352 MR. S. A. HILL ON THE WINDS OF NORTHERN INDIA, AND THEIR 
following pressure differences on days in May when the mean pressures at sea-level 
are alike :— 
Table VIII.—Pressure Differences in May dependent on Diurnal Variation only. 
Hour. 
Difference in 
15 deg 
Hour. 
Difference in 
15 deg. 
j Hour. 
Difference in 
15 deg. 
Hour 
Difference in 
15 deg. 
0 to 1 
+ 
•0112 
6 to 
7 
- -0153 
12 to 13 
+ -0246 ; 
18 to 19 
- -0133 
1 „ 2 
+ 
•0069 
7 „ 
8 
- -0103 
13 „ 14 
+ '0275 
19 „ 20 
- -0193 
2 „ 3 
— 
•0030 
8 „ 
9 
- ‘0051 
14 „ 15 
4 -0248 
20 „ 21 
- -0191 
3 „ 4 
— 
•0130 
9 „ 
10 
+ -0009 
15 ,. 16 
4 0176 
21 „ 22 
- -0127 
4 „ 5 
— 
•0190 
10 „ 
11 
+ -0088 
16 „ 17 
4 '0074 
22 „ 23 
- -0020 
5 „ 6 
— 
■0190 
11 „ 
12 
+ -0173 
17 „ 18 
- -0036 
23 „ 24 
+• -0077 
These differences, which are taken from the ‘Indian Meteorological Memoirs/vol. 1, 
page 326, would produce N.W. winds, indicated by the positive sign, from about 
9 h. 21 m. to 17 h. 7 m., and again from 22 h. 42 m. to 2 h. 12 m., the remaining hours 
having gradients for S.E. winds, indicated by the negative sign. Now, from the paper 
last referred to, we learn that, in the hot season, the resultant wind at Allahabad blows 
at all hours from points between S. 73° W. and N. 5°W., the double oscillation 
suggested by the Table being combined with and masked by a prevalent N.W. 
direction, though it is plainly indicated in the diagram formed by joining the ends of 
the lines representing the resultants for the several hours. The differences in the 
Table, in fact, are so small, not exceeding •0275-inch in about 813 geographical miles, 
or •002-inch per degree of the earth’s surface, that near the ground, where the friction 
coefficient is large, the winds due to them would be so light as to be almost imper¬ 
ceptible ; and, accordingly, we find that the double gyration of the wind vane, which 
would be produced by this cause, is almost entirely hidden by the influence of the 
more powerful currents of the higher atmosphere which descend in the middle of the 
day. The N.W. winds due to the diurnal inequality of pressure are, however, 
probably more than twice as strong in the afternoons as in the early mornings, the 
maximum pressure differences being in the ratio of 275 to 112, so that this cause 
combines with convective interchange between the lower and higher strata to make 
the velocity greatest about the hottest time of the day. 
On comparing the mean pressures for each month at several stations on the Upper 
Gangetic plains, we shall find, as has been stated in the introductory paragraphs, that 
their differences have no apparent relation to the mean wind velocity ; but the latter 
has, in the annual as in the diurnal period, a very distinct relation to the temperature. 
In the North-Western Provinces and Gudli there are four stations, for which we 
possess registers extending, with few interruptions, over nearly 18 years. When 
the monthly mean pressures of these are reduced to sea-level and the value of gravity 
at latitude 45°, we get the figures given in Table IX., the differences of which from 
place to place are strikingly small. 
