TROPICAL SUNLIGHT 525 



the total solar radiation per square centimeter of surface normal to the 

 rav of incidence. 



Unfortunately, measurements in the tropics with this instrument 

 are lacking and in Manila we have not yet been able to carry them out, 

 because the instruments which we have ordered have suffered great 

 delays in delivery. A large numl^er of available data have been gath- 

 ered by Dr. Herbert H. Kimball, of the Mount Weather Observatory, 

 and comparisons of the maximum intensity of solar radiation extending 

 from Cape Horn, on the south, to Treurenburg, on the north, show that 

 the variations are not great and those which appear, in the belief of 

 Kimball, are due to instrumental errors rather than to atmospheric con- 

 ditions. Angstrom gave some measurements from Teneriffe (20° 30' 

 north) and obtained practically the same figures as those quoted by 

 Kimball. Dr. Eudolph Schneider in February, in Vienna, found a 

 maximum higher than that given for Cape Horn and practically the 

 same as that at Katherinenburg, and the observations made by him for 

 the time close to the noon hour show a close resemblance to those in 

 Washington. Harvey X. Davis, of Providence, Ehode Island, obtained 

 practically the same results. Kimball, in discussing the annual march 

 of radiation, as compiled by him, states that " a rather surprising uui- 

 formity throughout the year (is shown) in the maximum intensity of 

 radiation, the December minimum being only 8 per cent, less than the 

 April maximum." Even if we take the annual total we find that War- 

 saw actually has 85 per cent, of the radiation received at Washington, 

 although it is 20 degrees farther north. 



Because data with the Angstrom pyrheliometer in the tropics were 

 lacking, we had recourse to animal experiments in Manila. In this 

 connection it should be remembered that although air temperatures in 

 some regions may be low and in others high, the effect of the sunlight 

 on solid objects, as in the case of the black-bulb thermometer, may be 

 very great and bear no relation to the shade temperature; so, for ex- 

 ample, Davos, Switzerland, shows an average maximum black-bulb ther- 

 mometer reading for three years of 53°. 8 with a highest absolute maxi- 

 mum of 67° in 1910, whereas the maximum in Manila in one year was 

 56° and at Helwan, Egypt, 70°. 8 during a period of three years. Alex- 

 andria during the same period gave 57°, Aswan Eeservoir in June, 

 1910, showed a maximum of 81°, and Ley, Thibet, with an altitude of 

 3,517 meters, a maximum black-bulb thermometer reading of 101°. 7 

 with a shade temperature of 23°. 9. These figures refer to maxima only 

 and do not take into consideration averages or the shade temperatures 

 which might be high or low. but it is evident that the occurrence of days 

 of extreme insolation is not so much a matter of latitude as of situation 

 and it is evident that even in the tropics we might come to averages 

 decidedly lower than in certain more northern climates. It is obvious 



