6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 59 
1910, had strongly indicated that the sun is really a variable star. 
The fluctuations in the amount of solar radiation seem to be of magni- 
tudes seldom exceeding 5 per cent, but to occur in irregular periods 
of 5 or 10 days. It is not easy to predict just what effects such changes 
in solar radiation would produce on terrestrial climate, but that they 
must have some influence, and quite possibly a notable one, seems 
highly probable. Hence if the supposed solar changes are real, it 
will doubtless be necessary for meteorological purposes as well as for 
general interest to observe the sun regularly at all times of the year, 
in order to detect and record its variations. 
Fic. 3.—Extra observers. Photograph by Abbot. 
But before going to the expense of such additional observing 
equipments as will be necessary for daily measuring the solar 
radiation, it is indispensable to prove definitely that the sun really 
varies. While the Mount Wilson work seems to recommend this 
conclusion very strongly, it is not perhaps impossible that local atmos- 
pheric conditions may have had some influence there, so that what 
seems most likely to have been solar changes may possibly have been 
of atmospheric origin. To exclude this possibility it is necessary to 
show that the same results would be reached by simultaneous obser- 
vations at another station far from Mount Wilson. 
