92 ROYAL SOCIETY OF CANADA 



this was the condition of matters during the observations now to be 

 ^ detailed. But it also occurs during a frosty wind, when there is no 

 hoar frost, which is formed only in a calm state of the atmosphere. 

 The formation of the gru during wind, and consequently without any 

 deposition of hoar frost on the ground, is especially to be noticed 

 in reference to Mr. Eisdale's explanation, as will be afterwards seen. 

 . It occurred to M. Hugi, as quoted by M. Arago, in tiie Aar, on the 

 16th Eebruary, 1837, with a west wind, after the river had been com- 

 pletely open on the 15th; and one. of Mr. Eisdale's correspondent» 

 ascribed its occurrence in one particular instance, which he related to 

 him, to the prevalence of a very sliarp northeast wind, which had blown 

 during the night of its formation. 



The following observations were made in the rivers Don and 

 Leochal. The former having an easterly course, is about 120 feet 

 broad, and a foot or two deep at the shallows and fords. The latter, 

 one of the small tributaries of the former, having a northerly course, 

 is about 20 feet broad, and a foot deep at the shallows. Both rivers 

 possess a like character of very clear water, and alternating rapids and 

 |>ools. The rapids in the Don are reaches, where the water falls two 

 or three, or more, feet, from a higher to a lower level, within a distance 

 of fifty or a hundred, or sometimes two or three hundred yards. They 

 are generally impeded Avith many large stones, some of them projecting 

 above the water. The depth varies greatly, but seldom exceeds two 

 or three feet. The pools between the rapids are oij an average much 

 longer reaches, in which there is little fall, and a greatly diminished 

 velocity of the stream, which often, in them. Hows so equably as to 

 give rise to no ripple on the surface. They too have in them large 

 stones, but fewer in numl)er. The depth in them too varies greatly, 

 from two or three to four or five feet. ~ The rapids and pools in the 

 Leochal are of a similar kind, but both much less deep in this smaller 

 stream. The bed of this river has, however, on the whole, a steeper 

 descent, and owing to this there is more broken water and spray in 

 the rapids. The character of alternating rapids and pools, in both 

 streams, is owing to the varying hardness of the granitic and micaceous- 

 schistose rocks in which their beds are formed. Where the rocks are 

 hard, there is a rapid; where more friable, a pool. In the parts of 

 the rivers observed, the original rocks themselves do not anywhere form 

 the immediate bed of the stream. That, to the depth o$ two or three, 

 or more, feet, is composed of the debris of these rocks, broken up 

 and sometimes much waterworn, and reduced to the size of a very large 

 gravel, by the action of the stream, Init not so small as to deserv^e to 

 be named sand. No part of the bottom is muddy. 



