5GG Report on the Magnetic Survey. [No. 6. 



When the hot winds are very violent, they diminish the power 

 of insolation very much owing to the quantity of dust they raise, 

 which very often, like thick aqueous clouds, completely hides the 

 position of the sun, and sometimes even produces a darkness like 

 the thickest fog. 



I noticed a peculiar coloration of the sun during dust-storms, 

 which is, I think, a regular phenomenon accompanying them, when 

 the air has lost a certain amount of transparency. 



In fogs the disk of the sun is red, or at least of a decided reddish 

 tint, when sufficiently darkened to be looked at without a dark 

 glass. In dust-storms the sky has also, as in fogs, a decidedly red- 

 dish colour, which in this case is that of the dust itself, but the sun's 

 disk is blue, a phenomenon evidently connected with the suspension 

 of solid particles in the air.* I observed this colour best on the 6th 

 of April at Futtehpore. The hot wind lasted from 12.45 to 6.10 

 p. m., and stopped very suddenly after sunset. The sun was very 

 much obscured as early as 1 p. m., and had then assumed this 

 blue appearance so decidedly, that it looked like the sun's disk seen 

 through a dark-blue glass, the shadow of a thin cylinder falling on 

 white paper was nevertheless well denned and reddish, showing that 

 the illuminated paper had received rays of the (complementary) 

 bluish colour. 



The blue colour of the sun, though the light was gradually much 

 diminished, lasted until 5.10 p. m., when the sun had a height only 

 of about 15 degrees : then the disk soon disappeared entirely be- 

 hind the clouds of dust. 



The temperature of the airf was corrected for index errors on 

 the 3rd of April— 



a series of most accurate meteorological observations, which not only embraced 

 the ordinary objects of observation, but in which, at the same time, a great variety 

 of experiments were combined with equal skill and ingenuity. 



* A similar dust-cloud passing perhaps at a small height above the ground may 

 explain the blue appearance of the sun mentioned in the Second Edition of Sir 

 John Herschel's Astronomy. 



f A room with tatties was easily kept at 25° to 27° C. in the centre of the 

 apartment, the wind being very strong. 



