336 REV. JAMES FARQUHARSON ON THE ICE 



the 7th. In the Don the ground-gru now covered all the bottoms of the pools as 

 well as of the rapids. It was of less depth in the deep still pool below the great 

 rapid ; but everywhere else it formed a great impediment to the stream, raising it so 

 much above its former level that it covered deeply the pieces of sheet-ice formed at 

 the edge on the 5th. New pieces of similar ice were now forming at the same places 

 on the more elevated surface. The Leochal was still more impeded by the gru than 

 the Don. 



But, what is worthy of particular notice, the clear spaces of the bottom, at the 

 piers, abutments, and embanking-walls of the bridge on the Don, and at the Phalaris 

 grass in the Leochal, still continued so, but were now considerably narrowed in their 

 lateral dimensions, the ground-gru having encroached upon them on the sides next the 

 streams. The temperature of the air was 24° Fahr. ; of the water, everywhere nearly 

 steady at 32°. 



Several circumstances occurred on some subsequent days which deserve to be no- 

 ticed, as throwing light, by the contrast which they exhibit, on the phenomenon now 

 under consideration. On the 8th of January there occurred a thaw, when the ther- 

 mometer suddenly rose to 47° Fahr. The rivers were speedily cleared of ice and 

 ground-gru, which last rose from the bottom and floated away with the stream. The 

 atmosphere at the time was considerably clouded, with a brisk S.W. wind. On the 

 9th of January the temperature of the air fell to 36° Fahr. ; and on the morning of 

 10th of January, with a temperature of the air at 29° Fahr., there was a fall of snow, 

 of about an inch deep, which ceased by 8 o'clock a.m. The snow that fell into the 

 rivers was observed to be entangled, and stuck fast, in irregular crushed masses, in 

 many parts of the rapids ; and there were collections formed of loose spiculse of a 

 muddy aspect, at the sides of the stones opposed to the streams, in the heads of the 

 pools, where the velocity of the currents was intermediate between that of the rapids 

 and that of the stiller parts of the pools ; but there was no appearance on any part 

 of the bottom resembling the symmetrical cauliflower-shaped ground-gru. On the 

 evening of the 10th the temperature of the air fell to 23°, and continued at from 23° to 

 21° till the morning of the 12th, with a densely clouded state of the sky. During this 

 time extensive sheets of surface-ice were formed on the pools of the Don, and many 

 of the pools of the Leochal were quite frozen over, but the ground-gru was nowhere 

 renewed; on the contrary, the masses of snow entangled in the rapids on the 10th 

 disappeared to a great extent, obviously floating away in the stream. In this state 

 of the river and weather, the collections of uncemented spiculae, on the faces of the 

 stones opposed to the streams in the heads of the pools, appeared in their places the 

 same as before, neither increasing nor diminishing in size. 



M. Arago, in his paper, refers to three circumstances, as partly, at least, expla- 

 natory of the formation of ground-gru in running water. 



1st. The inversion, by the motion of the current, of the hydrostatic order, by which 

 the water at the surface, cooled by the colder air, and which at all points of the tern- 



