RELATION TO THE DISTRIBUTION OE BAROMETRIC PRESSURE. 
351 
-have been obtained from three stations in Upper India—Jeypore, Roorkee, and 
Lucknow—but they have not been discussed yet. 
Table YU. —Diurnal Variation of Wind Velocity, 
Hours. 
Calcutta,. 
Agra. 
Miles per hour. 
Miles per hour. 
Midnight to 3 hours. 
3'28 
2-87 
3 
v 6 
2-96 
3-33 
6 
„ 9 
4-51 
4-25 
9 
„ 12 
6-12 
571 
12 
„ 15 
6-55 
6-82 
15 
„ 18 
5-65 
5-19 
18 
„ 21 
4-36 
3-49 
21 
,, 24 
3-87 
2-95 
Mean 
4-69 
4-33 
The figures in the Table represent annual means, They prove that the velocity 
attains its maximum about the hottest time of the day, and its minimum about 
midnight at Agra and 4 A.M. at Calcutta : the exact instants of the turning points, 
found by taking differences, being 3 h. 31 m. and 12 h. 58 m. at Calcutta, and 
Oh. 27 m. and 13 h. 13 in. at Agra. The anemograms for several years at Karachi 
have been discussed by Mr. F. Chambers and published in vol. 1. of the ‘Meteoro¬ 
logical Memoirs,’ but, as this station is on the sea-coast, where there is little 
frictional retardation, the hourly means do not exhibit anything like such a large 
variation in proportion to the mean velocity, which, at that station, is nearly 
17 miles per hour. The extreme hourly distances traversed are 14'4 niiles between 
5-| h. and 6lr h., and 20'9 niiles between 14-| h. and 15-| li. 
At all the stations in North India, for which the hourly variations of the wind 
resultants have been computed # a double diurnal oscillation, related to that of 
barometric pressure, has been more or less distinctly observed ; but the amplitude of 
this double oscillation, which may be taken to depend on the diurnal variation of 
pressure differences near sea-level, is extremely small in comparison with that of the 
single oscillation due to diurnal heating of the ground and consequent convective 
action. We have already seen, that, in the month of May, the pressure differences over 
the upper Gangetic plains are often evanescent; and it will be shown below that, in 
this region, the differences of the mean pressures are very small at all times of the year. 
It must frequently happen, therefore, that any east and west pressure gradient, which 
may exist, is due solely to the difference in phase of the diurnal variation owing to 
difference of longitude; the epochs of maximum and minimum, and the range of this 
variation being very nearly constant through many degrees of longitude. Thus, taking 
two points 15 degrees apart on the parallel of Allahabad, we should find the, 
* ‘Indian Meteorological Memoirs,’ vol. 1 (Papers 1, 9, and 10). 
