16 FKEER. 



come about the more readily, the greater the number of layers of the 

 atmosphere of different densities which exist at any given place. So, 

 in northern climates, where irregular and high winds may prevail, strata 

 might be produced in great numbers by meteorologic conditions, and as 

 each layer or stratum has a different density from the one above or below 

 it, it would take its part in the total amount of refraction. Where the 

 distribution is fairly equable as it is in the region of the trade winds, 

 we would expect a greater proportion of the waves of short length to pass 

 the atmosphere to the earth; at about 30° of latitude accurate observa- 

 tions will probably discover locations in which the sun has great actinic 

 power. At the equator, where the great mass of heated air rises to flow 

 toward the poles, while the air from the north and south passes in, we 

 may have what might be termed an atmospheric lens effect, serving to 

 concentrate the rays of the sun, as they pass from the rarer to the denser 

 medium, and thus giving more intense insolation. In this discussion 

 the modifying factors of diffused light, and of that reflected from the 

 earth, are not taken into consideration, obviously these will vary greatly, 

 and will be much influenced by local conditions. 



Although we may conclude, from the experimental data given in the 

 preceding portions of this paper, that insolation in our regions is really 

 of greater intensity than it is in northern climates, we must also note that 

 the measurements of value in determining this question are undertaken 

 on days of bright sunlight or of slight haze. It has already been shown 

 that the latter are often more "actinic" than the former, perhaps because 

 of reflection of the sun's rays which reach the earth, thus increasing the 

 total quantity of light. over that which would be present on a clear day. 

 This, however, does not account for the differences observed between two 

 equally clear days at the same time of the year, one of which may be 

 ''actinic," the other '"nonactinic." 



What causes this difference? Is it brought about by air currents of 

 differing density, causing refraction or even total reflection of a portion 

 of the light ? Is it due to some disturbance in the chromo- or photosphere 

 of the sun itself? Or, again, has the phenomenon of ionization of the 

 air, by absorbing the radiant energy for this purpose, a connection with 

 these conditions? I confess I am neither physicist, astronomer nor 

 meteorologist, and must leave the answer to others. It suffices here to 

 state that this is a fact which must be reckoned with in discussing the 

 total annual insolation of our regions. 



Again, when we consider many places in the Tropics, which are re- 

 garded, as a whole, as having a climate unsuited for continued residence 

 of European races, we must 'remember the large proportion of the year 

 in which the sky is overcast. Cloudy days, in distinction from those in 

 which the sum is but partly obscured by haze, naturally show the effects 

 of insolation to a much less degree than days of sunlight, the intensity 



