J r M A M J J A S .\ B J j 



I 1 \ I \ I I n I I I I 



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ON SOLAR RADIATION. 137 



the subject of solar radiation. I may now add that they 

 appear to have an intimate connexion with magnetic phe- 

 nomena. Among the tables given by the Astronomer 

 Royal in the volume of Greenwich Observations for 1859, 

 is one showing the monthly mean horizontal magnetic 

 force at Greenwich for the nine years 1849- 185 7, corrected 

 for secular variation. The line No. 2 in diagram E is a 

 E projection of the numbers in this 



table,, and the line No. i in the 

 same diagram shows the monthly 

 mean daily oscillation of tempe- 

 rature. It will be seen that every 

 rise or fall in one curve has a 

 corresponding rise or fall in the 

 other. It is, in fact, rare to meet 

 with so close an agreement be- 

 tween curves representing pheno- 

 mena so widely different as the 

 earth^s horizontal magnetic intensity and the mean daily / 

 temperature of the atmosphere. 



The conclusions arrived at from this discussion may be 

 briefly recapitulated as follows : — 



1st. That the calorific intensity of the sun^s light is 

 subject to periodical changes, the maxima and minima 

 of which correspond respectively with those of solar-spot 

 frequency. 



2nd. That the intensity of a ray of direct sunlight on 

 its arrival at the earth^s surface, in the latitude of Oxford, 

 is greater in April and September than in June, when the 

 sun^s meridian altitude is greatest. 



3rd. That the curve representing the mean monthly 

 values of solar radiation on cloudless days has its times of 

 maxima and minima corresponding with those of the curve 

 representing the mean monthly diurnal ranges of the mag- 

 netometer. 



