124 On the Ground-ice observed in 



rectly opposed to the laws of the propagation of heat. But it is 

 so. This evidence is not awanting ; and if the phenomenon of 

 ice in the bottom of water has only appeared recently as an 

 established fact in treatises on physics and meteorology, the 

 reason is, because their authors generally copy from each other, 

 because every one neglects what his predecessor neglected, and 

 because academical collections, in which many treasures remain 

 concealed, are very seldom consulted. 



In 1730, at an atmospherical temperature of — 9° centigr. 

 (15°8 Fahr.), Hales saw at Teddington, the surface of the 

 Thames, near the banks, covered with a layer of ice one-third 

 of an inch in thickness. There was also at the same time a se- 

 cond layer below, of greater thickness, which followed the depth 

 of the river, as it adhered to the bottom. This sheet was united 

 to the upper one even on the water-side ; but it was gradually 

 separated in proportion, as, in proceeding into the river, the 

 depth of the water increased. It was not so solid as the first, 

 and was mixed with sand, and even stones, which the flakes 

 sometimes carry with them in their movement upwards. 



This observation is defective, inasmuch as it was made too 

 near the bank. Those who do not know how imperfectly 

 every kind of soil transmits heat, might suppose that the 

 cold was communicated from the dry ground of the bank to that 

 which formed the bed of the river by means of conductibility. 

 It is unnecessary to discuss this difficult point, as it has no con- 

 nexion with many of the cases which are about to occupy our 

 attention. 



It is really surprising that those writers who have lately con- 

 sidered the subject of floating ice in a historical point of view, 

 have not alluded to some observations which were made in 

 France a good many years ago. 



At the close of December 1780, the temperature was very sud- 

 denly increased in the southern parts of France by a very strong 

 northerly wind. The thermometer sunk to 8° or 7° centigr. 

 below zero. Desmarest, member of the Academic des Sciences, 

 who, at that time, happened to be at Annonay, saw the bed of 

 the Deome covered with spongy ice. The frost commenced at 

 first on the margin of the river, where there was a depth of wa- 



