THE STUDY OF TROPICAL SUNLIGHT. 13 



liquid substance, such as the soil and water which form the 

 surface of the earth. The power of absorption of the air is 

 influenced by such factors as humidity, actual nuclei or droplets, 

 clouds, and other causes, just as it is by density; but moisture- 

 laden air . relatively does not absorb as great a proportion of 

 the rays of lower refrangibility as it does of higher. It is for 

 this reason that air temperatures at higher altitudes are lower 

 than in the lowlands, although the effects on solid objects, such 

 as the black-bulb thermometer, may be greater. This may be 

 shown by a comparison of some black-bulb thermometer readings 

 in different parts of the world, which I have gathered for other 

 purposes. At Davos, Switzerland (altitude 1,559 meters), the 

 average of maximum black-bulb readings for three years was 

 53°.8, with a highest absolute maximum of 67° in 1910. Compare 

 this with Manila, where the maximum for one year (1910- 

 1911) was 56°; or with Helwan, Egypt, where the highest ob- 

 served was 70°. 8 during a period of three years; or with Alexan- 

 dria, Egypt, with a maximum of 57° during the same period. 

 Of course, there are places on the edge of the desert, where 

 the atmosphere is exceptionally clear and where reflected light 

 is present in great proportion, that exceed these figures, so, for 

 example, Cairo, in May and August, 1909, shows a maximum of 

 79°. 5; and Aswan Reservoir, in June, 1910, of 81°. However, in 

 contradistinction to these desert places, we have another remark- 

 ably high black-bulb reading at high altitude, in Leh (Thibet), 

 (altitude 3,517 meters) of 101°. 7 with a shade temperature of 

 23°. 9. Of course, these figures refer to maxima only, and do 

 not take into consideration averages, or the shade temperatures, 

 which may 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, temperate climates. It is obvious that in any one of 

 the places mentioned, a living body might encounter days in 

 which it would be heated by solar radiation to a much greater 

 extent than in the Tropics, and the only question would be 

 whether the possibility of cooling, such as is brought about bj^ 

 low air temperature, low humidity, wind, or other means would 

 compensate to avoid the effects of such insolation. 



A body exposed to the sun absorbs a portion of the rays and 

 reflects a portion of them, the most perfect absorption being 

 that of as nearly ideally black a substance as" is possible. The 

 body would go on storing the energy so conveyed to it indefinitely. 



