A GREEN SUN AND ASSOCIATED PHENOMENA. 405 



maximum greenness, 160°7 was reached. The mean for the month was 147°*3, 

 against an average of 136°"5. The presence of either dust or condensed 

 moisture in quantities sufficient to have produced the observed effect would, it 

 seems certain, have intercepted the solar heat to a great extent, but vapour, 

 on the other hand, probably would not. Admittedly the question of the 

 absorption of radiant heat by aqueous vapour is still a moot one, but the recent 

 experiments made by Professor M'Gregor, by Professor Tait's new method, 

 show conclusively that the absorption cannot be nearly so great as many people 

 have supposed; and we find such an experienced meteorologist as Mr Blanford 

 stating, as the most probable conclusion from meteorological observations, 

 that "both air and vapour, as compared with other gaseous bodies, are 

 moderately good absorbers of heat, and in nearly equal degree."* The 

 minimum temperatures, which are perhaps a safer guide to the transparency 

 of the air, yield results similar to those of the black bulb thermometer ; and 

 observations of double stars showed that the atmosphere was clearer and 

 steadier than usual. We must therefore, I think, give up any theory involving 

 the presence of sufficient dust to render the sun green. Whether or not the 

 following sunset glows were due to the presence of dust I cannot discuss here, 

 but I would point out that an amount of dust sufficient to produce these effects 

 would probably not materially affect the transparency of the atmosphere. 



* Indian Meteorologist's Vade Mecum, part ii. p. 24. 



[Appendix. 



VOL. XXXII. PART II. 3 T 



