MR. H. F. BLANFORD ON THE WINDS OF NORTHERN INDIA. 
617 
a vapour-tension above that of saturation. But vapour must always tend to assume such 
a distribution ; and thus a current which has already been robbed of a large part of its 
vapour by passing over the Sahyadree ridge, even though rendered hygrometrically dry 
on redescending to lower levels, may gradually become resaturated in its higher strata 
by the upward diffusion of its residual moisture. After the first falls, moreover, if the 
current is uninterrupted, a portion of the precipitated rain, 'which may be roughly esti- 
mated between one fourth and one third, will be again taken up by evaporation and 
carried further inland, again to be precipitated. In this way probably also is to be 
explained the gradual advance of the rains up the Ganges valley to the Punjab, a pro- 
gress which occupies three or four weeks from the setting in of the heavy rainfall in 
Bengal. 
The rainless, or nearly rainless, climate of Sind and the plains of Bikaneer owes its 
character doubtless to that of the arid countries to the west and north-west. In the 
absence of any wind-observations in this region, this cannot be verified ; but a very pro- 
bable explanation is furnished by the following considerations. This region, including 
the Punjab, appears to be the seat of the lowest pressure in the rainy months; and it 
has been shown, from the wind-registers of the last-named province*, that during this 
period of the year there is a kind of cyclonic circulation of the winds around it. Accord- 
ingly (and this the wind- and rain-registers show) on its northern and eastern borders it 
receives a moderate rainfall from winds that reach it from the east and south-east, 
having travelled up the Ganges valley or across the Satpooras and the Malwa and 
Bundelkund plateau ; but on the south and west, if the rule there holds good, they must 
come from Baloochistan, Arabia, and Persia, all exceedingly dry countries. As far as 
I can speak from my recollection of a register kept at Khelat some years since, such 
rain as falls on the hills to the west of the Indus comes from the eastward. 
GENERAL SUMMARY. 
The north-east monsoon of Indian seas is produced by the cooling and condensation 
of a comparatively calm atmosphere over the land-surface of India. It has its origin in 
the plains of the Punjab, Upper and Central India, and Assam ; probably also on the 
southern slopes of the Himalaya, where the air, cooled by radiation and contact with 
the surface of the hills, flows down the large valleys to mingle with the similarly cooled 
ah of the plains. These currents are fed by an upper current, which I have termed the 
anti-monsoon. This is felt as a southerly wind on and over the south face of the Hima- 
laya, and descends on the plains of Upper and Central India, bringing the winter rains. 
There would appear to be two principal branches of this current, the course in each 
case being indicated by a higher temperature and lower pressure at the surface of the 
ground, as well as by the longer duration of southerly winds. One of these flows at a 
lower level opposite to the Gulf of Cambay, over a part of Kajpootana, and a portion at 
* Ante, Part I., pages 565 seqq. 
