4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 94 



calorie from those first computed, have been substituted for the broad- 

 casted Montezuma observations of the solar constant of radiation 

 for the past few years. 



Our first care was to reduce the St. Katherine results to the same 

 average scale as Montezuma. The scale depends on the adopted con- 

 stants of the pyrheliometers, and no favorable weather for determin- 

 ing these was available in Washington before the departure of the 

 Egyptian expedition. The scale correction was obtained by comput- 

 ing the average difference for i8o individual days of good quality 

 at both stations. The resulting mean difference, 0.034 calorie, was 

 subtracted from all St. Katherine daily values. 



Next the average mean difference of the daily results of good 

 quality common to the two stations was computed for each month 

 without regard for sign, as shown in table i. together with the num- 



Table I. — Monthly Averages of Daily Differences, Montezuma Minus 

 St. Katherine 



Year Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 



( IMean Value 7 



'''^^ 1 Number days 4 



J Mean Value 487 i4 12 9577 n "8 



'^■''^ ( Number days 4 7 'o 13 5 1 1 19 19 26 14 ' i 8 



( Mean Value 13 12 8 12 



'^^^ ( Number days 9 4 7 n 



General mean, 1S2 days, 0.0086 calorie. 



ber of days represented in each mean. The values given are in 

 thousandths of a calorie. 



Thus the general mean of all the daily dift'erences, numbering 182 

 in all and covering a period of 17 months, is approximately 0.45 per- 

 cent of the solar constant, taken as 1.940 calories. Omitting 6 aber- 

 rant values, it is 0.0079 calorie, or approximately 0.40 percent. Divid- 

 ing by V^, we find the average daily accidental error of a single 

 station is 0.30 percent. 



It was particularly important to determine whether the daily dif- 

 ferences, Montezuma minus St. Katherine, show a yearly period. 

 Accordingly mean values of them were computed monthly having 

 regard to sign. The results are given in table 2. They are expressed 

 in thousandths of a calorie. 



Table 2. — Monthly Averages of Daily Differences, Montezuma Minus 

 St. Katherine Taken with Regard to Sign 



Year Month Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 



1933 Mean +' 



1934 Mean +3-4 o +14 +12 —4 +1 -f2 -3 -9 +4 +2 

 193s Mean +13 — i —6 — 6 



