RELATION TO THE DISTRIBUTION OF BAROMETRIC PRESSURE. 
345 
tion of pressure, at high levels, be different from that observed near the ground surface, 
we may, on Koppen’s theory, expect to find in January a greater departure of the 
actual wind direction, from that given by the usual law, than we find in July. 
On the Upper Gangetic plains, and in the Punjab, the range of temperature in 
January is quite as high as in Bengal, though the skies are somewhat more cloudy. 
We do not there, however, find departures from Buys Ballot’s law to occur so 
frequently in this month ; probably because the disturbances of thermal equilibrium, 
causing convection currents, are less intense. For this there are two reasons. In the 
first place, whilst the diurnal range of the thermometer is but little greater on the 
plains than it is in Bengal, the range at the nearest hill stations is considerably 
greater than at Darjiling, so that the difference between high and low stations, at the 
hottest hour of the day, is not so considerable in North-western India as it is for the 
same difference of altitude in Bengal. Again, the temperature probably diminishes 
less rapidly with height at the North-western stations during January than it does in 
Bengal. Putting these two causes together, the effect is that the rate of upward 
decrease of temperature in the North-west is not by any means so rapid at the hottest 
time of the day as it is in Bengal. In the following Table, three pairs of stations 
are compared, so as to show the diurnal variation of the rate of temperature decrement 
with height during the month of January. 
Table V. — Temperature Decrements at different times of the day during January. 
Da.jil mg— Goa lpara. * 
Cliakrafa—Eoorkee. 
Murree—ffawal Pindee. 
6555 Feet. 
6165 Feet. 
4692 Feet. 
Difference of Meaus .... 
O 
21'5 
O 
13'4 
O 
8'4 
Rate per 1000 feet .... 
3-3 
2-2 
1-8 
Difference of Maxima 
29-5 
19-2 
152 
Rate per 1000 feet .... 
4'5 
3T 
3 2 
Difference of Minima 
13-5 
7-6 
D6 
Rate per 1000 feet .... 
2-0 
1-2 
0’4 
The vertical decrement of temperature in North Bengal at the hottest time of the 
day is, therefore, 43 per cent, more rapid than at north-western stations, namely, 
4’5° F. per 1000 feet, instead of 3'1° or 3‘2°, according to the Table, These rates of 
decrement are not those which actually obtain in the free atmosphere, since they 
include a certain variation with latitude ; and, moreover, there is reason to believe 
that, on mountain slopes, the decrease is considerably less rapid than that given by 
balloon ascents. The ratio between the rates for the two regions compared is, how- 
* This column is derived from Blanfokd’s ‘ Indian Meteorologist’s Yade Mecum,’ page 57. The others 
are from my paper on the Temperature of North-Western India, ‘ Indian Meteorological Memoirs,’ vol. 2. 
MDCCCLXXX VII. —A. 2 Y 
