GENEVA SOCIETY OF PHYSICS AND NATURAL HISTORY. 207 



time of low water, and it is from 5 to G centimetres (2 inches) greater in 

 summer at the time of high water. If the level of the water is sensibly 

 the same during the whole year at Chillon, Vevey, and Sdcheron, the 

 observations taken at Ouehy show a level lower by 2 centimetres (1 inch;, 

 and at Genthod, on the contrary, higher by 2 centimetres, the difference 

 remaining also sensibly constant during the whole year 1874. The obser- 

 vations taken in the three preceding years at Chillon, Vevey, and Ouchy 

 confirm the constant depression of level at the latter place relatively to 

 the other two. It would appear difficult to attribute this anomaly to 

 errors in the agreement of the zeros of the different scales; yet new 

 measures of verification will be taken to put the fact itself beyond all 

 question. 



M. Plantamour, in conjunction with Professor Hirscb, presented the 

 results of the latest operations of leveling; these results are recorded 

 in the fifth number of the Topography of Switzerland, published by the 

 Federal Geodetic Commission. 



Professor Thury presented a memoir on photometry, and on an astro- 

 nomical photometer of his own invention ; this memoir was inserted in 

 jSTo. 202 of the ArchiveSj^oy ember, 1874. Professor Thury's photometer is 

 constructed upon the system of the extinction of the light of a star, an 

 ^extinction produced by the reduction of the aperture of the object-glass 

 by the help of a diaphragm, or by the reflection of pencils of rays 

 by a system of mirrors. The diaphragm in this apparatus consists of 

 16 thin rectangular plates, capable each of sliding in the direction of 

 its length and in the direction of the center of the object-glass. By a 

 very ingenious mechanism, the movement of these IG plates is so co-ordi- 

 nated that their small interior sides form a regular jjolygon, the diame- 

 ter of which can vary from zero to the diameter of the object-glass. In 

 order to prevent the inconveniences resulting from false disks, produced 

 by too great a diminution of the aperture of the objective, a diminution 

 which might be necessary to bring about the extinction of a brilliant 

 rstar. Professor Thury has recourse to the interposition of one or two 

 Iblack mirrors perfectly plane, and receiving the incident ray at an angle 

 <;3f 45°. The extinction of the light can be produced either by the dimi- 

 nution alone of the aperture for a star of moderate brilliancy, or by the 

 interposition of one or two mirrors, according to the brightness of the star, 

 and not reducing the opening below 0.23 of the total aperture. In each of 

 these three cases the position of the eye-glass is of course different. 



Professor Thury also presented a memoir on the experiments he made 

 in connection with Dr. Minnich, relative to a disengagement of electri- 

 «city from the thermal waters of Baden (Aargan). These experiments, 

 which appeared in No. 205 of the Archives, January, 1875, furnish the 

 means of verifying the existence of a very decided electric current, which 

 is revealed by the galvanometer, when by means of electrodes the water 

 of the Limmat is placed in communication with the thermal spring, the 

 ^direction of the deviation indicating that the mineral siiring is electrified 



