THE GASEOUS PORTCONS OF THE ATMOSPHERE. I 



aqueous pressure would then be found to increase with the 

 rising temperature up to some particular time ; after which, 

 as temperature declined dew would form in the lower regions, 

 and the cold of the night would return to the earth or sea 

 the water which had been taken from it by the heat of 

 the day. The total aqueous pressure would then be shewn 

 to have, like daily temperature, one rise and one fall in the 

 twenty-four hours. 



But the simplicity of this process is broken in upon and 

 destroyed by another effect that is produced during the day. 

 When vapour, in the day, is condensed by cold in the upper 

 regions of the atmosphere, heat, previously latent, is set free, 

 and this heat attaches itself to and expands the gases that are 

 in the part, and renders them lighter than the adjoining gases 

 at the same elevation which have not been disturbed by the 

 condensation of vapour. The warmed gases being thus ren- 

 dered lighter are forced upwards by those that are colder and 

 heavier, when through further expansion fresh condensation 

 takes place, a new heating of the gases occurs, and the process 

 is repeated in an ascending column of air and vapour. The 

 vapour thus carried up by the gases then exerts less incumbent 

 pressure on that which remains below, and this favours further 

 evaporation, and thus permits more vapour to pass from the 

 surface into the atmosphere to increase its total weight and 

 pressure. But this increase of vapour near the surface is, at 

 some part of the process, counterbalanced by the lightening 

 of the gases. These, as they expand by heat, remove a part 

 of their mass which previously pressed on the surface within 

 the locality, to an adjoining part, and the gaseous pressure in 

 the column is thereby reduced.* We have then two altera- 

 tions taking place in the atmosphere at the same time, which 



* Perouse says — " A few days after our departure from Teneriffe we left those 

 serene skies only found in the temperate zone; instead of which a dull whiteness, 

 between fog and cloud, always prevailed. The horizon was contracted within less 

 than three leagues, but after sunset the vapour was dissipated and the nights 

 were constantly fine," — P. 19. 



