REPORT ON ATMOSPHERIC CIRCULATION. 11 



of the vapour pressure, is of the simplest description. The following are the deviations 

 from the mean daily humidity, 80 per cent., over the North Atlantic, from the 

 observations made on that ocean in 1873 (Plate I. fig. 5) : — 



Hence the maximum humidity takes place from midnight to 4 A.M., and the minimum 

 from noon to 4 p.m., in other words, when the temperature of the air is at the daily 

 minimum and maximum respectively, the curve of humidity being thus simply inverse 

 to that of the temperature. These are, substantially, the prominent phases of the 

 curve of humidity for all climates and seasons, subject, however, to a slight increase 

 in sea-side climates during the hours of the day of the prevalence of the sea breeze. 



The significance of this constituent of climate lies in its relations to the 

 diathermancy of the air, and to the dust particles everywhere present in it, and 

 consequently to the all-important questions of solar and terrestrial radiation. It is 

 assumed, with high probability, that perfectly pure and dry air, or air quite free from 

 aqueous vapour and dust particles, permits rays of heat to pass through it with at 

 most no more than a very slight increase to its temperature. "We are yet without 

 exact information as to whether a mixture of the air with aqueous vapour as a pure gas 

 only, is equally diathermanous with dry air. Whether this be so or not, it may be 

 regarded as certain that the atmosphere never interposes between the earth and the 

 sun a purely gaseous aerial screen, but that it everywhere, even when apparently quite 

 clear, contains minute particles of dust, and water either in the fluid state, or in the 

 solid state as small spicules of ice. 



Next to the winds, the aqueous vapour of the air, in its amount and relation to 

 solar and terrestrial radiation, and in the different ways in which in different localities 

 it is partitioned through the hours of the day and months of the year, plays the most 

 important part in giving to the various regions of the globe their infinitely diversified 

 climates. 



Oscillations of the Barometer. Tables III. and IV., App. pp. 7-48. — The general 

 character of the diurnal oscillations of atmospheric pressure is shown by figs. 6 and 

 7 of Plate I. Fig. 6 represents the mean oscillation for Batavia, lat. 6° 11' S., 

 long. 106° 50' E., and fig. 7 a strictly ocean oscillation in the Pacific in lat. 1° 10' S. 

 and long. 150° 46' "W. Both figs, show two maxima about 10 a.m. and 10 p.m., and 

 two minima about 4 a.m. and 4 p.m. respectively. The two situations are near the 

 equator, the one on the coast of Java and the other in mid ocean. In the latter the 



