564 Report on the Magnetic Survey. [No. 6. 



The haze, during the experiments at TJmballa, though generally 

 accompanying the hot winds, modified the heat, particularly for 

 the boiling thermometer ; also the small clouds, though very thin, 

 and not covering the sun at the moment of the reading, had caused 

 a sensible depression in the black thermometers on the wool ; the 

 other black bulb, freely suspended, being much less sensible for the 

 rays of the sun, since the objects against which it radiated changed 

 their temperature but very little {see " Surface of Ground,") went 

 on steadily rising. 



The great heat of the air lasting till sun-set is very characteristic 

 of days with hot winds in general ; also the surface of the ground 

 keeps remarkably warm during the first hours after sunset though 

 it loses 25 degrees C. during the night. 



Notwithstanding this great variation, no particle of dew is de- 

 posited in these regions during nearly three months, the wet bulb 

 thermometer sinking even 10 degrees lower during the night than 

 the dry one. 



The power of the sun's rays was also determined at Benares and 

 Cawnpore. 



At Benares the boiling point thermometer, in an arrangement 

 like the one just described, was exposed on the 3rd of April. 



It stood at 12 H. 30 M., 78°.15 Centigrade.* 



1 „ 5 „ 82.60 



1 „ 10 „ 84.23 



1 „ 15 „ 84.08 



The maximum seems to fall decidedly after 12, (the sun's power 

 decreasing less rapidly than the loss of radiation is diminished by 

 the general increase of the temperature of the air continuing till 2 

 or 3 P. M.) The maximum on normal days, without clouds, and 

 with very light wind, seems to be reached pretty regularly at Hi. 

 10m. to \h. 20m. p. m. 



At lh. 29m. the wind at Benares became more violent, about 6.5 

 metres per second, and felt very hot. To my surprise the thermo- 

 meter immediately went down, the thermometer with the white 

 bulb in the sun also fell, though very little. Evidently the in- 

 struments had gradually surrounded themselves with a stratum of 



* The readings are corrected for index errors. 



