1875.] W. F. Blanford — On recent Variations of the Sun's Heat. 121 



number and extent of the spots on the sun's surface, and to Professor 

 Koppen's discovery, published in 1873, that the temperature of the lower 

 atmosphere in the tropics varies almost inversely as the number of the spots, 

 he noticed the papers of Mr. Joseph Baxendell published, in 1867, some 

 years before either of the above, in the Memoirs of the Literary and Philoso- 

 phical Society of Manchester. 



Mr. Baxendell had endeavoured to ascertain whether the sun's heat 

 radiated to the earth underwent any periodical variation corresponding to 

 the variation in the state of his surface, and, by an indirect but ingenious 

 treatment of his data, had succeeded in showing that there was good reason 

 for an affirmative conclusion. Then, remarking on the peculiar advantages 

 for investigating all solar phenomena offered by India, and that it was very 

 desirable that Mr. Baxendell's results should be confirmed by more direct 

 evidence, he proceeded to give the results of the observed temperatures of the 

 exposed black-bulb thermometer during the last seven years at Port Blair 

 and ten stations in India. These observations showed an average rise of 

 nearly six degrees in the temperature of the sun's radiation between 1868 

 and 1870, and a decline of about three degrees since 1872, 1867 having been 

 the epoch of minimum spots and 1871 the maximum. This represented the 

 effects of the sun on the land surface, and was probably only a fraction of the 

 total variation at the limits of the atmosphere. 



Comparing this result with that obtained by Professor Koppen, it 

 followed that, as the heat of the sun increased, so the warmth of the lower 

 stratum of air diminished, and vice versa, — a conclusion somewhat unexpect- 

 ed, and at first sight apparently anomalous. Mr. Blanford suggested that 

 the explanation of this anomaly might perhaps be found in Messrs. Mel- 

 drum and Lockyer's law of the rainfall, supposing it to be fairly established. 

 The Indian temperature records show that, both on short and long periods, 

 the temperature of the lower atmosphere bears a certain inverse ratio to the 

 rainfall, and it was easy to assign physical reasons for this observed fact, 

 since, coeteris paribus, the more frequent the rainfall, the greater must be the 

 quantity of cloud which intercepts the direct solar rays, and the greater the 

 evaporation from the earth's surface, which also has a great cooling effect. 

 Since the greater part of the earth is covered with water, the chief effect 

 of increased radiation from the sun will be to increase the evaporation ; there- 

 fore, as the subsequent effect, the quantity of cloud and the fall of rain ; which 

 is Messrs. Meldrum and Lockyer's result. The heat set free by the con- 

 densed vapour must doubtless raise the temperature of the upper strata of 

 the air, those viz., in which cloud is formed, but the lower strata will be 

 affected in the opposite manner, which is the result obtained by Professor 

 Koppen. 



He concluded by pointing out the vast importance of the fact, once 



