16 THE VOYAGE OF H.M.S. CHALLENGER. 



A selection from the hourly barometric oscillations of the Challenger is included 

 in the table, from which it is seen that on small islands, such as Ascension and 

 St. Helena, and, during the rainy season, of such localities as Havana, Bombay, Hong 

 Kong, and Zi-ki-wei, the amounts and times of occurrence of the maxima and minima 

 closely agree with what were observed on board the Challenger over the open sea in 

 like latitudes. 



The influence of the land, in dry climates, in increasing the amount of the 

 oscillation is most strikingly shown at Jacobabad, where pressure rises at 9 a.m. to 

 - 097 inch above the mean, and at 4 p.m. falls to 0'090 inch below it, thus showing 

 the large range of nearly two-tenths of an inch. At Aden, where the climate is dry 

 at all seasons, the fall from the morning maximum to the afternoon minimum is 

 0'084 inch in January, whereas in August it amounts to 0"163 inch, or nearly double 

 that of January, when the sun occupies a 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 same peculiarity is seen in the corresponding seasons of Havana, 

 Hong Kong, and Zi-ki-wei. At Dodabetta, 8640 feet high, the relatively lower 

 morning minimum and retardation of the morning maximum, which characterise the 

 curves of High Level Stations in the higher latitudes, are well illustrated. 



Among the most valuable of the physical results arrived at from the observations 

 made on board the Challenger — valuable from the important conclusions to which 

 it leads — is the fact that the diurnal range of the mean surface temperature of the 

 sea does not anywhere exceed a degree Fahrenheit, whilst the diurnal oscillations of 

 the barometer occur over the open sea as well as over the land surfaces of the globe. 

 It follows, therefore, 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. 



This consideration leads at once 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 that they are primarily caused by the 

 direct heating by solar radiation and cooling by nocturnal radiation to the cold 

 regions of space, of the molecules of the air and of its aqueous vapour, these 

 changes of temperature being instantaneously communicated through the whole mass of 

 the atmosphere from its lowermost stratum resting on the surface to the extreme limit 

 of the atmosphere. There are, as has been shown, important modifications, affecting 

 the amplitude and times of occurrence of the four principal phases of the phenomena, 

 observed over land surfaces, the temperature of which is superheated during the day 

 and cooled during the night, as observed in climates widely different as regards the 



