew Books. 



Ferguson's Percentage Unit of Angular Measurement, with Loga- 

 rithms; also a Description of his Percentage Theodolite and 

 Percentage Compass. By John Coleman Ferguson. Longmans, 

 Green & Co. : London,' 1912. 



T^HE fundamental idea is to measure angles not by equal incre- 

 -*- ments of arc, but by equal increments of the tangents of the 

 ares. The tangent of 45° is divided into 100 equal parts ; and the 

 radial projection of these subdivisions on the corresponding circular 

 arc gives the graduation on the circular rim. Graduations begin at 

 the cardinal points, JST, S, E, W, pass round in both directions, 

 and meet half-way in each quadrant. The method of subdivision 

 is familiar in quadrant galvanometers ; but it has never before been 

 applied to surveying. There is no doubt that many operations are 

 greatly simplified by the use of this method of graduation. The 

 tables of logarithmic sines, cosines, tangents, and secants differ 

 from ordinary tables in having as the argument the natural tangent 

 instead of the arc. 



L XXXIV. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 207.] 



December 20th, 1911.— Prof. W. W. Watts, Sc.D., LL.D., M.Sc, 

 F.R.S., President, in the Chair. 



nPHE following communications were read: — 



1. 'The Glacial Sections at Sudbury (Suffolk).' By the Eev. 

 Edwin Hill, M.A., E.G.S. 



The sections round Sudbury w^ere described in two Geological 

 Survey Memoirs : since the date of publication of these much more 

 has been disclosed. A list is given of the principal sections now 



* As Lenard has shown to be the case for the luminous " centres" in 

 phosphors. 



