[BARNES] ANCHOR-ICE FORMATION 99 



In regard to the former of these points, I cannot say what M. 

 Arago wonld have deduced from it, had it been established in one 

 way or the other. The observations made on the Don on the 5th of 

 January show that the temperature of the whole water was not quite 

 down to 32° Fahr. when the ground-gru was forming in large quantity. 

 In regard to the latter, the little icy particles seen by Mr. Knight, 

 the same condition belongs to them that belongs to the circumstances 

 professedly adduced by M. Arago, as explanations; that is, they occur 

 as well when the ice is forming on the surface only as when it is form- 

 ing on the bottom. They account well, however, for the collections 

 of frozen matter seen by him at the sides of the stones opposed to the 

 stream, in parts where its velocity had a certain modification. 



And here I may advert to the explanation ofl'ered by the Kev. 

 Mr. Eisdale, in his paper already referred to. Prom the information 

 he received, he was led to believe the ground-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 descended, brought into contact with the rocks by the agitation 

 cf 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 tbe 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 

 3Ir. Eisdale, from the existence 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 spiculae 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 

 plow 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. j\IcKeever, quoted by M. Arago, had gone far to illus- 



