286 THE MECHANICS OF THE EARTh's ATMOSPHERE. 



their form and appearing and disappearing, and such as also occur with 

 clouds formed by adial)atic expansion, especially during thunder storms. 



These different methods of cloud formation by direct cooling, by adia- 

 batic expansion, and by mixture can of course also occur side by side 

 in the most varied combinations, as is expressed in the extraordinary |j 

 diversity of cloud forms. 



It seems to me very important in the study of these forms to keep 

 these different processes in view, since only then can Ave hope fluallyJ 

 to attain a thorough knowledge of these forms. 



Above all, as Hellmann has a])propriately expressed it, it is necessary: 

 to la}' the foundation for a "physiology of the clouds'' before we can 

 hope to attain to a truly satisfactory arrangement and nomenclature.*' 



But further work will still be necessary before this problem is solved,, 

 since on the one hand the question becomes more complicated the 

 nearer we approach to it, and since on^ the other hand it ai)pears s*' 

 extraordinarily- difficult to realize experimentally even a[)proximately 

 those conditions under which the formation and dissolution of cloud»« 

 take place in the atmosphere. 



Beautiful and praiseworthy as are the ex])eriments that Vettin has- 

 made with clouds of smoke, still we must be very careful about the con- 

 clusions which we would draw from them as to the formation of the real 

 clouds. All exi)erimeuts with smoke, when looked at properly, give 

 only pictures of the movements in dry air, since the condensation and 

 evaporation as well as the processes of compression and expansion are- 

 excluded, and we therefore are working under conditious such that in 

 the real atmosphere no formation of clouds would occur. 



But it is precisely because of these processes (condensation, evapora- 

 tion, compression, and expansion) that we can not consider the motion 

 of a cloud as a measure of the motion of the air, for not only do clouds 

 hang apparently motionless on the mountains, whereas in fact strong 

 winds are streaming through them {e. g. Fiihn cloud-bank, the Table- 

 cloth of the Table mountain, the Cloud-cap of the Helm- wind) but it 

 even hai)pens to aeronauts that they pass through clouds while moving 

 in a horizontal dii'ection. This latter is however only possible when 

 the cloud has a velocity different from that of the air in which it tloats, 

 since the balloon itself has only the power of vertical motion. 



The cloud is in fact not a body that can be driven forward as such 

 by the air unchanged, but is a form in a process of continuous forma- 

 tion and disappearance, and can have as a whole motions entirely dif- 

 ferent from those of the particles nf which it consists. 



On account of the increased interest with which at the present time 

 we are studying the forms and motions of the clouds, it seemed to me 

 important to call attention to all these points sinwe we must have these 

 in mind when we attempt from the external appearance of the clouds to 

 draw any conclusion as to the i)rocesses which in individual cases de- 

 termine their growth or dissolution and therefore also their form. 



* Compare also O. Volger iu Gaea, IduU, vol. ii, pp. 05-75. 



