6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



I was led by several circumstances to think that the nocturnal 

 radiation to the sky would be found to be a function of the water- 

 vapor content of the atmosphere and, as a consequence, observations 

 were made with wet and dry thermometers simultaneously with the 

 measurements of the radiation. In order not to introduce unneces- 

 sary influences that might modify this expected effect, it was con- 

 sidered important always to observe under a perfectly clear sky. It 

 was found that a few scattered clouds, far from the zenith, seldom 

 seemed to have any appreciable influence upon the radiation, but, in 

 order not to introduce conditions of the effect of which one could 

 not be quite sure, all the observations made at Bassour and used in 

 this paper were made under a perfectly cloud-free sky. The climatic 

 conditions were favorable for this program, and observations were 

 taken almost every night under a clear sky. Observations were also 

 made of the radiation to different parts of the sky, this study being 

 considered as of special interest in connection with the general 

 problem. 



It was my purpose also to make an investigation of the influence 

 of altitude upon the radiation to the sky, and in fact some prelimi- 

 nary measurements were carried out with a view to the investigation 

 of that problem. Thus I made observations one night in the valley 

 of Mouzaia les Mines, situated at the foot of the peak of Mouzaia 

 among the Atlas Mountains, about 15 miles from Bassour. The 

 height of the valley above sea level is 540 meters. Simultaneously 

 Dr. Abbot observed at Bassour (1,160 m.) on this particular night, 

 as well as during the following one, when I took measurements on 

 the top of Mouzaia (1,610 m.). The result of these observations 

 will be found among the investigations of the California expedition, 

 one of the purposes of which was to consider more closely the 

 problem of the influence of altitude upon the radiation of the atmos- 

 phere. For assistance with the practical arrangements in connection 

 with the expedition to Mouzaia my hearty thanks are due to M. de 

 Tonnac and M. Raymond, property owners. 



As the most important result of the observations in Algeria it 

 was found that the water vapor exerted a very marked influence 

 upon the nocturnal radiation to the sky ; a change in the water- vapor 

 pressure from 12 to 4 mm., causing an increase in the nocturnal 

 radiation amounting to about 35 per cent, other conditions being 

 equal. From the observations it was possible to arrive at a logically 

 founded mathematical expression for this influence. 



