60 Proceedings of Royal Society of Edinburgh. [sess. 
also] cooled, a system of descending air-currents sets in over the 
whole face of the country bounding the deep valley. The direction 
and velocity of these descending currents are modified by the 
irregularities of the ground, and, like currents of water, they con- 
verge in the bottom of the valleys, which they fill to a considerable 
height with the cold air they bring down from the sides of the 
mountains. This cold and relatively dense air rises above the 
barometers which happen to be down in the valley, with the result 
that a higher mean pressure is maintained during the night. In 
summer, when the daily range of temperature reaches the maximum, 
the pressure during the coldest time of the night is maintained 
0*040 inch higher at Gries than it is in open situations in that part 
of Europe. On the other hand, during the day these deep valleys 
become highly heated by the sun, and a strong ascending current 
of air is thereby formed, under which pressure falls unusually low. 
Thus, while at Vienna the afternoon minimum falls 0026 inch 
below the daily mean, at Gries the amount of the fall is 0*058 inch, 
and at Cordova 0*061 inch. 
The general result is, that in these deep valleys atmospheric pres- 
sure stands much higher during the night and falls much lower 
during the day than is elsewhere the case. The amounts increase 
in proportion to the daily range of temperature ; or, strictly speaking, 
to the amounts the temperature falls below the daily mean during 
the night, and rises above it during the day. The object of this 
paper is to show that the same rule holds in comparatively shallow 
valleys such as that of the Thames. 
Mr Francis C. Bayard has calculated, for the five years 1876-80, 
the diurnal range of barometric pressure for nine stations in the 
British Islands, including the two Observatories at Greenwich and 
Kew. The paper has recently been published by the Meteoro- 
logical Council, in which the Tables give the diurnal range to 
the ten-thousandths of an inch. The diurnal range for these two 
places, which are only seven miles apart, being for the same five years, 
are therefore strictly comparable, and the fourth decimal renders 
possible a more exact comparison of the results. 
The following are the departures from the daily means at Green- 
wich and Kew for June, from 9 a.m. to Koon, in ten-thousandths 
of an inch : — 
