1888-89.] Dr A. Buchan on Atmospheric Circulation. 789 
extensive tract of land to eastward or south-eastward. A table was 
given showing, for fourteen stations, a gradual retardation of this 
phase of the diurnal pressure in June, from 7 a.m. at Culloden, to 
11 a.m. at St Petersberg, and finally to 3 p.m. at Sitka. 
As regards the land surfaces of the globe, the great range hitherto 
observed between the morning maximum and the afternoon 
minimum is nearly two-tenths of an inch in the arid climate of 
Jacobabad. At Aden, where the climate at all seasons is dry, it is 
0084 inch in January, whereas in August it amounts to 0*163 inch, 
or nearly double that of January, when the sun occupies a much 
lower place in the sky. On the other hand, at Bombay, during the 
dry season in January, the range is 0*119 inch, but during the wet 
season in July, though the sun’s position is then nearly vertical, the 
range is only 0*067 inch. 
The “Challenger” observations show that the atmosphere over 
the open sea rests on a floor or surface, subject to a diurnal range 
of temperature so small as to render the temperature practically 
constant both day and night, and also that the diurnal oscillations 
of the barometer occur over the open sea equally as over the land 
surfaces of the globe. This consideration leads to the all-important 
conclusion that the diurnal oscillations of the barometer are not 
caused by the heating and cooling of the earth’s surface by solar 
and terrestrial radiation, and by the effects which follow these 
diurnal changes in the temperature of the surface, but are primarily 
caused by the direct heating by solar radiation, and cooling by 
nocturnal radiation of the molecules of the air and its aqueous 
vapour, and the dust particles suspended in it, these changes of 
temperature being instantaneously communicated through the w r hole 
mass of the atmosphere, from its lowermost stratum resting on the 
surface to the extreme limit of the atmosphere. The all-important 
bearing of these considerations of the theory of the diurnal oscilla- 
tions of the barometer was explained at length. 
The peculiarities of the diurnal barometric tides iu deep valleys, 
and those at high-level observatories, such as Obirgipfel and Ben 
Nevis, were described and discussed. 
During the cruise, observations of the force of the wind were 
made on 1202 days, at least twelve times each day, 650 of the days 
being on the open seas and 552 near land. As regards the open sea, 
VOL. XVI. 13/2/90 3 E 
