THE GASEOUS PORTIONS OF THE ATMOSPHERE. 



9 



the pressure of the upper vapour on the lower is reduced, — 

 the lower vapour consequently expands more freely, its 

 tension becomes less, and the dew-point sinks. In short, 

 we have in this case all the common indications from tension 

 of a reduction, and not an increase, having taken place in 

 the quantity of vapour existing in the atmospheric column. 

 It has, however, been shewn that these indications are all 

 effects of a process (cloud formation) which is really taking 

 additional quantities of aqueous matter into the higher parts 

 of the atmosphere ; whilst the same process, by relieving 

 the water which is in the moist sand below of part of the 

 vapour pressure which had previously existed on it, allows 

 fresh vapour to rise from the sand and to pass more freely 

 into the air. Thus as the cumulous cloud is forming, and 

 in proportion to the rapidity of its formation, the tension of 

 vapour diminishes near the surface almost up to the time 

 when rain begins to fall from the cloud; shewing that a 

 reduction of the tension of vapour may be accompanied by 

 an absolute increase in the weight of the aqueous portion 

 * of the atmosphere, and proving that the tension near the 

 surface is not a true indicator of the quantity of aqueous 

 matter in the incumbent column. 



What has been just stated is closely connected with the 

 diurnal sea breezes that are found on many coasts, and with 

 the daily rise and fall of the barometer. The sun when 

 advancing towards the zenith heats the surface of the land in 

 those parts more than that of the adjoining sea, and produces 

 more energetic evaporation from any water that may be on 

 the land than that which takes place from the cooler sea. 

 The vapour produced from the land therefore penetrates the 

 higher regions of the air sooner than that which rises from 

 the sea, and some of the former is condensed sooner than the 

 latter, without forming palpable cloud. At the commence- 

 ment of this condensation some portion of heat is liberated, 

 and the air over the land is thereby warmed and expanded, 



C 



