IgO ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



"mother-of-pearl" clouds (pi. 4) and found them to be in the strato- 

 sphere, about 15 miles above the ground. 



The strangest of clouds (pi. 5) are those observed in the middle 

 of the night, or twilight, which occur at a measured altitude of 60 

 miles. The heights have been measured in Norway " by the same 

 band of observers and at many of the same stations as those used in 

 the determinations of the distance of auroras from the earth's sur- 

 face. These clouds are not iridescent, and they move westward at 

 about 100 miles an hour. 



It will be gathered that the study of the Northern Lights is bound 

 up with other physical phenomena in the upper regions of our 

 atmosphere, and that progress can best be made, as in other branches 

 of science, by advance on a broad front. 



^ Carl St0rmer, Astrophysica Norvegica, vol. 1, no. 3, February 1935. 



