96 ROYAL SOCIETY TjF CANADA 



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

 that was not very rapid, below the iittle falls or very rapid parts of 

 the river.'' 



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

 was comparatively 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 Phalaris arundinacea 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 observ- 

 citions, was particularly ascertained. Tliat of the air at sunrise, about 

 an hour before the observations commenced, had been 23° Fahr. ; but 

 it was rising rapidly during their progre^ss, and was at 36° Fahr. 

 before their conclusion. The temperature of the water m the Don 

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

 traced as depending 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 temperature Avas 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 Tth 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 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 



