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III. On Ground Gru, or Ice formed, under peculiar circumstances, at the bottom of 
Running Water. By James Farquharson, LL.D. F.R.S., Minister' of the Parish 
of Alford. 
Received March 11, — Read March 25, 1841. 
In a paper of mine on Ground Gru, or ice formed at the bottom of running water, 
which was honoured with a place in the Philosophical Transactions*, I had inferred, 
from a great many conditions attending a remarkable occurrence of the phenomenon 
in the rivers Don and Leochal, in the beginning of January 1835, as well as from its 
occurring only when the air is at the time quite clear, that it is caused, when the 
water has gone down in temperature to the freezing point, by the bottom of the water 
being cooled to a still lower temperature, in the same manner as the surface of the 
dry land, under a clear sky, is cooled down below the temperature of the air, as first 
demonstrated by the experiments of Dr. Wells. 
As the accuracy of the conclusion at which I arrived respecting the question has 
been controverted, I respectfully request the Royal Society to permit me to present 
to them brief notices of some recent occurrences of ground gru, in the same rivers to 
which I formerly referred, the conditions of which seem to me strongly to confirm the 
accuracy of the views I presented regarding the cause of the phenomenon ; and also 
to answer some of the objections which have been brought against it. 
Cold weather commenced on the 20th December 1840 (on which night the ther- 
mometer went down to 31°), and continued with frost every night, yet never below 
26°, and with frost also through most of the day, till the 31st of the same month. 
By the 2Gth December, surface ice in considerable quantity was formed on the edges 
of the small river Leochal, and the temperature of the water was down to the free- 
zing point. Down to the evening of the 28th the weather was cloudy, and there was 
no appearance in the river of anything resembling ground gru ; but on that night 
the sky suddenly became clear ; and before the morning of the 29th, the bottoms of 
all the rapids of the little river were thickly coated by the ground gru. The gru dis- 
appeared as speedily as it had formed, when, on the 29th, a close cloud, depositing 
slight showers of snow, again covered the whole sky, and continued till the tempe- 
rature of the day and night rose above freezing. 
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 
* Part II. for 1835, p. 329. 
