GENEVA SOCIETY OF PHYSICS AND NATURAL HISTORY. 217 



on the plain of Eynard near Eolle. From the different details observed 

 and the topograpbical position of tlie poplars, M. Colladon concludes : 

 1st, that the poplar is more likely to be struck than other trees; 2d, that 

 when several poplars are standing in a group or in a row, the one near- 

 est to water is the first struck, and this ftict may be made available in 

 discovering springs; 3d, the upper part of the poplar always remains 

 intact, because its branches are good conductors of the electricity; 4th, 

 the lightning passes between the sap-wood and the old wood, and escapes 

 near the foot of the tree, laterally, by passing through the sap-wood leav- 

 ing great spots around the holes through which it has passed. 



Professor Forel, of JMorges, announced to the society the first results of 

 his observations of the seiches (variations of level), made at Merges by 

 means of a recording limuimeter. The lake here presents longitudinal 

 and transverse seiches, according as the periodic undulations follow the 

 great or small axis of the lake. The normal seiches of Merges have a dura- 

 tion of ten minutes, whatever their amplitude may be. There are other 

 seiches, less frequent, at Merges, having a duration of 70 minutes and of less 

 amplitude, notexceedingonecentimetre (four-tenths of aninch). M. Forel, 

 by comparing these with those of Lake Constance and Lake Neuchatel, 

 thinks that those of 70 minutes' duration are to be considered as longi- 

 tudinal. Others, observed at Veytaux and Chillon, and which he acci- 

 dentally recognized at Merges and Evian, whose duration is about 35 

 minutes, might be the longitudinal seiches of the large lake, oscillating 

 from Chillon to the bar of Promenthon, and vice versa; those observed 

 at Geneva of a duration of 30 minutes are probably the seiches peculiar to 

 the small lake oscillating between the port of Geneva and the limit of 

 the large lake; and, finally, the ordinary seiches of Merges and Iilvian, of 

 10 minutes' duration, are the transverse seiches of Lake Leman. The 

 register indicates the passage of a steamboat, 20 minutes in advance, by 

 waves of three-quarters to one minute in duration, and these waves, after 

 the passage of the boat, continue to repeat themselves for an hour and 

 a half at intervals of from one to two minutes. 



2. Physics and mechanics. — Professor Soret communicated to the society 

 the result of his study of the phenomena of diffraction in the circular 

 gratings examined by him in the special case of equidistant circles. This 

 result presents some analogy with those considered in a communication 

 of the previous year. They produce a powerful concentration of the 

 luminous rays along the whole length of their axis, and are capable of 

 giving images of objects quite similar to those obtained through small 

 holes. 



The same member, in making a communication on halos, and especially 

 on parahelia by reflection in crystals of ice, adverted to the fact that 

 Bravais had considered two cases only, the prism with an elongated 

 hexagonal base, and the star or regular hexagonal prism with re-enter- 

 ing angles. M. Soret added two new cases of parahelias, the prism of 

 icicles with six regular sides, and prisms with elongated base, in which 



