On the Formation of Bottom Ice. 411 



was the temperature of the water before this ice formation 

 took place. 



In December, 1720, the Falls of Trolhattan were stopped 

 for nine days by bottom ice, and a serious flood was the result. 

 The rivers in Norrland often freeze up, through several masses 

 of bottom ice rising up from the depths to the surface of the 

 water, and then freezing together into one solid body of ice. 

 When these masses of ice rise from the bottom they often 

 bring up with them the largest sunken trees, and even stones 

 of a considerable size, and break through the coat of ice which 

 had been formed on the surface. This has several times also 

 been observed on the Rhine. Dumatrel remarked, that when 

 the surface of the Seine was covered with ice, the bottom of 

 that river was also covered with a compact body of ice about 

 thirteen lines thick. In the river Aar a good deal of ice has 

 often been seen to come up from the bottom, where it had 

 doubtless been formed. This ice consisted of round flat 

 pieces, which rose to the surface of the water with the flat 

 side in a vertical direction, often three feet above the water, 

 when it sank to the surface and laid in an horizontal position. 

 On one occasion several islands of bottom ice formed in the 

 river, of which the largest had a diameter of more than a hun- 

 dred feet. These islands, in the shape of cones, were fastened 

 to the bottom by a gelatinous formation of ice. In the har- 

 bour of Pillau, an iron chain, thirty-six feet long, was lost in 

 eighteen feet of water. Several years after the same chain was 

 observed floating on the surface of the water, encrusted in a 

 very thick mass of bottom ice. 



Several hypotheses have been advanced to account for the 

 formation of this bottom ice. One circumstance we must bear 

 in mind is this, that bottom ice never forms in still fresh 

 water, but only in such water as is, or has been, in motion just 

 before the ice formation takes place. This observation is ne- 

 cessary, to give a clear idea of the phenomenon. In a mass of 

 water which is mixed, on account of its motion the several 

 layers of water, of different temperatures, cannot arrange them- 

 selves according to their original specific weight ; but the 

 temperature of the water at the bottom may be the same as 

 of that on the surface. The experiments of Professor Wilcke, 

 in ] 769, respecting ice formations in fresh water, which had 

 been cooled below freezing point, elucidated this. Professor 

 Wilcke found, amongst other things, that if cold water at 

 freezing point is poured into a colder glass, or if cold quick- 

 silver is stirred up in it, or cold shot poured into it, a quantity 

 of little ice figures rise up in the water, which appear as if 

 they were the commencement of the formation of a mass of ice. 

 These figures consist of little circular thin flat clear plates of 



