B. TERRESTRIAL PHYSICS AND METEOROLOGY. 69 



as a floor is so compact and so smooth as to furnish excel- 

 lent skating. The water that does not freeze runs off to 

 the lower portions into a mass of debris, and appears as a 

 spring on the side of the mountain containing the cave. 

 A number of observations give the mean temperature of 

 the cave at 0.86 C, while outside it was +3.53 C. Dr. 

 Krenner states various reasons why the temperature can re- 

 main so low, and thus render possible the persistence of ice, 

 as follows : The cave has only one very small entrance, from 

 which it runs downward throughout its entire extent ; the 

 water that is not frozen has an opportunity to flow off with- 

 out stagnating, or melting the ice ; the position of the en- 

 trance is such that the sun never reaches it, therefore it 

 must be comparatively cool ; a current of cold air passes 

 upward through the cave, tending to produce low temper- 

 ature. 



THE DEPOSITION OF FINE SEDIMENTS. 



Dr. T. S terry Hunt states, in reference to the question of 

 the deposition of fine mud in the Mississippi, that the de- 

 posited matter requires from 10 to 14 days to subside; but 

 that if sea-water or salt or sulphuric acid be added to the 

 turbid water it becomes clear in from 12 to 18 hours. Thus 

 is explained the rapid precipitation that occurs when the 

 river water mixes with the salt waters of the Gulf of Mexico. 

 The cohesion of water diminishes when it holds saline matter 

 in solution, as was said by Guthrie and was verified by Dr. 

 Hunt. He found that the addition of eight parts of chloride 

 of calcium to 1000 parts of water reduces the size of drops to 

 one ninth, and the precipitation of suspended clay is made 

 very rapid when a strong solution of salt is employed. 12 



.4-,X. 9 ; 27r. 



SO-CALLED TIDES IN GREAT LAKES. 



The question of the so-called tides in the greater lakes of 

 North America is likely to receive some elucidation from the 

 researches of Dr. Forel, of Lausanne, who has for several 

 years been investigating what are known as the seiches of 

 the Lake of Geneva, this term being applied locally to a cer- 

 tain oscillatory movement occasionally seen to occur on the 

 surface of the lake. Forel agrees with previous observers in 



