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



to show that the former was fed by the latter. No doubt my steam-cloud 

 would induce paleness of sky, an evil whicli you speak of as prevailing at 

 Calcutta ; and it is liighly probable that such steam-clouds, not by any 

 means prominent, were driven over Mussoorie for days and days in the dry 

 weather at least. What were they composed of ? not smoke and not dust 

 as far as I could judge. 



" As to the haze, to all appearances, dust haze, being visible between 

 showers, as you mention, we have noticed that here too. I have water 

 barrels at the corners of our house ; they are fed exclusively by iron pipes 

 from a clean iron roof. After a few showers had fallen, I had the barrels 

 well cleaned in my presence ; the water was clear. Subsequently a heavy 

 fall of rain occurred, I examined the barrels, expecting the water to be 

 quite clear ; instead, the water was charged with yellow clay ; and yet, 

 after the first showers, I should have thought that the air was too saturated 

 with moisture, not to arrest dust a long way below 7,000 feet. 



" Again last year, in tlie dry weather, I was watching day after day for 

 actinometric weather ; the hills were obscured or dimmed by haze, obviously 

 dust haze. I can see the Chor where I write ; between that mountain and 

 this, the dust haze was quite plain ; suddenly there was a change in the haze 

 about 2 p. M one day, it was a sheet ; it began to roll about in waves and 

 I may say visibly changed into clouds of vapour, which rose like ordinary 

 clouds, leaving me a clear view of the Chor, &c., looking g[uite blue. Note 

 there was no rain.^^ 



It results from what has been said above, that (excepting on the coast) 

 up to a certain point, which cannot be strictly defined, a humid condition 

 of the atmosphere tends to increase the readings of the sun-thermometer 

 and the actinometer ; indirectly by reducing the (dust ?) haze which in 

 dry weather forms a absorbing stratum of many thousands of feet in thick- 

 ness, and directly by causing the formation of cloud masses which when 

 clustering round the sun, reflect the solar rays and add the effect of 

 the reflected to the direct radiation. On the other hand the amorphous 

 cloud which exists at great elevations in dry weather and especially in the 

 winter and spring months, and is generally only appreciable by its lowering 

 and blanching the sky tint, is also a potent absorber. The sheets of Pallio- 

 cirrus and pallio-cumulus which are result of a highly humid condition, 

 and are especially the clouds of the rainy season, are of course the most 

 impervious of all solar screens. 



Since then, the athermancy of the atmosphere is enhanced by such 

 opposite conditions of dryness and humidity, and, at present, we have no such 

 records of these conditions as might enable us to frame a law of numerical 

 concomitance, and thus apply an empirical correction to our actinometric 

 results, it might seem almost hopeless to seek for evidence of any variation 



