68 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



I. That there is proportionality between the radiation and the 

 energy of the current, used for compensation, down to very low values 

 of both of them. 



This is a very important point, as far as the utility of the instru- 

 ment is concerned. The truth of the statement is clear from the fact 

 that we can add up small portions observed and get a sum equal to 

 the total quantity observed. 



II. That the way in which the distribution curves have been extra- 

 polated down to 90 zenith angle must be nearly correct. 



III. That the sky must have been very uniform during the time of 

 observation. If this had not been the case, it would not have been 

 possible to calculate the total radiation from observations upon a 

 single vertical circle. 



From the diagrams it is to be concluded that the maximum of 

 radiation from a horizontal surface toward rings of equal angular 



Table X 



Observer 



Homen 



Angstrom i 1 



Angstrom 2 1 



Angstrom 3 2 



Angstrom 4- 



Angstrom 5 2 



x Mt. Whitney (4,420 m.). 



1.5 



3-6 



3.8 

 5-0 



7-1 



2 Bassour (1,160 m.). 



width takes place in a direction that makes an angle of between 35 ° 

 and 45 with the zenith. An increase of the water- vapor density of 

 the atmosphere shifts this maximum nearer the zenith; with de- 

 creasing density the maximum approaches a limiting position of 45 °, 

 which it would have if no absorbing and radiating atmosphere 

 existed. 



In table X, which is obtained by measuring the corresponding 

 areas in figure 12, the ratios are given between the values of the 

 radiation within various zones, obtained from the observations, and 

 the same values as calculated from the simple sine-cosine law, that is, 

 for the case where a horizontal surface radiates directly to a non- 

 absorbing space. Hereby the radiation is assumed to be unity for 

 zenith angle o°. Between 8o° and 90 the radiation is only between 

 0.5 per cent and 2.0 per cent of the total radiation. The influence 



