of the Aqueous Portion of the Atmosphere. 431 



iieight the two become the same. In the forenoon, therefore, 

 the lower air has its temperature removed progressively fur- 

 ther from the dew-point, but when it ascends, it approaches 

 the dew-point of the higher strata, until at last, at some 

 height, condensation takes place and cloud is formed. When 

 this occurs, the vapour that is in the air below the cloud, 

 being partially relieved from incumbent vapour pressure, 

 ascends more freely from the lower to the higher regions, 

 where the cloud is forming. Thus it is the rise of tempera- 

 ture near the surface that increases evaporation and raises the 

 dew-point, and the vapour produced by this evaporation ex- 

 pands and forces its way upwards by its own laws of expan- 

 sion and diffusion. But in ascending it cools by expansion 

 1° for, say, every 500 yards, whilst it has to pass through the 

 gaseous atmosphere, a medium which is made colder by its 

 own law of cooling, 5° for every 500 yards of elevation ; 

 therefore, as the vapour ascends, it must at some height reach 

 a temperature low enough to condense a part of it and form 

 cloud. On the formation of the cloud taking place, a part of 

 the vapour that is in the atmosphere is converted into globules 

 of liquid (water), and the pressure of this condensed vapour 

 on that immediately below it nearly ceases: for these globules 

 of water, unlike the vapour from which they have been 

 formed, do not rest upon or float in the vapour atmosphere 

 alone, but also on \.\\q, gaseous portion of the atmosphere, which, 

 from its superior quantity and density, will sustain the greater 

 part of the weight of this floating water. The lower vapour, 

 relieved from a portion of that which previously pressed on it, 

 expands upwards more rapidly, and ascends sometimes so 

 Ireely as to prevent such an accumulation as shall further 

 raise the dew-point, although evaporation continues active 

 below. Indeed the pressure from above may be so far re- 

 moved by cloud formation, and the ascent of the vapour be 

 rendered so free and rapid as to lower the dew-point, as took 

 place both at Zurich and on the coast of the Baltic. The 

 processes which have been here described may be traced by 

 attending at the same time to the dew-point and the heights 

 of the ordinary and the wet-bulb thermometers. These are 

 exhibited in the Plymouth registers and diagrams, presented 

 to the British Association by Mr. S. Harris. 



By reference to these, it may be seen that at Plymouth the 

 difference between the dry and wet-bulb thermometers is, at 

 five in the morning, say about 1° of Fahrenheit. This dif- 

 ference increases until one in the afternoon, when it is, say, 

 4°; evaporation must therefore have gone on with increasing 

 activity during this time ; and at three o'clock, that is two 

 hours after the time of highest temperature, the diff*erence 



