1888-89.] Dr A. Buchan on Atmospheric Circulation. 787 
deals with the diurnal, and Part II. with the seasonal phenomena of • 
meteorology. 
Diurnal Phenomena . — An examination of the temperatures ob- 
served by the “Challenger” proves that nowhere over the ocean 
does the mean daily fluctuation of the temperature of the surface 
amount to a degree Fahrenheit, the extremes being from about 0° *3 
in high latitudes to 0°'9 in the tropics. Thus the 'atmosphere over 
the ocean rests on, or blows over, a surface the temperature of which 
is practically uniform at all hours of the day. This small variation 
is a prime factor in meteorology, particularly in those discussions 
which relate to the diurnal phenomena of atmospheric pressure and 
winds. 
The temperature of the air over the open sea shows a daily 
variation of 3° *2, being four times greater than that of the sea over 
which it lies ; but when the “ Challenger ” was near land, the varia- 
tion rose still further to 4° *4. This larger variation in the daily 
temperature of the air, as compared with that of the sea, is a point of 
much significance in atmospheric physics, from the light it casts on 
the relations of the atmosphere and its aqueous vapour to solar and 
terrestrial radiation. 
The phases of the elastic force of vapour over the open sea occur 
at the hours of the maximum and minimum temperatures of the sea 
and the air. On nearing land, however, this no longer holds good; 
but owing to the influence of the land breeze, the time of minimum 
humidity is delayed from 4 to 6 a.m., and owing to the sea breeze 
and its effects, the amount of the aqueous vapour falls to a secondary 
minimum from noon to 2 p.m. As regards the relative humidity, 
the maximum occurs from midnight to 4 a.m., and the minimum 
about 2 p.m., this curve being thus inverse to that of the tempera- 
ture ; and it may be added, that this is substantially the curve of 
the relative humidity for all climates and seasons. 
The phenomena of the double diurnal barometric tide appear in 
their simplest form in the centre of the Pacific, or in the midst of 
the largest water surface of the globe. The following are the 
variations of pressure from September 1 to 12, 1875, in mean lat. 
1° 8' S. and long. 150° 40' W., the mean pressure for the time being 
29,928 inches : — 
