Epidemics. 147 



Oblique and vertical rays of the sun much more materially 

 affect the heat on the surface of the earth than the duration 

 of light. The rays are most vertical in the tropical regions 

 of the earth, and the least in the Arctic regions, where, on 

 the contrary, the rays attain to their greatest obliquity. In 

 summer the rays are always more vertical than in the winter 

 or the cold and rainy seasons; hence the direction of the 

 rays of the sun is most important, in relation to diurnal heat. 



Humidity and high barometrical pressure affect the develop- 

 ment of positive heat in the highest degree ; hence valleys 

 are hot with high barometrical pressure, and mountains are 

 cold-, where rarefaction is great, and barometrical pressure is 

 diminished. And as positive heat, emanating from the rays 

 of the sun, is a vivifying agent, and the chief of all known 

 agents, it follows that long-extended plains, where rarefaction 

 is very limited, and yalleys well watered by rivers and 

 streams, will give the largest per centage of vegetable and 

 animal organization, and in equal latitudes high mountains 

 will be most scanty in their yield of organized products. 



But as year by year the same amount of sunlight, whether 

 checked by clouds or not, is not the question ; but the sun 

 shines from above upon the same lands, and gives light year 

 by year for the same length of time, and at the same period 

 of the year, in every place alike throughout all ages there- 

 fore, so far as relates to diurnal light, no part of the earth 

 receives more one year than other, nor is the slightest 

 amount of abatement to be found in solar distribution of 

 light in any one part of the earth at the same period of time. 



Spots upon the sun affect the magnetic currents on the 

 earth somewhat, when present, but how this magnetic 

 disturbance affects growth and vegetation at present we 

 are in perfect ignorance, and among disturbing agents 

 on the earth's surface, as yet, no positive inference of any 

 kind can be drawn. 



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