NARRATIVE OF THE CRUISE. 
999 
globe renders it highly probable that nowhere over the ocean does the mean daily 
fluctuation of the temperature of the surface amount to 1 F. Hence the atmosphere 
over the ocean may be regarded as resting on or blowing 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 atmospheric 
pressure and winds. 
“ The temperature of the air over the sea on the same 126 days has been 
examined, and the amplitude of the daily variation is seen to be 3° '21, or nearly four 
times greater than that of the sea over which it lies. During this time the 
Challenger was near land on 76 days, and on these days the diurnal variation 
was 4° ’38, thus showing a larger range in the temperature of the air when near land 
than when out in the open sea. This larger variation in the daily temperature of the 
air than in that of the sea over which it lies is a point of no small significance in 
atmospheric physics from the important bearings of the subject on the relations of the 
atmosphere and its aqueous vapour to solar and terrestrial radiation. 
“ With respect to the diurnal variation in the elastic force of the aqueous vapour of 
the air, the observations made in the North Atlantic, at a distance from land, from March 
to July 1873, give a mean elastic force of 0'659 inch, falling to the minimum 0'639 inch 
at 4 a.m., and attaining the maximum 0‘679 inch at 2 p.m. Thus the phases of the 
elastic force of vapour occur at the hours of the maximum and minimum temperatures of 
the sea and the air. On approaching land, however, the curve of the elastic force no 
longer follows the corresponding phases of the temperature of the sea and the air. The 
disturbance caused by proximity to land in the distribution through the day of the 
aqueous vapour in the lower stratum of the atmosphere is very striking. Under the 
influence of the land breeze, the time of the minimum humidity is delayed from 4 to 6 a.m. ; 
and under the influence of the sea breeze and its effects, the amount of the aqueous 
vapour shows a secondary minimum from noon to 2 p.m. The latter minimum occurs at 
the hours when the surface of the land is most highly heated, when the ascending current 
of heated air rising from it is therefore strongest, and the resulting breeze from the sea 
towards the land also strongest. This diminution in the amount of the aqueous vapour 
observed on board the Challenger near land, points clearly to an intermixture with the 
body of air forming the sea breeze of descending thin air-filaments or currents to supply 
the place of the masses of air removed by the ascending currents which rise from the 
heated surface of the land. 
“ From the same observations it is seen that the daily maximum of the relative 
humidity occurs from midnight to 4 a.m., or when the temperature of the air is at the 
minimum, and the minimum humidity at 2 p.m., when the temperature is at the maximum, 
the curve of relative humidity being thus inverse to that of the temperature. This is 
substantially the curve of humidity for all climates and seasons. 
(narr. chall. exp. — vol. i.— 1885.) 
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