HERMANN VON HELMHOLTZ 



be found at surfaces of separation between two layers 

 and along the borders of rotating masses. 



Helmholtz drew special attention to the manner 

 in which, according to the kinetic theory of gases, 

 viscosity and interchange of temperature take place 

 across such surfaces of separation. We may suppose 

 that in the upper regions of the atmosphere there 

 are contiguous layers of air of different densities, 

 temperatures and velocities. If no watery vapour 

 condenses at the surfaces across which interming- 

 ling takes place nothing will be seen. But should 

 there be condensation of vapour, clouds will be 

 formed ; and then, just as wind raises waves on 

 the surface of the ocean, so cloud waves will be 

 formed on the side of the less dense layer. The 

 action will not take place in a quiet steady manner, 

 but by fits and starts, being, indeed, a case of dis- 

 turbed equilibrium. The effect is well seen in the 

 beautiful wavy appearance of barred cirrhi clouds, in 

 which the shining white bars correspond to the wave- 

 troughs, and the thin translucent bars, through which 

 the light of the blue sky penetrates, to the wave crests. 

 Helmholtz, at Cap d'Antibes, measured the velocity 

 of the wind with an anemometer, and counted the 

 number of waves on a given surface of sea, and thus 

 obtained data on which to base his calculations. He 

 thus found the connection between the strength of the 

 wind and the length of the waves. The water waves 

 were compared as to length with cloud waves. A 

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