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III. On Ground Gru^ or Ice formed, under peculiar circumstances , at the bottom of 

 Running tVater. By James Farquharson, LL.D. F.R.S.y 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 2Cth 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 ctose 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 

 1 1th 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. 



