RELATION TO THE DISTRIBUTION OF BAROMETRIC PRESSURE. 
361 
20‘SO inches sends a long loop down the valley of the Ganges to the north of Delhi. 
The next two isobars are very similarly curved, but they extend further to the east 
over known regions, and are seen to curve southward again over Bengal and the head 
of the Bay. The others probably form closed curves of a roughly triangular form 
round a centre lying in the western Deccan near the town of Sholapur. 
The system of winds, pertaining to this distribution of pressure at a height where 
the friction coefficient is very small, would be S.W. over the Bombay coast, Sindh, 
and Bajputana, S. or S.W. at most of the Himalayan stations, N. or N.E. at Murree, 
S.E. along the foot of the Central Himalaya, N. W. in Bengal, W. over Central India 
and the Gangetic plains, N.E. over nearly the whole of the Madras Presidency, and 
E. or S.E. in Travancore and Cochin. Now in almost every instance where the 
observed prevailing wind direction for this month is inconsistent with the distribution of 
pressure at sea-level it is in accordance with this upper system of wind currents. 
A possible solution of another question of great interest is suggested by the chart. 
The cause of the disturbances which produce the winter rainfall of the Punjab, North- 
Western Provinces, and Bajputana, and the snows of the North-west Himalaya, is not 
yet fully understood. Mr. F. Chambers has suggested that these precipitations may 
be due to disturbances of the same nature as the winter storms of high latitudes 
entering India from the west,* while Mr. Blanford attributes their origin to the 
disturbance of atmospheric equilibrium by the upward diffusion and condensation of 
water vapour formed locally.f It is difficult to understand how a cyclonic storm, if it 
be of the same nature as those formed in the Bay of Bengal at the turn of the seasons, 
can cross a mountainous country like Afghanistan ; but that whirling storms do cross 
the Bocky Mountain region is a common experience in America. 
Without presuming to dogmatise, therefore, about a subject which I have not 
sufficiently studied, I would suggest that it is possible such storms may be formed in 
more than one way, and that some of them at least may travel for long distances if 
they do not originate in a region of relatively low pressure in some upper stratum of 
the atmosphere. Such whirling storms, crossing the Indus in the month of January, 
might travel along the axis of lowest pressure directly into Kashmir, or they might 
pass along the Y-shaped depression in the Valley of the Ganges as far as Behar ; but, 
owing to the higher pressure in West Bengal and Nepal, they could seldom penetrate 
to Assam, except by the roundabout way of Thibet, in which case their chance of 
survival after twice crossing the Himalayas would be infinitesimal. Now these possible 
paths suggested by the form of the isobars are precisely those most frequently taken 
by the disturbances which bring the winter rains. 
The chart for May shows that the distribution of pressure at 10,000 feet elevation 
during this month is extremely simple if we leave out of consideration local minima 
* ‘ Nature,’ vol. 23, page 400. 
t ‘ Journal of the Asiatic Society of Bengal,’ vol. 52, Part 2, and ‘ Indian Meteorological Memoirs,’ 
vol. 3, page 93. 
3 A 
MDCCCLXXXVII.—A. 
