178 M. W. Siemens on the Admissibility of the 
continual condensation takes place ; but, at the heights in 
which according to experience the formation of clouds occurs, 
it must still be too inconsiderable to bring about the observed 
precipitation. The reason of the condensation that takes 
place in ascending air-currents lies essentially in this — that 
the aqueous vapour is intimately mixed with the air, and that 
in the ascending current it does not take the adiabatic tem- 
perature belonging to itself, but that of the greatly prepon- 
derating mass of air with which it is mixed. Now, as the 
air is cooled much more quickly with increasing height than 
the vapour, the latter is cooled below the adiabatic tempera- 
ture belonging to it ; and this diminution of temperature 
gives rise to its condensation if the point of saturation of the 
vapour is overpassed. 
This conception is apparently contradicted by the circum- 
stance that aeronauts have repeatedly proved that strata of 
warmer air frequently overlie colder ones, while the law of 
adiabatic expansion requires a continuous decrease of pressure 
and temperature. But this is easily explained by the dis- 
similar constitution of the earth's surface, owing to which 
the ascending current frequently, and in many places, has a 
much higher temperature and contains a far greater quantity 
of aqueous vapour than in others. If the amount of water 
held by such a hot ascending mass of air is so great that part 
of it is separated during the ascent and falls as rain, the air 
met with in the upper strata of the atmosphere is still further 
heated by taking up the latent heat of the aqueous vapour ; 
and thereby its volume and upward impulse are augmented ; 
and the final result must be a stratum of relatively warm and 
comparatively waterless air, which is then pushed by its expan- 
sion over colder air, but which, from containing more aqueous 
vapour, is lighter *. These departures from the rule that 
the temperature and density of the atmosphere decrease as the 
height increases, while the contained water must increase, 
are easily accounted for. The latter must be the rule for 
the higher latitudes at least, since the masses of warm air 
containing relatively much water continually ascending in the 
calms, on their way to higher latitudes, after loss of their 
greater heat by radiation, it is true for the most part sink 
to earth again as descending currents ; but they must also 
in part reach the high latitudes as an upper equatorial current. 
* Rronig lias already proved that tlie aqueous vapour mixed with an 
ascending current produces by its condensation no diminution, hut an 
augmentation of volume, as the latent heat of the vapour enlarges the 
volume of the air hy a much greater amount than the volume of the con- 
densed vapour before its condensation (Fortschr. d. Phys. xx. p. 626). 
