CHAPTER I 

 PROGRAM AND HISTORY OF THE EXPEDITIONS 



It is appropriate to begin this paper with a survey of the external 

 conditions under which the work upon which the study is based was 

 done. Most of the observations here given and discussed were 

 carried out during two expeditions, one to Algeria in 1912, the other 

 to California in 1913. An account of these expeditions will give an 

 idea of the geographical and meteorological conditions under which 

 the observations are made, and it will at the same time indicate the 

 program of the field work, a program that was suggested by the 

 facts referred to in the historical survey of previous work and by 

 the ideas advanced in the chapter on the theory of atmospheric 

 radiation. 



In 1912 I was invited to join the expedition of the Astrophysical 

 Observatory of the Smithsonian Institution, led by its Director, Dr. 

 C. G. Abbot, whose purpose it was to study simultaneously at Algeria 

 and California the supposed variations of the radiation of the sun. 

 In May of that year I met Dr. Abbot at Bassour, a little Arab village 

 situated about 100 miles from Algiers, in the border region between 

 the Atlas Mountains and the desert, lying at 1,100 meters above sea 

 level. This place had been selected by Dr. Abbot for the purpose 

 of his observations on the sun, and on the top of a hill, rising 60 

 meters above the village, his instruments were mounted under ideal 

 conditions. The same place was found to be an excellent station 

 for the author's observations of the nocturnal radiation. A little 

 house was built of boards by Dr. Abbot and myself on the top of the 

 hill. This house, about 2 meters in all three dimensions, was at the 

 same time the living room and the observatory. The apparatus used 

 for the nocturnal observations was of a type which will be described 

 in a later chapter. Its principal parts consist of an actinometer, to be 

 exposed to a sky with a free horizon, a galvanometer, and a milliam- 

 meter. At Bassour the actinometer was mounted on the roof of 

 the little observatory, observations of the galvanometer and the 

 ammeter being taken inside. The horizon was found to be almost 

 entirely free. In the north some peaks of the Atlas Mountains rose 

 to not more than half a degree over the horizon, and in the south- 

 east some few sandy hills screened off with their flat wave-like tops 

 a very narrow band of the sky. 



3 



