38 
DR. FARQUHARSON ON GROUND GRU. 
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 the 
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 mid-day on the 8th, and at 7° below zero on the night of the 8th. I ex- 
amined 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 the 
dry land, when the surface of the earth becomes colder than the air, (which we ex- 
plain by a radiation of heat from the surface of the earth into the clear sky, or by 
impulses of cold from the sky to the earth,) with only this difference, that there was 
an additional transparent fluid 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 Cyclo- 
paedia, 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. Farquharson 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 reasoning 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 consequently a body of moving water so 
deep as to be impervious to light, and particularly when covered, as in the case of the 
Neva, with a sheet of ice three feet thick, and as much more snow, must present an 
insurmountable obstacle to the radiation of heat from the bottom of the river.” 
Now, in the first place, with respect to shade ; I was so far from forgetting that it 
is an obstacle to radiation, that, on the contrary, in my observations in 1835, I had 
shown by very many instances, that shade had prevented the formation of ground 
gru, just as it prevents dew. Wherever shade intervened to prevent radiation from 
the bottoms of the rivers Don and Leochal, there no ground ice was formed ; while 
the unshaded parts of the bottoms were coated with it. My explanation thus mainly 
rested upon the fact that shade prevents radiation. In the next place, with respect 
to wind ; the writer in the Cyclopeedia himself forgets the difference of the statical 
conditions of air and water in connexion with temperature. Air becomes heavier by 
diminution of temperature. Water under 39° Fahr. becomes lighter by diminution 
