1886.] S. A. Hill — Solar Thermometer Ohservations at AUaJiahad. 321 



Year. 



Solar Constant. 



1876 



82-8° 



1877 



851 



1878 



85-2 



1879 



83-6 



1880 



82-7 



1881 



81-8 



1882 



79-6 



1883 



78-6 



1884 



80-4 



1885 



831 



If these results are to be trusted, tliey sliow that, during tLe cycle 

 of visible change on the sun's surface, there is a cyclical variation of 

 large range in the heat emitted by him, the maximum of emission coin- 

 ciding with, or perhaps slightly preceding, the phase in which his surface 

 is most uniformly bright, and the minimum coinciding with the phase 

 of greatest disturbance, which is also apparently that of greatest absorp- 

 tion of the photospheric radiation by the sun's atmosphere. For, in 

 1878, the surface of the sun was less disturbed by spots than in any 

 other year of the ten, while the spots were most frequent and occupied 

 the greatest area' in 1883. This conclusion, which, so far as the mini- 

 mum phase of the sun-spots is concerned, was deducible from the 

 observations published in my paper of 1883, is confirmed by the aver- 

 age variation of the intensity of solar radiation all over India, as de- 

 duced in the annual meteorological reports, though probably, on account 

 of the somewhat rough and ready way in which the observations are 

 combined in those reports, the variation is not so uniform as it would 

 be if a more elaborate method of reduction were adopted ; it is also 

 confirmed by the variation of temperature all over India and probably 

 in other tropical countries, Koppen having long since shown that the 

 temperature of the torrid zone varies inversely with the frequency of 

 Bun-spots; I have also found it distinctly confirmed by a series of 

 ground-temperature observations made at Allahabad, and, so far as 

 evidence collected only on this side of the world goes, it may be con- 

 sidered as fairly established. It is desirable, however, that contem- 

 poraneous observations made in America, or in some other distant region, 

 should be investigated with a view to testing the truth of this variation 

 in the sun's heating power ; for, though a variation in the direction here 

 indicated seems to me not opposed to anything we know of the constitu- 

 tion of the sun, but rather to be expected from the changes observed on 

 his surface, the antecedent probability is very small that the variation 



