218 On the Winds of Northern India. [Feb. 26, 



to -^ of an inch for 400 iniles over the land, and -^ of an inch over the 

 sea, which suffices to maintain the steady current of the south-west 

 monsoon. 



In July the minimum of pressure is reached without important relative 

 change. In August a rise begins, greater over Northern India, which 

 continues during September and October, when the uniformity of 

 pressure is once more approximately restored. 



It is apparent that the distribution of pressure follows, within certain 

 limits, that of temperature, in an inverse ratio of intensity. Thus the 

 region of high pressure in the cold months, which lies across Northern 

 India from E-oorkee to Cuttack, coincides approximately with the area over 

 which the isothermals, then approximately parallel to the circles of latitude, 

 are bent downwards, or the temperature of which is lowest -relatively to 

 the areas to the east and west of it. Again, during the hot half of the 

 year, as the isothermal lines advance, first turning their branches south- 

 wards and leaving an area of higher temperature in Central India between 

 them, which eventually is inverted towards the west over the Punjab, so 

 a pomewhat corresponding change occurs in the lines of equal pressure, 

 which at this season may be said to be distributed on lines generally 

 following the meridians, but with a loop more or less deeply concave 

 towards the west. 



The author then discusses at some length the manner in which these 

 changes of pressure arise, and to what extent they are dependent on the 

 changes in the proportion of aqueous vapour in the air, and concludes 

 that the vapour indirectly greatly influences the pressure by carrying 

 heat from the lower to the upper strata, and by arresting solar and ter- 

 restrial radiation, thus equalizing the temperature of the air-column, but 

 that its power of changing the density, by reason of the displacement of 

 the heavier air-particles, is relatively small, and in some cases unimportant. 

 In general terms the changes of temperature are the principal causes of 

 the variations of pressure. 



4. Certain Effects of Winds. Inquiry is made whether any dynamic 

 heating or cooling of the air can be traced by reason of winds descending 

 to a lower or rising to a higher level, with the conclusion that no such 

 effects are discernible, and that certain explanations given of the winds 

 of India (by other writers), based on such a conception, are erroneous. 



Evidence is adduced which is held to establish that anti-monsoon cur- 

 rents blow in the upper strata of the atmosphere, at the various seasons 

 of the year, and at varying elevations, causing corresponding modifica- 

 tions in the general temperature, the operation of all winds being to dis- 

 tribute the temperature peculiar to them. To the descent of the anti- 

 monsoon current from the south, the author is disposed to attribute the 

 rains of the cold weather. 



Attention is also directed to the greater velocity of the wind-currents 

 near the sea, the westerly winds increasing in force as they approach 



