[BARNES] * ANCHOR-ICE FORMATION 107 



In comparison with this, I would refer to a series of frosty days 

 from the 1st to the 11th of February, 1841, with a temperature the 

 same as from the 22nd to 31st December, 1840, never descending below 

 26°. The water of the river descended to the freezing temperature, 

 and surface ice was formed in large quantity on the edges of both the 

 Leochal and the Don. A dense cloud covered the sky during the 

 eleven days and nights, and no ground-gru appeared in the rivers. 



A remarkable occurrence of ground-gru took place in both the 

 rivers from thie evening of the 7th to the morning of the 9th January, 

 1841, with a completely clear sky during the time. The thermometer 

 was at 2° below zero on the night of the 7th, at 9° at midday on 

 the 8th, and at 7° below zero on the night of the 8th. I lexamined 

 particularly the state of the Don, during this extreme and clear frost, 

 before it abated on the morning of the 9th. The bottom of the river 

 was everywhere coated by an immense quantity of ground-gru, excepting 

 where it was partially shaded by bridges, or lofty banks close to the 

 stream. In the partially shaded places the bottom was clear of gru. 

 Thus, this remarkable formation of ground-gru took place under exactly 

 such circumstances as those in which hoar frost or dew takes place 

 on thje dry land, when the surface of the earth becomes colder than 

 the air (which we explain by a radiation of heat from the surface 

 of the earth into the clear sl^y, or by impulses of cold from the sky 

 to the earth), with only this difference, that there was an additional 

 transparent iluid over the bottom of the river, namely, the water; and 

 thus also a shade prevented the formation of ground-gru in the river, 

 as it does that of hoar frost or dew on the land. 



In noticing the objections to the explanation I have given of the 

 cause of ground-gru, I shall confine myself to those brought forward 

 by a writer in the Penny Cyclopaedia, under the name of Ground 

 Gru, which I have seen only very lately, although I believe they have 

 been published for some years. He says, the explanations of the 

 formation of ground-gru, given by Dr. Farquliarson and Mr. Eisdale, 

 are least of all satisfactory, and adds, "The former gentleman says 

 it is the result of radiation, and endeavours to substantiate his reason- 

 ing upon the principles of the formation of dew, seeming to forget 

 entirely, that Dr. Wells maintains expressly, that wind and shade are 

 alike obstacles to radiation; and that cousequently a body of moving 

 Avater so deep as to be impervious to light, and particularly when cov- 

 ered, as in the case of the Neva, with a sheet of ice three fetet thick, 

 and as much more snow, must present an insurmountable obstacle to 

 the radiation of heat from the bottom of the river." 



