Febeuaet 4, 1910] 



SCIENCE 



193 



currents in mountain valleys. The fact of the 

 nocturnal descent of air on mountain sides 

 and along the floors of mountain valleys is 

 familiar, and in some cases a deepening of the 

 current during the night has been noted. The 

 present report describes the method of occur- 

 rence of the lateral drainage only. The ob- 

 server found that during the daytime, pro- 

 vided the sun shone, a distinct set of the air 

 toward the valley bottoms was noticeable in 

 the shadows of cliffs, while in the sunlight no 

 movement was discernible. One case, where 

 a vertical cliff cast a well-defined shadow, 

 showed that by going even so short a distance 

 as twenty-five feet, one moved from uncom- 

 fortable heat into a cooling breeze. This 

 descent of air in the shadows was undoubtedly 

 due to a cooling similar to the more often ob- 

 served nocturnal phenomenon, though on a 

 very small scale. 



A similar control over nocturnal winds was 

 noted by the writer a few years ago near the 

 foot of the niecillewaet Glacier, in the Canad- 

 ian Selliirks. The valley of the Illecillwaet 

 River, which flows northwestward from the 

 glacier, is very steep walled. This, with the 

 presence of the ice, affords ideal conditions 

 for nocturnal downcast winds. About sunset 

 on the day in question, the writer was stand- 

 ing near the foot of the glacier, but somewhat 

 upon the east side of the valley. The air was 

 perfectly calm, and the temperature in the 

 full sunlight gave no indication of the pres- 

 ence of the ice. The west side of the valley 

 was already in shadow. As the edge of this 

 shadow crossed the valley floor, a distinct 

 movement of foliage within the shadow be- 

 came evident. The zone of movement 

 widened, keeping pace with the advance of the 

 shadow; and as the edge of the latter passed 

 the observer on its way up the east wall of the 

 valley, the edge of the zone of foliage move- 

 ment lagged a hardly perceptible distance be- 

 hind, and was seen to move up the slope to the 

 limit of the bushes. Possible movement be- 

 yond this point was rendered invisible by the 

 distance and character of vegetation on the 

 higher slopes. Almost at the instant of the 

 passing of the shadow edge, a gentle puff of 



cold wind down off the glacier announced the 

 beginning of the nocturnal descent of air. 

 HaM an hour later, at the hotel some distance 

 down the valley, the night wind was already 

 blowing moderately and the temperature had 

 dropped many degrees. 



It is improbable that the upper limit of 

 foliage movement indicated the depth of the 

 down-valley current in " mid-stream." The 

 rapidity of ascent of the shadow would call 

 for the sudden beginning of movement of a 

 mass of air so large that it could not possibly 

 have been cooled thus quickly throughout. 

 Instead, the upper limit of a relatively thin 

 sheet of cooling air which was moving more 

 or less directly toward the valley bottom, was 

 indicated. 



Observation may prove that this lateral 

 movement, while showing near its upper lim- 

 its a fairly direct downward course, turns 

 more and more obliquely down the valley 

 under the influence of the drag of the air- 

 stream proper. Careful study might also 

 show whether the surfaces of such down-val- 

 ley currents assume the slight convexity noted 

 in the case of water-streams, or whether the 

 constant lateral accessions of air tend to pro- 

 duce a diminishing concavity of surface as 

 the stream slowly deepens during the night. 

 B. M. Varney 



Habvaed Univeesity, 

 January 6, 1910 



8CIENTIFI0 BOOKS 

 Outlines of Chemistry : A Text-Boole for Col- 

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 In a clear and interesting style the author 

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 Professor Kahlenberg has accomplished his 

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