FORMED AT THE BOTTOM OF RUNNING WATER. 335 



for a greater distance immersed in the water, and were intercepted by, and mecha- 

 nically retained against, the faces of the stones by the action of the stream at the 

 heads of the pools. Further down, and in stiller water, where no such intercepted 

 heaps were seen, their buoyancy had, no doubt, by degrees, overcome the cohesion 

 and raised them to the surface ; and in fact, in the still water, many minute icy frag- 

 ments were floating in the surface. 



Mr. Knight, the celebrated botanist, quoted by M. Arago, has obviously, in part, 

 but not completely, distinguished between the " frozen matter which reflected a sil- 

 very kind of whiteness," which covered the stones in the rocky bed of the river, and 

 " floating spiculae under water," which he found to " accumulate much more abun- 

 dantly upon such parts of the stones as stood opposed to the current, where that was 

 not very rapid, below the little falls or very rapid parts of the river." 



In the smaller stream of the Leochal, the quantity of ground-gru was compara- 

 tively much more abundant, occupying the bottoms both of the pools and rapids in 

 close masses, and in the latter, at many parts, forming such an impediment as to 

 urge the water over its usual banks. But there were two remarkable exceptions. 

 One of the pools flows close to the foot of a steep bank about fifteen feet high, and 

 in the side next the bank there was little ground-gru. In a rapid, which at a turn 

 of the river has an easterly course, there was a very dense fringe of Phalar-is arundi- 

 nacea standing, with its dense foliage of withered leaves, in the south edge of the 

 water. Its height was four feet, and it extended fourteen feet in length along the 

 stream. At the foot of it the bottom of the rapid was clear of ground-gru to the 

 breadth of three feet. 



The temperature of the air and water, at the time of these observations, was parti- 

 cularly ascertained. That of the air at sunrise, about an hour before the observations 

 commenced, had been 23° Fahr. ; but it was rising rapidly during their progress, and 

 was at 36° Fahr. before their conclusion. The temperature of the water in the Don 

 varied from 32° to 33° Fahr. ; but the variation could not be distinctly traced as de- 

 pending on the depth or velocity, as there was a temporary variation in the same 

 place, both in the pools and rapids. At one of the small streams, returning from 

 under the sheet-ice on the little pools at the edge of one of the rapids, the tempera- 

 ture was nearly steady at 33° Fahr. In the Leochal the temperature was nearly 

 steady everywhere at 32° Fahr. 



By 10 o'clock A.M. on the same day, a cloud obscured the whole sky, and at 2 o'clock 

 P.M. the temperature of the air was 40° Fahr. At this time much gru rose from the 

 bottom and floated down the streams of both rivers. The relaxation of the frost, 

 however, was of very brief continuance. Before sunset the temperature of the air 

 was again down to 31° Fahr., with a perfectly calm air and clear sky ; and the clear 

 sky continued till the evening of the 7th of January, the thermometer during the two 

 intermediate nights being at 23°, and during the intermediate day at 26°. 



The same parts of the Don and Leochal were again examined at 10 o'clock a.m. on 



MDCCCXXXV. 2 X 



