GENEVA SOCIETY OF PHYSICS AND NATURAL HISTORY. 229 



potassium and of magnesium is also a product of the mother-liquors which 

 is not deliquescent, and which jiresents some advantages over the 

 chloride of magnesium. It gives a temperature of — 32° to — o^° C. 

 (— 260 to —290 F.) 



M. Turrettini showed that the temperature to which may be carried a 

 solution of salt or of chloride of magnesium in water is the same as that 

 produced by salt or by this chloride when mixed with snow: thus the 

 solution of chloride of magnesium freezes only at — 34° c. ( — 29° F.) 



M. Turrettini also exhibited and explained the diplograph, a writing- 

 machine for the use of the blind, invented by M. Ernest Eecordon of 

 Lausanne, and constructed by the Genevese Society for the Manufacture 

 of Physical Instruments. This machine forms, at the same time, char- 

 acters in relief for the blind, and ordinary printed letters for those who 

 can see; so that the blind can correspond with each other, and those 

 who have sight with the blind. 



2. Meteorology and terrestrial physics. — Professor Plautamour presented 

 to our society a resume of the principal results of a work upon the baro- 

 metric observations, the hygrometric state of the atmosphere, the nebu- 

 losity, the rain, and the wind observed at Geneva during the fifteen years 

 comprised between 1881 and 1875. These researches, which were con- 

 siderable in extent and important in results, were published in the twenty- 

 fourth volume of our Memoirs, under the title of N'ouvelles etudes sur le 

 climate de Geneve (Recent investigations of the climate of Geneva), and 

 were discussed by Professor Gautier in the Archives^ 1877, t. 58. They 

 formed a sequel to a volume published by M. Plantamour, in 1803, upon 

 the climate of our country from 1826 to 1860. 



Dr. Forel while pursuing, as we know, with great perseverance his 

 investigations of Lake Leman, succeeded in discovering the form of the 

 seiches — oscillations of the water in the direction of the length and the 

 width of the lake. This exposition is equally applicable to the movement 

 of the waters of the lakes of Brienz and of Wallenstadt. The obser- 

 vations which served as the base of these calculations were made with 

 a limnimeter register. (See Arehives, 1870, t. 56, 57, and 59.) 



To carry the study of this oscillation of the water further than can be 

 done in a single locality, Professor Plantamour examined this move- 

 ment at Secheron near Geneva. He recognized its existence, but it was 

 produced here in a contrary direction from that at Merges. When it 

 was low at Merges it was high at Geneva and vice versa. Professor 

 Plantamour caused a limnimeter register to be established at Secheron. 

 {Archives, 1877, t. 58.) 



Dr. Forel has also studied the transparency of the waters of Lake 

 Leman. He determined it by measuring the depth at which a white 

 disk ceased to be visible in the water, or by measuring the minimum 

 depth at which a photographic plate experienced no alteration. A 

 white disk of 25 centimetres (10 inches) in diameter disappeared, from 



