364 Observations made on a Botanical Excursion. [Oct. 



Soane, the sun always rose behind a dense fog-bank. This was when 

 close to Parus Nath, and the effect of a slight east wind, forming, first 

 a stratus amongst the mountains to the west, which gradually rose, 

 obscuring the whole sky with cirrho-cumulus. On all other mornings 

 the sun-rise was clear and cloudless ; though through a visible haze. 



At 9|- a. m. the black-bulb Thermometer rose in the sun to 130°. 

 The morning observation before 10 or 11 a.m. always gives a higher 

 result than at noon, though the sun's decimation is so considerably less, 

 and in the hottest part of the day it is lower still (3|- p. m. 109°,) 

 an effect no doubt due to the vapors raised by the sun, and which 

 equally interfere with the Photometer observations.* The N. W. winds 

 invariably rise at about 9 a.m. and blow with increasing strength till 

 sunset ; they are no doubt due to the rarefaction of the air over these 

 heated plains, and being loaded with dust, the temperature of the atmo- 

 sphere is raised by the passage of a warm body, which at the same time 

 that it varies the temperature in the shade, depresses the black-bulb 

 Thermometer. The increased temperature of the afternoon is there- 

 fore not due wholly to the accumulation or absorption of caloric from 

 the direct sun's rays, but to the passage of a heated current of air 

 derived from the much hotter regions to the westward. It would be 

 interesting to know how far this N. "W. diurnal tide extends ; and if it 

 crosses the Sunderbunds or upper part of the Gangetic delta ; also the 

 rate at which it gathers moisture in its progress over those damp regi- 

 ons. Of its excessive dryness at Benares, Prinsep's observations give 

 ample proof, and I shall compare these with my own observations, both 

 in the valleys of the Soane and Ganges, and on the elevated plains of 

 Behar and Bengal and of Mirzapur. 



Observations with the black-bulb Thermometer, though confessedly 

 imperfect, are of considerable interest, and that they have attracted little 

 notice in India is evident from a paper of Capt. Campbell, f who men- 

 tions that in Lat. 18° N. 43° is the maximum effect he ever obtained, 

 and that Dr. Baikie has shown 24° to be the maximum on the Neel- 

 ghery mountains in January. In February and March I have repeated- 

 ly observed a difference of upwards of 50°, and on one occasion of 68°. 

 These were in Lat. 25° N. On the Kymaon hills (alt. 1104 ft.) 



* See Analysis of Observations. 



t Calcutta Journal of Nat. His. v. 2. p. 185. 



