Meteorological Observations. 55 



may be explained in the same way ; the only difference is, 

 that the necessary cooling before a fog can take place, is 

 occasioned in this case by a light northerly wind of suffici- 

 ently low temperature setting in at night-fall, producing the 

 same effect as radiation in reducing the temperature of the 

 atmosphere. 



Fogs during the day in the plains of India rarely happen, 

 because the temperature of the atmosphere at the surface is 

 such as to raise the dew-point above the influence of ordi- 

 nary changes of temperature. 



The diurnal variation of temperature in Calcutta during 

 the north-east monsoon amounts to 30° Fahr. and the dense 

 mists which float along the surface about sun-rise may 

 be ascribed to the absorption of more moisture into the 

 atmosphere from low tracts during the day when the tempe- 

 rature is highest, than can be held in an invisible form at 

 sun-rise when the temperature is lowest, while these causes, 

 together with others dependent on the state of the pre- 

 vailing winds occasion mists. 



Such changes are however only observed in the open 

 air. In houses the variation of temperature is checked to 

 within a range of 10° Fahr. and where houses closely 

 adjoin each other, their effect is communicated to the open 

 air so as to prevent the formation of mists and fogs in their 

 immediate vicinity. In removing a thermometer from our 

 house to the open air beyond its influence at sun-rise, we 

 have found the mercury suddenly fall from 75° to 64° or from 

 64° to 50°, according to the season. It is only such houses 

 as are built of brick or stone that display this effect in 

 resisting diurnal extremes of temperature in Bengal. For- 

 est trees however have a similar effect but in a minor degree, 

 a fact the recollection of which might be useful in forming 

 new stations where houses of the best description cannot be 

 built at once, and fine forest trees often occur. 



The diseases here most common to man during each 

 of the three seasons, may be stated to be fevers during 



