606 
MR. H. F. BLANFORD ON THE WINDS OF NORTHERN INDIA. 
of radiation quoted at page 594 show that while more solar heat reaches the ground of the 
plateau in May (and it is the same in April), the nocturnal radiation is scarcely greater 
than on the plains of the delta, if the extreme temperatures of radiation may be accepted 
as a criterion of the quantities of heat received and emitted. And since the column of 
the atmosphere over the former is 2000 feet shorter than over the latter, it may be 
inferred that more heat is retained in the atmosphere over the plateau than in that 
above the same level over the plains. In any case, these low plateaux appear to have 
the effect of locally reducing the pressure of a humid atmosphere, since the Table of 
S. L. pressures and the charts for the months May to September show that a trough of 
pressure exists to the south of the Gangetic plain throughout the rains. 
Throughout the rains the seat of lowest pressure appears, both from the direct 
evidence of the Agra and Koorkee barometers and the indirect evidence of the winds, 
to be in the Punjab ; and this is also the seat of the highest temperature at that season. 
But it is also to be noticed that the commencement of the rains, between May and 
July, is marked by a fall of temperature of not less than 15° at Nagpore and nearly 12° 
at Jubbulpore, while the atmospheric pressure also falls by '087 inch at the former and 
T28 inch at the latter. Here the explanation is probably to be found in the diffusion 
of heat from the lower to the higher strata by the influx of water-vapour, the tension of 
which increases from 0*539 inch to 0*776 inch at Nagpore, and from 0*439 to 0*654 
at Jubbulpore. It has been shown in the Table at page 596, that after March the 
tension of water-vapour at all elevated stations up to 7000 feet rises more rapidly than 
near the ground surface, and also in the Table at page 589 that the temperature becomes 
relatively higher — owing, as has also been shown with much probability, to the diffusion 
and condensation of water-vapour. I shall presently prove, I think irrefragably, that 
high temperature is the most influential cause of the fall of pressure ; and if sufficient 
time be allowed for the communication of heat from lower to higher levels, a low pres- 
sure may be produced without any very copious accession of vapour, since the Punjab, 
which is the region of least rainfall and highest temperature, is that towards which the 
winds tend throughout the rains. 
There is yet another cause which may affect the pressure to an appreciable extent, 
and to which I have not yet referred. It is one not usually considered by meteorologists, 
although Mr. Espy has resorted to it in explanation of the diurnal tides, and more 
recently Mr. Laughton has assigned to it more importance than I should be disposed 
to concede to it in atmospheric physics*. This is the dynamic pressure of the atmo- 
sphere in motion. It follows from elementary mechanical laws that wherever the 
motion of a current of air is checked or diverted, pressure must be produced ; and the 
only question in the present connexion is whether, and to what extent, it will be appre- 
ciable. A case in which, if in any, it would, I think, be sensible is that in which the 
lower winds diverge from a circumscribed area or ridge of high pressure, the supply 
being necessarily drawn from above, and maintained by currents flowing in, in the upper 
* Phys. Geog. p. 329. 
