Page:Popular Science Monthly Volume 80.djvu/533

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TROPICAL SUNLIGHT

although in the longer days in the temperate zone the sunshine reaching the earth when the sun is near sunrise or sunset is only a small proportion 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 consider, 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 exposed, 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 very brief résumé 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 locality, the season of the year, and other causes. Experiments carried on in Manila also seem to show that the Malay and the Negro possess relatively 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 sunlight, and the total ionization brought about by radioactivity may be of influence in controlling the electro and other meteorological phenomena, 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 shown by the spectrograph, does not extend beyond 291 , still it may be possible that we receive 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.