PAPER BY PROF. HELMHOLTZ. 91 



surfaces is unstable, and sooner or later they break up into whirls that 

 lead to general mixture of the two strata. This statement is confirmed 

 by the experiments with sensitive flames and by those in which by 

 means of a cylindrical current of air blown from a tube we make a sec- 

 tion in a flame and thus make visible the boundary of the moving and 

 the quiet mass. If, as in our case, the lower stratum is the heavier it 

 can be shown that the perturbations must at first be similar to the 

 waves of water that are excited by the wind. The process is made 

 evident by the striated cirrus clouds that are visible when fog is pre- 

 cipitated at the boundary of the two strata. The great billows of water 

 that are raised by the wind show the same process which is different in 

 degree only, by reason of the greater difference of the specific gravi- 

 ties. The severer storms even turn the aqueous billows to breakers, 

 that is to say, they form caps of froth and throw drops of water from 

 the upper crest high into the air. Up to a certain limit, this process 

 can be mathematically deduced and analyzed, on which subject I pro- 

 pose a later communication. For slighter differences of specific gravity 

 the result of this process must be a mixture of the two strata with a 

 formation of whirls and under some circumstances with heavy rainfall. 

 An observation of one such process under very favorable circumstances 

 I once made accidentally upon the Eigi and have described.* 



The mixed strata acquire a temperature and moment of inertia whose 

 values lie between those of the component parts of the mixture, and its 

 position of equilibrium will therefore be found nearer the equator than 

 the position previously occupied by the colder stratum that enters into 

 it. The mixed stratum will descend toward the equator and push 

 back the strata lying on the polar side. Into the empty space thus 

 created above, the strata from which this descending portion has been 

 drawn stretch upwards, and thus their cross section must be dimin- 

 ished. Wherever the lower layers are pushed apart by descending 

 masses of air, as is well known, there arise anti-cyclones ; wherever 

 cavities or gaps arise by reason of ascending masses of air, there arise 

 cyclones. Anti-cyclones and the corresponding barometric maxima 

 are shown, with very great regularity, by the meteorological charts t 

 aloug the very irregularly varying limits of the northeast trade in the 

 Atlantic Ocean— in the wiuter, under latitude 30°; in summer, under 

 40° latitude. On account of the inclined position of the strata, the 

 rain that frequently forms by reason of the mixture of air (Dove's Sub- 

 tropical Rain) falls somewhat farther northward because the water must 

 iall down almost vertically. J 



* See Proceedings of the Physical Society in Berlin, October 22, 1886. 



t Daily Synoptic Weather Charts. Published by the Danish Meteorological Insti- 

 tute aud the German Seewarte, Copenhagen and Hamburg. 



i [The results stated iu the above paragraph were subsequently greatly modified 

 by Helmholtz. See Section v of his second memoir, or page 98 of these Transla- 

 tions. — C. A.] 



