RELATION TO THE DISTRIBUTION OF BAROMETRIC PRESSURE. 
except September, and from Table XXL it would appear impossible to predict in May or 
June what will be the prevailing pressure difference later on in the year. The 
irregularities in the pressure differences deduced from the observations of single 
months may, however, as already pointed out, be due in great part to the uncertainty 
of the assumed rate of decrease of temperature, which, while probably nearly true, or 
at all events giving consistent results when applied to the average of many years’ 
observations for any calendar month, or even to the mean for five or six consecutive 
months of a single year, may be very considerably in error when applied to the 
observations of one month standing by itself. 
On making a retrospective survey of all the evidence put forward in the preceding 
pages, it will, I think, be generally admitted that the hot winds of Northern and 
Central India cannot be satisfactorily explained by the distribution of pressure at the 
earth’s surface, and that Koppen’s hypothesis of convective interchange between the 
upper and lower strata is probably the true explanation of them, (1) because the 
vertical distribution of temperature is such that convective action must take place; 
(2) because the diurnal variation of the intensity of these winds and their characteristic 
dryness suggest such an origin for them ; and (3) because the distribution of pressure at 
10,000 feet above sea-level in May is such as to produce winds of the observed direction. 
The distribution of the upper currents as suggested by the charts also elucidates 
many other obscure points in the wind system of India, and the same hypothesis of 
the origin of the westerly winds gives a rational explanation of the law worked out by 
Mr. Blanford connecting the spring snowfall of the North-west Himalaya with the 
rainfall over the plains during the succeeding summer monsoon. 
If these conclusions have not been all completely established, as I am by no means 
desirous of asserting, they have, I think, been shown to be sufficiently probable to 
warrant a more complete examination in detail by the determination, if possible, of 
more trustworthy rates of temperature decrement with height than those assumed in 
this paper, and by the systematic preparation of high-level pressure charts for the 
whole of India, month by month. 
Description of the Plates. 
Plate 19 shows the isobars or lines of equal pressure at sea-level over India and the 
Bay of Bengal for the months of January, May, July, and October ; typical respectively 
of the cold, the hot, and the rainy season, and the autumn transition period. The 
pressure, indicated by each line, is represented by the figures attached to it, those for 
successive lines differing by - 05 inch. The pressures have all been corrected for 
variations of gravity with latitude. The charts also give the prevalent wind directions 
at the more important stations, these “ resultant ” directions having been computed by 
means of Lambert’s formula from the observations of many years. 
Plate 20 gives the isobars for a stratum of the atmosphere 10,000 feet above sea-level. 
MDCCCLXXXVII.-A. 3 C 
