46 Proceedings of the Asiatic Society. | Fes. 
of the causes of this class of phenomena. One locus of such 
places of minimum pressure for the northern hemisphere in the 
hotter months appears to be a large tract of central Asia, extending 
down into, or rather having what I may call outliers in, our 
own Indian peninsula. I have already referred to this in men- 
tioning the cause of our Monsoons. The part which this region of 
low barometer plays in governing the course of the periodic winds 
is only vaguely ascertained ; and its possible influence as an ele- 
ment in the generation of our circular storms has not yet, I believe, 
been made the subject of serious inquiry. It would appear proba- 
ble that the barrier to horizontal motion which is presented by the 
Himalayas must to a large extent exclude the barometrical condi- 
tion of the atmosphere over Central Asia from being any significant 
element in the motion of the lower strata of the atmosphere over 
the peninsula of India, at any rate over those tracts which are com- 
paratively close to the hills. The Himalayan range, if assumed to 
be of the effective height of 10,000 feet only, (probably the effective 
height is almost double this), would in truth be a dam to at least 
one-fourth of the whole material of the atmosphere, and to much 
more than that proportion of the therein contained vapour. For 
strata above this height, no doubt, any difference which might 
exist between the northern and southern pressures would become 
active ; but it may, perhaps, be questioned whether there is much 
difference at a high level in any degree proportional to that which 
is found to obtain near the earth-surface ; for assuming the relative 
smallness of weight in the trans-Himalayan atmospheric column to 
be in any considerable degree due to the absence of vapour, it is 
probable that this element affects the density of the lower part of 
the column especially. If, however, in consequence of the exist- 
ence of the Himalayan mountain range, there is at one period 
of the year, so far as regards the Gangetic trough and the 
higher part of the Bay of Bengal, a motive force operative upon 
the upper strata of the atmosphere which has no effect or com- 
paratively little effect upon the lower, there must thus arise 
by a sort of torsion such a divergence of currents in the body 
of the atmosphere as would be favorable to the formation of 
local centres of minimum pressure and consequent vorticellary 
