ON SOLAR RADIATION. 



135 



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3, diagram C, may be taken to be a fair representation, is 

 unlike that of any other thermometric element. It has, 

 however, a remarkably close resemblance to the curve re- 

 presenting the monthly changes of one of the magnetic 

 elements, namely, that of the monthly means of the diurnal 

 ranges of the magnetic needle. In the volume of the 

 Greenwich Observations for 1859, the Astronomer Royal 

 has given a table showing the monthly means of the diurnal 

 ranges of the magnetometer at Greenwich from ten years^ 

 observations, 1 848-1857. The numbers in this table are 

 as follow : — 



January 9*5 



February ii'3 



March 13*1 



April 14*0 



May . 

 Juue . 



July ., 

 August 



127 



12*6 



I2'7 

 12*6 



September 13*5 



October i2"2 



November 9*3 



December 8*2 



D 



The line No. 2, in diagram D, is a projection of these 



numbers, and the line No. i is a 

 repetition of curve 3, diagram C. 

 It will be seen that the maxima 

 and minima of the one correspond 

 exactly with those of the other; 

 and we are therefore entitled to 

 conclude that the two phenomena 

 are intimately connected, and that 

 the causes which produce varia- 

 tions in the intensity of solar radiation also affect the di- 

 urnal oscillations of the magnetic needle. 



It was supposed by Sir William Herschel that the emis- 

 sion of heat from the sun varied according to the greater 

 or less frequency of solar spots ; and he attempted to sup- 

 port this view by a comparison of the prices of grain in 

 years when the solar spots were numerous with those in 

 years when few or no spots were seen ; but this kind of 

 evidence was generally regarded as unsatisfactory, and it 



