NO. 9 AREQUIPA PYRHELIOMETRY ABBOT 3 



values may be called the atmospheric transmission coefficients at 

 the given station for the first, second, and third air masses. As 

 shown by Forbes and many subsequent writers, aj<a 2 <a 3 , when, 

 as with the pyrheliometer, a complex beam including' many wave- 

 lengths is observed. 



Confining ourselves altogether in treating of atmospheric trans- 

 parency to the consideration of the quantity a 2 for the station Are- 

 quipa, as we shall do in this paper, we propose to investigate its 

 dependence on the amount of atmospheric humidity, and on the 

 season of the year. We hope that the observations may be continued 

 long enough to give good correlation factors in these respects, so 

 that in future years abnormal changes like those caused by volcanoes 

 will reveal -themselves, and their climatic influences may be studied. 

 Remarks on the influence of the dust from the Katmai eruption of 

 1912 will appear below. 



A second object of the work is to connect by empirical formulae 

 the values of intensity of solar radiation, atmospheric transmission, 

 and humidity as observed at Arequipa with the values of the solar 

 constant of radiation outside the atmosphere determined by the 

 spectro-bolometer at Mount Wilson. Thus it is hoped to employ 

 Arequipa observations to indicate variations of solar emission of 

 radiation. 



No sufficient object to justify printing all Arequipa pyrheliometer 

 values seems to exist. We therefore abridge the results as shown 

 in the following table. Generally observations were secured with 

 secant Z values as small as 1.3, and often as small as 1.05. To give 

 the best possible comparable values of pyrheliometer measurements, 

 we have interpolated the values for air mass I.2. 1 In addition we 

 give the values for 1.0 and 2.0 air masses whenever this can be done 

 with fair certainty. From these latter values come the transmission 

 coefficients a 2 . The humidity was determined sometimes by swing- 

 ing wet and dry thermometers, sometimes by the hydrograph. We 

 have compared results by the two methods, and have expressed all 

 in terms of pressure of aqueous vapor in millimeters of mercury. 

 The values given in the table are the mean values for the interval 

 *of time covered by the pyrheliometer measurements of each day. 

 The letters A, M, and P signify morning, noon, and afternoon, re- 

 spectively. In the two final columns, after the date and the initials 

 of observers and remarks, are given empirical determinations of the 

 solar constant of radiation, of which more will be stated hereafter. 



1 We shall use the term " air mass " in this paper as the equivalent of secant 

 Z, taking no acount of barometric pressure. 



