1856.] 



Report on tlie Magnetic Survey. 



;g7 



At 12, 



1, 



2, 

 3, 

 4, 

 5, 

 6, 

 The tempera 



Dry. 



Wet. 



38.2 



19.9" 



38.1 



19.0 



37.3 



19.3 



37.3 



19.3 L 



37.3 



19.2 



36.7 



19.1 



36.4 



18.9^ 



Direction of the 

 hot winds West, very- 

 strong, from 12.45 to 

 6.10. 



;ure of the air diminished, as occasionally noticed in 

 the observations above-mentioned, with the increase of the wind 

 and discoloration of the air. 



A coincidence worthy of notice is the sudden cessation of the 

 wind after sunset, which took place at every one of my observa- 

 tions. Generally speaking, it is characteristic of the hot winds that 

 in the beginning of the hot season the wind always ceases very 

 near sunset, the night is very calm, and towards the morning an 

 Easterly breeze is not unfrequently felt. 



The cause of the hot winds ceasing with sunset seems to be the 

 great depression of temperature during the night, coinciding with 

 the Westerly direction of the wind. In consequence the wind 

 meets, soon after sunset, with surfaces covered with a stratum of 

 air cooled by the vicinity of the radiating surface of the ground ; 

 the hot wind passes over this stratum and is therefore not felt near 

 the surface. 



It is only during the middle, and towards the end of the hot 

 season, that the hot winds blow till 2 or 3 o'clock in the morning, 

 when the rains stop them suddenly. 



The hour of the day at which the hot winds commence, is variable 

 from the beginning to the end of the hot season. In the first 

 months they begin to blow at 12 or perhaps 1 p. m., then the period 

 becomes gradually earlier, till a short time before the rains they 

 begin at 6 or 7 a. m., leaving only a few calm hours in the very 

 early part of the morning. 



The quantity of carbonic acid in the air during the hot winds is very 

 small. It reached, in an experiment at Allyghur,* scarcely T -Jo part 



* The quantity of air examined was 50,000 cubic centimetres, the increase of 

 I weight of all potassium tubes reached was not a full milleogramme. 



