NATURAL HISTORY OF GENP^VA. 167 



izatiou by diffusion of liglit. He has observed a trace of illumination 

 upon a cone of solar light concentrated not onl} in transparent liquid 

 or solid bodies, water-crystals, &c., but even in brilliant flames. The 

 particles of incandescent charcoal, which these contain, therefore, still 

 exert their reflective power contrary to the idea given by M. Hirn. This 

 luminous trace is always completely polarized for an angle of vision of 

 90O. 



Professor Forel, of Morges, has given us the pleasure of listening to an 

 account of the beautiful researches he has' undertaken on Lake Lemau, 

 for several years, ou the transmission of light in water; and has 

 shown us impressions obtained upon photographic paper immersed at 

 diflerent depths. He found that 130 to 100 feet of depth in summer was 

 the limit, beyoml which the transmitted light is too feeble to exercise any 

 action upon the chloride of silver. In winter the water of the lake is 

 much more transparent, and the limit extends to 200 feet. 



General Dufour described the effects of a thunderbolt on a poplar tree 

 upon his estate near Geneva; a large furrow was made in it from the 

 top of the trunk, 80 feet in height, to the bottom. On this occasion I\J. 

 de la Harpe cited a fact which he had witnessed. A bolt fell upon a 

 group of poplars, leaped from one tree to another, passed through sev- 

 eral, and made cylindrical holes in them of from 3 to 4 inches in diameter. 



Terrestrial physics, meteorology. — Professor Plantamour presented a 

 memoir upon the observations of latitude, of azimuth, and of the pendu- 

 lum, made on the Righi, the Weisseustein, and at Berne, in 1807, 1808, 

 and 1809. In a comparison of the observations of the pendulum on the 

 Gebris and the Weissenstein, the weight was found to be 50^017 ^^^^ than 

 in the first of these localities. M. Plantamour attributed this to the 

 nature of the rocks, which are of cone-in-cone at Gebris, and calcareous 

 at the Weissenstein. The same member announced to us the conclu- 

 sion of the great work of the leveling of the polygon, traversing twice 

 the Alps, at the St. Gothard, and at the Simplon retarded by an error, 

 which, for a time, could not be accounted for, but which was finally 

 found to be due to a ftilse figure in the record of one of the sides of the 

 polygon. 



The variations, extreme this year, of the waters of the Lake of Geneva, 

 attracted the attention both of the public and of the engineers. Gen- 

 eral Dufour proved that the high waters of 1873, however, remained 

 below those of 1792 and of 1817, and M. Plantamour added below those 

 of 1840. A remarkably low level followed during the winter of 1873- 

 1874, but the public were mistaken in believing an assertion in a certain 

 journal that the lake had never been lower. 



These questions in regard to the level of the lake, which are frequently 

 renewed, have formed the object of a special work by Professor Planta- 

 mour, founded upon thirty-six years of observations, from 1838 to 1873. 

 The author first proceeds to the verification, at several points, of the 

 marks successively employed, and the level of the lake having been re- 



