THE STUDY OF TROPICAL SUNLIGHT. H 



Station and latitude. Intensity. 



Cape Horn, 55° 31' S. 1.47 



Washington, 38° 54' N. 1.44 



Montpelier, 43° 36' N. 1.60 



Modena, 44° 39' N. 1.37 



Kief, 50° 24' N. 1.39 



Warsaw, 52° 13' N. 1.35 



Hald, 56° 25' N. 1.82 



Katherinenburg, 56° 50' N. 1.58 



Pavlovsk, 59° 41' N. 1.48 



Upsala, 59° 51' N. 1.35 



St. Petersburg, 59° 56' N. 1.47 



Treurenburg, 79° 55' N. 1.29 



These variations are not great, and such as appear, are at- 

 tributed by Kimball to instrumental rather than to atmospheric 

 conditions. Angstrom ^'-^ publishes some results from Teneriffe 

 (20° 30' north) in which he compares Guimar (360 meters al- 

 titude) with Alta Vista (3,352 meters altitude) and obtains 

 1.38 at noon for Guimar and 1.618 for Alta Vista, the latter 

 higher figure is to be expected owing to the altitude. The 

 maximum observed by Dr. Rudolph Schneider at Vienna (48° 13' 

 north) -° was 1.524 in February, and figures ranging from 1.00 

 to 1.455 are quite frequent; indeed, the observations for the 

 time close to the noon hour in Vienna, although averaging some- 

 what lower, bear a remarkable resemblance to those in Washing- 

 ton, when we consider that Kimball worked only on clear days 

 and Schneider made observations on days of partial cloud and 

 even of fog. Mr. Harvey N. Davis, working at Providence, 

 Rhode Island, in ten months observed a maximum of 1.328 in 

 March, and in general his figures also bear a striking resemblance 

 to those obtained in Vienna. Kimball, in discussing the annual 

 march of radiation as compiled by him, states that "a rather 

 surprising uniformity throughout the year (is shown) in the 

 maximum intensity of radiation, the December minimum being 

 only 8 per cent less than the April maximum." The departures 

 by months from the average quinquennial mean show that there 

 is a considerable variation by years, amounting to a minus 

 quantity of as much as 18 per cent on the average for the year 

 1903. This diminution was widespread and such low times are 

 periodic; the same is probably true of high periods, so that the 

 absolute amount of insolation on the earth's surface may vary 



" Astrophys. Joum. (1899), 9, 342. 



^ Jahrh. d. k. k. Zentralanstalt f. Meteorol. u. Geodyn. (1906), n f. 43, 12. 



