138 MR. JOSEPH BAXENDELL ON SOLAR RADIATION. 



4th. It seems probable that the heating rays of the sun 

 consist of two kinds, differing considerably in intensity, 

 and being subject to periodical changes, the times of maxi- 

 mum of one kind, and those of minimum of the other, cor- 

 responding respectively to the times of maximum frequency 

 of solar spots. 



5th. That the oscillations of mean daily temperature 

 are intimately connected with the changes which take place 

 in the earth^s horizontal magnetic intensity. 



I have said that the results derived from the Greenwich 

 observations of solar radiation were anomalous and unsa- 

 tisfactory. It may, therefore, be necessary to state that 

 these observations appear to have been made under cir- 

 cumstances not favourable to the accurate determination 

 of the intensity of solar radiation at all seasons of the year. 

 The values derived from them appear to be too high in 

 summer, and too low in winter. Take, for instance, any 

 winter month, say December 1857. The mean difference 

 for the month between the maximum in sun and the maxi- 

 mum in shade' was only i°'j ; at Oxford it was 6°*i. The 

 highest value during the month was 4° '5 at Greenwich ; at 

 Oxford it was 19°. At Greenwich there were only seven 

 days on which the difference exceeded 3° ; at Oxford there 

 were seventeen : and yet at Greenwich the month was 

 unusually fine and dry, only 0*36 of an inch of rain fell, 

 and several days appear to have been nearly, if not quite, 

 cloudless. Under these circumstances it is difficult to un- 

 derstand why the black-bulb thermometer, if properly 

 exposed, did not register much greater differences. In 

 1859 there was a remarkable and unaccountable falling 

 off in the summer values, and the mean for the year 

 was decidedly lower than the mean of any of the three 

 preceding years; but at Oxford it was higher. I can 

 only account for this remarkable difference by supposing 

 that some change was made in the position of the solar- 



