243 Mr. W. S. Jcvons on the Forms of Clouds. 



and motion. Liquids are in many respects very unlike gases, the 

 latter being chieily distinguished by the property of elasticity ; 

 but in the atmosphere this property cannot be directly productive 

 of force or motion, or even the modification of force and motion, 

 because the air, being only confined by the superincumbent air, 

 is always at perfect freedom to assume the density and elastic 

 force due to the pressure of that air. Elasticity is, as it were, 

 always self-adjusting, and never called into play, so that free air 

 will resemble in its motions a very rare liquid, and any part of 

 the atmosphere will be subject to the same hydrodynamical laws 

 as the interior of a body of liquid. 



3. But assuming that we understand the mere motions which 

 take place in the production of a cloud, it is quite another thing 

 to show whence the forces which produced those motions are 

 derived. In our miniature experiments such conditions are 

 known beforehand, because we have prepared our liquids of dif- 

 ferent specific gravities by dissolving various weighed quantities 

 of some heavy soluble substance in water ; and we can project 

 the solutions thus prepared into each other with any desirable 

 velocity or direction by the aid of a simple apparatus to be pre- 

 sently described. But to explain, according to the known com- 

 plex properties of the atmosphere, the origin of those differences 

 of specific gravity and of those motions which we have established 

 to exist in the formation of a cloud, is a distinct and more diffi- 

 cult part of the subject. 



4. It is necessar}^, so to speak, to translate the conditions of 

 our liquid experiments into the language of air, water and heat, 

 which are the only elements actively concerned in the production 

 of meteorological phsenomena. In fact, every complicated change 

 which heat, itself only a mode or disguised form of force, may 

 occasion in air or water, and every disturbing effect which air 

 and water, — the latter in no less than three distinct forms, the 

 solid, liquid, and gaseous, — may mutually have upon each other, 

 will have to be taken into account before we can lay down the 

 vera causa of an atmospheric cloud as distinctly as we can an- 

 nounce the conditions of our miniature one. 



My experimental arrangements and results may be briefly 

 explained and easily understood, and my readers must judge for 

 themselves whether the representation which they afford of the 

 internal motions of clouds be satisfactory. But even if so, it 

 will still be open to them to accept or dispute the suggestions, 

 founded upon less simple principles, which I shall offer concern- 

 ing the causes of those motions. 



The Section-glass. 



5. The apparatus which I employ may be conveniently termed 



