REPORT ON ATMOSPHERIC CIRCULATION. » 



dry air materially obstructs the free diffusion of the aqueous vapour, the law of the 

 independent pressure of the vapour and the dry air of the atmosphere holds good only 

 approximately. The aqueous vapour, however, constantly tends to approach this state. 

 The important conclusion follows, that the hygrometer can never indicate more than 

 the local humidity of the place where it is observed. While then in certain cases the 

 amount of vapour indicated by the dry and wet bulb readings is far from the truth, yet 

 in averages, particularly long averages, a close approximation to the real humidity of 

 the locality is attained if the hygrometer be at all tolerably well exposed and carefully 

 manipulated and observed. 



Aqueous vapour is being constantly added to the air from water, snow, and other 

 moist and frozen surfaces. The rate of evaporation is greatest when the air is driest or 

 freest from vapour, and least when it is nearest the point of saturation. As air expands 

 under a diminished pressure, its temperature consequently falls, and it continues to 

 approach nearer the point of saturation, or to become ruoister; and as it contracts 

 under an increased pressure, its temperature rises, and it recedes from the point of 

 saturation, or becomes drier. Hence ascending currents of air become moister with every 

 addition to the ascent, and descending currents drier as they continue to descend. 



The pressure exerted by the aqueous vapour in the atmosphere, or, as it is 

 usually called, the elastic force of vapour, is expressed in decimals of an inch of the 

 mercurial barometer. It indicates the quantity of aqueous vapour in the air at the 

 place of observation, and in this light may be viewed as the absolute humidity of the 

 air as there observed. It cannot, however, be regarded as indicating the pressure due 

 to the aqueous vapour of the whole atmosphere over the place of observation, since we 

 are still very ignorant of the distribution of the aqueous vapour with height. Now the 

 diurnal variation in the elastic force of vapour in the air is seen in its simplest 

 form over the open sea. Grouping together all the hygrometric observations made 

 on board the Challenger in the North Atlantic, at a distance from land, from March 

 to July 1873, eighty-four days in all, there being for that time a mean elastic force 

 of 0"659 inch, the following is the diurnal variation (Plate I. fig. 3) : — 



Thus the minimum, —0*020 inch, occurs at the hour when the temperature of the 

 surface of the sea and air resting over it falls to the daily minimum ; it then rises to 

 the mean a little after 9 a.m. ; to the daily maximum, +0'020 inch, at 2 p.m., when the 

 temperature of the sea and air are also near the daily maximum ; and falls to the mean 



(PHYS. CHEM. CIIALL. EXP. — PAET V. — 1889.) 2 



