338 REV. JAMES FARQUHARSON ON THE ICE 



the groiind-gru does not occur but when there is a hoar frost on the ground ; and he 

 explains the ground-gru to be particles, or crystals as he afterwards names them, of 

 hoar frost precipitated into the water, retaining there the shapes in which they de- 

 scended, brought into contact with the rocks by the agitation of the water, and 

 forming nuclei for the accumulation of ground-gru. Could it be proved that such 

 crystals are precipitated into the water, they would serve no more for explanation 

 than the icy particles of Mr. Knight. We have learnt, indeed, from travellers in 

 high northern regions, that, in certain states of cold and moisture of the air, such 

 crystals, as Mr. Eisdale assumes, are there seen and felt floating in it ; but nothing 

 of that kind was observed in January last ; and when Mr. Eisdale, from the exist- 

 ence of spiculae of hoar frost on the ground, would infer the like may be formed in 

 the air to fall into the water, he neglects to take into the account, that the spiculse 

 of hoar frost have not fallen from above, but that their symmetrical arrangement, 

 round on all sides of the bodies on which they are found, and their slow increase, 

 prove they have been deposited on their places by a gradual deposition of invisible 

 watery vapour, owing to the substances to which they are attached being cooled 

 below the temperature of the surrounding air, by the radiation made known to us 

 by the experiments of Dr. Wells. Besides this we have to remark, that the ground- 

 gru sometimes takes place, agreeably to the information of one of Mr. Eisdale's own 

 correspondents, in a windy state of the atmosphere, at which time no hoar frost is 

 seen. 



The interesting experiments of Dr. Wells just referred to enable us to give, after 

 all, a very satisfactory explanation of the ground-gru ; and Mr. M'Keever, quoted 

 by M. Arago, had gone far to illustrate it by means of them, although he had over- 

 looked some conditions necessary to be taken into the account for a complete expla- 

 nation. M. Arago, however, entirely rejects the explanation of Mr. M'Keever, 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. 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 



* 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 observation 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. 



