1874.] Winds of Northern India. 215 



mum temperature, the one in the Punjab and the other in Assam, and, 

 with some exceptions, the isothermals nearly conform to the parallels of 

 latitude. In the hot months a focus of heat is formed in Central India, 

 round* which the isotherms are bent, the temperature on the coasts and 

 in the northern plains being considerably lower than that of the interior. 

 Finally, during the rainy season the seat of highest temperature is in the 

 Punjab, the coolest regions then being those of the maximum rainfall, 

 and consisting of two tracts extending from the coasts of Bombay and 

 Bengal, along the course of the monsoon currents. 



The author then refers to the distribution of temperature in a vertical 

 direction, as ascertained from observations made at the mountain-stations. 

 He points out apparent anomalies in the differences of temperature due 

 to difference of altitude in the mountains of North-western India and 

 those bordering on Bengal, and suggests, as a probable explanation, the 

 variation of hygrometrical condition of the air in the two regions, 

 remarking that the continual upward diffusion and condensation of water 

 vapour must tend to equalize the upper and lower temperatures, and that 

 this tendency will be the greater as the approach to saturation is closer. 

 The subject, however, is admitted to be one that requires further exami- 

 nation, and particularly with respect to the operation of nocturnal radia- 

 tion and diurnal absorption of heat the remark being also made that the 

 available observations give the local temperature near the surface of the 

 mountains, and do not properly represent the condition of the free 

 atmosphere at corresponding elevations. 



2. Vapour-tension, Humidity, and Rainfall. In the regions under 

 discussion, the lowest vapour-tension occurs almost everywhere in 

 January, when the temperature is lowest. The lowest mean tension for 

 any month is about O2 inch, observed in the Southern Punjab, the corre- 

 sponding minimum in Bengal being about O5 inch. The increase of 

 tension begins early in the districts near the sea, and continues regularly 

 and rapidly till the setting in of the rains ; but in the drier regions of 

 the interior, where the w r est winds prevail throughout the spring and 

 hot-weather months, the rise of tension is slow, probably not more than 

 is due to the actual rise of temperature acting on the local vapour supply. 

 The increase at the commencement of the rains, in June or July, when 

 the southerly winds begin to be felt, is very marked and sudden ; and 

 equally so is the fall after September, when the southerly is replaced by 

 the northerly current. 



As regards variation of tension due to elevation, the conclusions of 

 former observers are confirmed, that the ratio of decrement follows 

 generally the increase of elevation, but with a marked addition to the 

 relative tensions at the higher stations in the hottest and wettest months. 



Passing to the humidity of the air, it is shown that the period of greatest 

 dryness falls later in the year the greater the distance from the sea, 

 measuring along the course of rain-carrying wind-current. On the coast 



