106 ROYAL SOCIETY OF CANADA 



On Ground-gru^ or Ice Formed, under teculiar circumstances, 



AT THE BOTTOM OF RUNNING "WaTER, 



By James Farquharson, LL.D., F.E.S., 

 Minister of the Parish of Alford. 



From Philosophical Transactions, Vol. 131, p. 37 (1S41.) 



Tn 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 

 remarkable occurrence of the phenomenon in the rivers Don and Leo- 

 chal, 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 phenom- 

 enon; 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 thermometer 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 26th December, 

 surface ice in considerable quantity was formed on the edges of the 

 small river Leochal, and the température of the water was down to the 

 freezing 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 disap- 

 peared 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 temperature of the day and night rose above freezing. 



^ Part II, for 1835, p. 329. 



