TROPICAL SUNLIGHT 529 



although in the longer days in the temperate zone the sunshine reach- 

 ing the earth \yhen the sun is near sunrise or sunset is only a small pro- 

 portion of that at midday. As a result we have in the tropics greater 

 absorption and radiation from the earth's surface as a result of direct 

 exposure to the sunlight to augment the influence of the sun's rays, so 

 that, as it has been shown that the heat factor is the chief one to con- 

 sider, this increment due to radiation from the earth will be of decided 

 influence. This will naturally vary with different regions according to 

 the hours and intensity of insolation and the color of the surface ex- 

 posed, being least with green surfaces of vegetation and greatest with 

 rocks, or red, clay soil, such as is common in India under the name of 

 laterite. 



Another factor needs to be considered, and that is the evenness of 

 the tropical' climate, which is devoid of severe contrasts, such as are 

 given by the winters in northern climates, yet Chamberlain's results 

 seem to indicate that this has but little effect. 



I have endeavored in this short article to give a verv brief resume 

 of the most important points which, up to the present, have been 

 brought out. Any one can see that the subject under investigation is 

 so complex and that it is influenced by so many factors that general 

 conclusions at the present time are premature, excepting in so far as they 

 are borne out by experimental evidence. Obviously relative humidity 

 is of great influence on evaporation and varies with geographical local- 

 ity, the season of the 3'ear, and other causes. Experiments carried on 

 in Manila also seem to show that the Malay and the Negro possess rela- 

 tively more sweat-glands than the European. The formation of ions in 

 the air, the proportion of such ions, if any, due to the effect of the sun- 

 light, and the total ionization brought about by radioactivity may be of 

 influence in controlling the electro and other meteorological phenom- 

 ena, and we have also begun work in this direction, but as yet are not 

 in a position to publish the results. 



Although the spectrum of the sun, as shoAvn by the spectrograph, 

 does not extend beyond 291 /x/a, still it may be possible that we rece've 

 rays the nature of which we have not yet determined and which, with 

 our present physical technique, we can not determine. These may also 

 be factors in the phenomena of insolation. 



