lOO ROYAL SOCIETY OF CANADA 



trate it by means of them, although he had overlooked some conditions 

 necessary to be taken into account for a complete explanation. M. 

 Arago, however, entirely rejects the explanation of Mr. McKeever, and 

 it is fair to set down the terms in which he does so. 



After having shown that the ground-gru cannot be explained by 

 the action of the moon/ according to the sailors, nor by the friction 

 of running water producing more heat at the surface than at the 

 bottom, nor by referring its source to the smaller tributaries of the 

 streams, nor to different layers of ice formed at the several surfaces, 

 when the water in the river, from whatever cause, is in a state of 

 varying fullness, all of which have been assigned as causes of the 

 ground-gru, M. Arago proceeds : 



"^ We come now to Mr. McKeever, who, confining himself closiely 

 to the most subtle principles of the tlieory 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 bot- 

 toms 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 of radiation; when the atmospherical tem- 

 perature is very low, they of course freeze the water which touches 

 them. It is unnecessary to examine here whether heat radiates through 

 a thick layer of water, as Mr. McKeever supposes, as the most simple 

 observation is sufficient to overthrow it. Where is the person who has 

 not observed that the strong radiation, which the Irish philosopher 

 admits, would be more plainly manifested, or as completely, in still 

 water than in running water? But no one has seen a piece of still 

 water frozen at the bottom." - 



But there is nothing more easy of experimental proof than that 

 heat radiates through water. I do not mean, however, to vindicate 

 the reasoning of Mr. McKeever respecting the more powerful radiation 

 of it from stones and from mud. His reasoning respecting the matter 

 is, on his own part, conjectural, to explain the readier formation of 

 gru on a stony or gravelly bottom; but the gru also forms on a muddy 

 bottom, a fact which M. Arago notices, when he brings the attachment 

 of mud to the under side of the floating flakes as a proof that they 

 have been formed at tlie bottom. Mr. McKeever was driven to his 



^ This explanation of the sailors is a confirmation of what I have stated, 

 that the gru never appears but under a clear sky. The constant observ- 

 ation of the sailors has associated, in their minds, the shining of the moon 

 with the ground-gru; but the moon never shines, to excite great attention, 

 but in a clear sky. 



^ Edinburgh New Philosophical Journal, vol. xv, pp. 132, 133. 



