SUN'S HEAT — ABBOT. 147 



It has long been known that the aurora borealis, or northern lights, 

 have been particularly active at times of sun-spot maximum, and as 

 these lights are electrical disturbances in our atmosphere, it has come 

 to be believed that the sun furnishes not only light rays but also 

 bombards us with electric ions. Electric ions are known from labora- 

 tory experiments to promote the formation of clouds. Hence, it is 

 quite possible that the electric bombardment of the earth by the sun, 

 being more vigorous at times of sun-spot maximum, tends to promote 

 cloudiness; which in turn indirectly, by reflecting away solar radia- 

 tion, actually diminishes the amount available to warm the earth, 

 although the direct tendency of increased solar activity is to increase 

 the earth's supply of radiation. 



The short irregular fluctuations in the sun's radiation amount 

 sometimes to 3 or even 5 or 7 per cent within a few days. Although 

 they seem to be slightly associated with the rotation period of the 

 sun, as if rays at different strength were sent out by the sun in differ- 

 ent directions which, after a full rotation of the sun accomplished in 

 26 or 27 days, come round again in the direction of the earth, yet in 

 general these fluctuations are nonperiodic and accordingly not pre- 

 dictable. Lately their cause has received a very unlooked-for but 

 probable explanation by comparison of the solar observations of the 

 Smithsonian Institution at Calama, Chile, with photometric observa- 

 tions of Doctor Guthnick of the Observatory of Berlin, Germany. 

 Employing a photo-electric cell, Doctor Guthnick compared the 

 brightness of the planet Saturn with the star Regulus during Janu- 

 ary, February, March, April, and May, 1920. Shining by reflected 

 sunlight, Saturn must vary if the sun varies. Doctor Guthnick, how- 

 ever, on comparing his results with those reported from the Smith- 

 sonian Institution on the brightness of the sun, could see no corre- 

 spondence between the small solar and planetary fluctuations which 

 occurred. In his comparison, however, he assumed that whatever 

 changes might occur in the sun would make themselves felt in all 

 directions simultaneously. This is not necessarily so, for if the sun 

 should be surrounded by an obscuring atmosphere thicker in some 

 directions than others, rays of different intensity would go out to 

 different quarters; so that the earth, being in a certain direction from 

 the sun, might receive rays of a different strength from those which 

 were emitted in the direction of the planet Saturn. As the sun ro- 

 tates upon its axis once in about 27 clays (the actual time differs for 

 different parts of the sun), the ray which reached the earth would 

 sweep around perhaps in one, two, or three days to or from the posi- 

 tion of Saturn at the time of Guthnick's measurements, so that one 

 would expect Saturn's reponse to the change noted in the solar radia- 

 tion perhaps two or three days later, or two or three days earlier, 

 exactly according to the relative positions of the two planets. 



