94 



Sierra Club Bulletin. 



could be seen floating off to the southwest, low down on 

 the Sierra flank. But with the rising of the warm day 

 breezes the smoke would gradually advance up the valley, 

 becoming denser by degrees, until by nine or ten o'clock 

 one could scarcely see across from rim to rim. This con- 

 dition would prevail all day until with the afternoon re- 

 versal the down-valley wind would set in again and take 

 the smoke back with it. Much to the chagrin of the writer, 

 who at the time was engaged in the survey of the valley 

 and depended on the clearness of the air for his long- 

 distance sights, this daily smoke invasion persisted for 

 four long months with scarce an interruption. It may be 

 imagined that he came to understand the phenomenon 

 right well. 



Oddly enough it is precisely upon this daily atmos- 

 pheric seesaw that one of the Yosemite's chief attractions 

 depends. As is well known, one must go to Mirror Lake 

 at an early morning hour, if he wishes to see it at its best. 

 The surprised and usually somewhat vexed tourist who 

 finds he must arise at an impossible hour in order to enjoy 

 a perfect reflection, little dreams that what he is under- 

 taking really amounts to keeping a tryst with the early 

 morning reversal out on the shores of Mirror Lake ; and 

 that, unless he be quite punctual he will miss it because of 

 its almost momentary briefness. Yet such is actually the 

 case. The stillness of the water surface sets in just as the 

 down-valley draft dies out; but as soon as the upper 

 cliffs of Tenaya Canon become sufficiently insolated, up- 

 drafts begin to stir the air again, and a faint tremor forth- 

 with steals over the lake. Accepting the correctness of 

 this explanation, one is tempted to believe there might be 

 another calm corresponding to the afternoon reversal, — 

 an ever so much more convenient hour for the tourist. 

 But alas, experience has shown that this cannot always 

 be depended on. The reason is, no doubt, that in the 

 afternoon there is no well-defined pause in the circulation 

 of the air of Tenaya Canon, because of the presence of 

 great shadows on its north side which send down eddying 

 breezes at various times. 



