80 H. F. Blanford — Some further results of the sun-thermometer. [No. 4, 



A feature common to all these stations, and, at first sight, sufficiently 

 remarkable, is that, at all, the month in which the maximum insolation, 

 (or the least atmosplieric absorption) occurs, is one characterized by a high 

 proportion o£ cloud, indicating comparatively high humidity in certain 

 of the higher atmospheric strata. In the case of Ha&aribagh and Pesha- 

 war, the most cloudy months of the whole year, (or as far as is shewn 

 in the tables,) are also those in which the insolation is greatest ; at 

 Sironclia and Sirsa, the greatest insolation occurs in the month imme- 

 diately following that of most cloud ; and, both at Sirsa and Jessore, the 

 average cloud propoi-tion, at the epoch of the former, differs by only 

 an insignificant amount from the maximum. At Allahabad, the maximum 

 insolation temperature coincides with a secondary cloud maximum, (that 

 of the winter rains). It appears, therefore, that the rule, at stations in 

 the interior of the country, is, in a measure, the reverse of that winch 

 we have found to hold good for the coast region ; and that a humid state 

 of the cloud-forming strata of the atmosphere, as indicated by the 

 cloud proportion, is coincident with more than average diathermancy. 



The association of a high degree of insolation with a highly humid 

 state of the atmosphere has been prominently noticed both by the late Baron 

 Hermann von Schlagintweit* and Mr. J. Park Harrison,f and each has 

 suggested an explanation. That put forward by Mr. Park Harrison is 

 based upon experimental results, which, as far as they go, appear to be 

 perfectly valid. He finds that, when clouds are clustered about the sun, 

 without obscuring it, the (probably reflected) heat, from the illuminated 

 clouds, raises the equilibrium temperature of the sun thermometer, some- 

 times by several degrees ; and moreover, that " the action does not appear 

 to be confined to days on which there is visible cloud, for even on cloudless 

 days, (so called) very high readings of solar radiation appear to be due 

 to the presence of opalescent vapour," and that " an apparent increase of 

 solar radiation occurs, as the sun enters a white cloud, of sufficient tenuity 

 to allow free passage to its rays." Now with respect to the effect of 

 visible clouds about the sun, it is very probable that many cases, which 

 may be observed in the original registers, in which the maximum insolation 

 temperature exceeds by several degrees that attained on other days in the 

 same month, may.be due to this cause. But observation with the actino- 

 .meter shows that diffused amorphous cloud, which simply lowers the tint of 

 the sky, making it pale and sometimes almost colourless, far from increasing 

 the insolation, greatly reduces it. J And it is the frequent presence of 



* Proc. Roy. Soc. vol. XIV, p. 111. 



t Proc. Roy. Soc. vol. XV, p. 356 ; also vol. XVII, p. 515 and Phil. Mag. 4th 

 Ser. vol. 39, pp. 70 and 299. 



X Abundant evidence of this is afforded by the actinometric observations made at 

 Alipore and printed by the Solar Physics Committee of the Royal Society in Appendix 

 of their report. 



