532 0)1 tlie Ground fee observed in 



In 1742, when this strange theory saw the light {Observa- 

 tions sur les Ecrits modernes^ t. xxxi.), the thermometer was in 

 the hand of every person, and of course it could have been 

 easily ascertained that, during a hard frost, river water is in 

 general colder at the surface than at the bottom. But, as Mon- 

 taigne says, even in the facts which are laid before them, men 

 willingly amuse themselves in seeking for reasons rather than 

 truth ; they abandon things and fly to causes. 



To reconcile the theoretical objection which Nollet has made 

 to the popular opinion respecting ice at the bottom of water, 

 with the observations which incontestibly establish that the 

 greater part of the flakes which have been broken up have been 

 immersed for a longer or shorter period, and that their inferior 

 surface rests on a muddy bottom, it has been thought that the 

 origin will be found in the small streams which run into large 

 rivers. There, it is said, the water being shallow, the ice should 

 soon find itself in contact with the ground or mud with which 

 the bed is covered. As to the flakes of ice which rise beneath 

 the water, which sailors bring up with their hooks from a depth 

 of some feet, their existence is explained by remarking, that, 

 after a sharp frost followed by the commencement of a thaw, 

 there is sometimes a great increase, to which a new frost suc- 

 ceeds, so that there is in the river, but especially near the banks, 

 two layers of ice superimposed at a distance ; the one at the 

 height of the first level of the water, the other at the height 

 which this level has attained on the rise of the water. This 

 theory, which refers to a peculiar case, does not explain, in any 

 point of view, the observations just made, and in which natu- 

 ral philosophers have actually seen ice formed on the surface of 

 pebbles placed at the bottom of the water in the beds of certain 

 rivers. 



We now come to Mr M'Keever, who, confining himself closely 

 to the most subtle principles of the theory of heat, has not, on 

 this account, been more fortunate than his predecessors. 



According to this author, the rocks, stones, and gravel which 

 generally cover the bottom of rivers, have powers of radiation 

 superior to those of mud, perhaps on account of their peculiar 

 nature, but chiefly because they have rough surfaces. Thus rocks, 

 in large or small masses, will become much cooler in consequence 



