52 li. F. Reid—Studies of Miiir Glacier. 



crystalline rock above and the softer slate l)eloAV, and is prol)- 

 al^ly due to the different rates of erosion of these rocks. 



Meteorological Notes. 



The prevalent wind on the Alaskan coast is from the south- 

 west, but the glacier, by cooling the air in contact with it, pro- 

 duces a cold wind which slides down its slope. Thus a north- 

 easterly wind blew continuously at our camp except occasionally 

 when a strong southerly gale overcame it. On the Avestern trib- 

 utary the wind was from the west and in Main valley from 

 the northwest ; in fact, everywhere it flowed down the slope of 

 the glacier. Its influence on the temperature was very marked. 

 The mean temperature during July and August was 45°. 1 F., 

 about 10° lower than that at Juneau during the same period, 

 although this latter place is only about 35 miles further south- 

 ward. At no time during our stay, however, was a freezing tem- 

 perature reached. 



This cold wind did not usually extend very high ; frequently 

 mist could be seen moving northward not 1,000 feet above our 

 camp, where the flag was streaming toward the south. The 



Figure 3 — Diagram illustrating Refraction. 



temperature was higher on elevations than lower down . At V 

 (3,000 feet) the thermometer was once observed 6°.7 C. (12° F.) 

 higher than at camp ; also, at the same time, on the top of Tree 

 mountain (2,700) the temperature was 4°.3 C. (7°.7 F.) higher 

 than at camp. The increase of temj)erature with altitude causes 

 an unusually rapid decrease of density in the atmosphere, with 

 a corresponding increase in refraction, thus producing the mirage 

 which is so common here. It is noticeable only Avhen both the 

 observer and the object are in the cold layer. A ray of light 

 may reach the observer after following a horizontal path, or after 

 rising slightly and then being refracted down again. The result 

 is to make the object appear stretched out and to give it increased 

 height. We often saw islands with apparently vertical sides; 

 the icebergs in Glacier ]xay were magnified vertically so as to 



