RELATION TO THE DISTRIBUTION OF BAROMETRIC PRESSURE. 
373 
direction of the wind is powerfully influenced by the distribution of pressure at high 
levels, it is more important, as far as the weather of the winter months is concerned, 
to know the variations of pressure from the normal at the highest hill-stations than at 
any place on the plains. In May there are considerable fluctuations over both regions, 
the greatest range being shown by Ajmere and the next greatest by Ranikhet; but 
in July the variations over the mountains, which are always more or less moist 
and equable in temperature at this season, are very small, the ranges not exceeding 
•043 inch in the five years compared, while at Ajmere and Sutna they amount to 
'144 and ’120 inch. In October, as we should expect from the fact of the mean 
temperature of the mouth being about the average of the year, and the normal 
distribution of pressure very uniform at all levels, the range of the variation is less 
than in any of the other three, and is greatest at Hazaribagh, which, during the first 
half of the month, is on the border of the region where the rains still penetrate after 
they have ceased in Upper India, while it also lies near the track of some of the 
October cyclones. 
The great variability of pressure at 10,000 feet over Central India and Bajputana 
during the rainy season, which depends chiefly upon the still greater variability of 
temperature in the same region, is a good instance of the general tendency for any 
established set of meteorological conditions to persist by the interaction of cause and 
effect. An unusually high pressure in the upper atmosphere over this region in the 
months of May and June probably produces, as we have seen, westerly and north¬ 
westerly winds ; these, being dry winds, neither cool the earth’s surface by precipitation 
and subsequent evaporation, nor, by the interposition of a screen of clouds, prevent its 
temperature from rising high in the day-time ; then, in consequence of the high day 
temperature in this region as compared with the Himalayas, which are more or less 
cloudy and moist, the pressure at 10,000 feet or other high levels remains relatively 
excessive; and again, in consequence of the high baric gradient thus conserved, the 
westerly winds continue to blow on, until some disturbance originating at a distance 
supervenes, or, as in 1877, until after the autumnal equinox, when the night begins 
to exceed the day in length, and the loss of heat by radiation to the clear sky over¬ 
balances the gain. 
The mean gradients or pressure differences, measured more or less directly across 
the Gangetic plain, in each of the months compared in Table XVIII., are given in 
Table XIX., the figures representing thousandths of an inch, and the positive sign 
indicating a gradient for westerly winds, i.e., meaning that the higher pressures 
belong to the southern stations, such being the most frequent condition. 
