368 MR. S. A. HILL ON THE WINDS OF NORTHERN INDIA, AND THEIR 
earliest writings on this subject Mr. Blanfokd seemed to incline to the notion that 
these north-west winds were a nearly direct effect of the cold to the north of India, 
their immediate cause being the (apparent) high pressure of the mountain region when 
the actual pressures observed were reduced to sea-level. It has been pointed out in 
the introductory paragraph, however, that the result of such a distribution of actual 
pressure would be to cause easterly instead of westerly winds, in accordance with the 
usual deflecting influence of the earth’s rotation. In his last paper (‘ Proceedings of 
the Ptoyal Society,’ vol. 37, 1884) this idea that the north-west winds are a direct 
effect of Himalayan cold has been dropped, as will be seen from the following 
quotation, and he anticipates the explanation on the hypothesis of convective inter¬ 
change, which I now attempt to give. At page 18 he says: “The question then 
presents itself, ‘ What is the origin of the dry westerly current ? ’ The supposition 
that the indraught from the south-west furnishes more than a small portion of the 
stream is at once negatived by the fact that, even at Karachi and Bhuj, southerly winds 
do not preponderate over northerly until May, and even then almost inappreciably—at 
Rajkot not before June ; and the very fact of the great dryness of the west and north¬ 
west winds militates against the idea that any considerable portion of their air-mass 
can be drawn from the sea. Neither is it derived to any considerable extent from the 
valleys and lower slopes of the surrounding hills. There is no permanent drainage of 
air from these hill-slopes, and strong winds blowing outwards from the larger valleys, 
like the dadu of Hurdwar, are local and exceptional phenomena, restricted to certain 
hours of the day. At all the hill-stations of the outer North-West Himalaya, as far as 
the existing registers show, southerly winds preponderate over northerly all through 
the year ; and, although this is probably due in some measure to the fact that the night 
winds have not hitherto been registered, it suffices to show that, up to a level of 7000 
feet, there is no steady outflow of air from the hills to the plains. 
“ There remains then only the supposition that these w inds are fed by the descent 
of ah* from an upper stratum, viz., from a current moving at a considerable elevation 
from west to east. And that this is their true explanation several facts seem to testify. 
In the first place, they are characteristically winds of the day time, their movement 
being at a minimum (almost or quite a calm) in the morning hours, and indeed up to 
9 or 10 o’clock in the forenoon—then increasing with temperature, and falling again 
towards evening; and, secondly, such observations as have been made on the decrease 
of temperature with elevation show that, in the dry weather, the vertical decrement 
is such as is incompatible with the vertical equilibrium of an air column, being con¬ 
siderably more than 1° in 183 feet. The diurnal variation of tire movement is then 
probably to be accounted for on Koppen’s hypothesis, viz., the interchange of the 
higher and lower air strata, by convective movements which do not affect the existing 
horizontal movement of the higher atmosphere, so that the air'of the latter, after its 
descent, preserves for a time its original eastward motion. The hypothesis of con¬ 
vective interchange receives further support from the character of the diurna 
