2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 82 



of solar radiation made at observatories thousands of miles distant 

 from each other, one in the northern hemisphere and the other in 

 the southern hemisphere ; so that the chance of both being afifected 

 by the same weather changes becomes very small. The solar radia- 

 tion reaching the earth is measured in calories per square centimeter 

 per minute, and averages about 1.940 calories. Table i shows a com- 

 parison of observations of solar radiation made simultaneously in 

 northern Chile and in the United States (first in California and then 

 in Arizona) during the years 1918 to 1924. The table shows the 

 frequency of different values observed in the United States for each 

 increase of .010 calorie in Chile. 



Table i. — Comparison of Solar Radiation lvalues in Chile and the United States 



(y umber of Cases) 



Values in Values observed in Chile 



United , ^ ^ 



States 1. 910-9 1.920-9 1.930-9 1-940-9 1-950-9 1.960-9 



1.890-9 166600 



1.900-9 II II I 000 



1.910-9 20 25 II 5 40 



1.920-9 18 38 21 5 4 2 



1.930-9 7 23 29 II II 



1.940-9 4 6 12 15 16 4 



1.950-9 4 10 10 13 6 



1.960-9 o I 3 4 7 5 



1.970-9 0' I I 3 2 



if there were no relation between the measurements at the two 

 stations, the observed values would be scattered through the dif- 

 ferent classes at random. The tabulation shows that a random dis- 

 tribution does not exist ; but for each group of observations in Chile, 

 there is a maximum near the same values in the observations in the 

 United States. There is, therefore, a progressive displacement of 

 the maximum frequency as the solar values increase from i. 910-9 

 to 1.960-9, or nearly three per cent of the mean value. The probable 

 error of the measurements is ± .006 calorie ; so that the solar varia- 

 tion during the interval covered by the observations was more than 

 eight times the probable error of each group of observed values. 



Since variability in solar radiation has been questioned by some 

 investigators, it is well to state that the evidence of this variability 

 rests on three fundamental and independent facts : 



(i) The changes in radiation are alike when measured at two 

 widely separated stations, allowing for variations from a middle 

 value due to errors of observation. 



