HELMHOLTZ IN BERLIN 



glacier flows down a valley, showing the curves and 

 stream lines as in the true glacier. It is a curious 

 sight to watch corks, if left to themselves for a 

 long time on the surface of the wax, ultimately 

 float on it, just as bodies left on the surface of a 

 glacier may by-and-by be found deeply embedded in 

 the apparently brittle substance. 



In one of his lectures Helmholtz gives an explana- 

 tion of waterspouts. He shows that the whirling 

 water forms a vertical tube full of air. Probably his 

 most interesting contribution to meteorological physics 

 is contained in two papers on the movements of the 

 air published in 1888 and 1889. This was followed 

 by one in 1890 on the Energy of Winds and Waves. 1 

 In these he develops the mathematical theory of the 

 formation of cloud strata. It is said that in one of 

 his Alpine excursions he saw, from the summit of the 

 Rigi, the grand spectacle of a table-land of clouds 

 below him, and the appearance suggested that of 

 waves on the sea seen from the height of a rocky 

 coast. The formation of cloud waves was suggested 

 from the formation of water waves, the fundamental 

 idea being that a plane surface of water over which a 

 wind of uniform velocity is blowing is in a state of 

 unstable equilibrium, and that waves thus originate. 

 Here we have two fluids, air and water, of different 

 densities, gliding over each other. While friction in the 

 upper regions of the air must be very slight, it will 



1 Whsenschaftl. Abhandlungen, Bd. iii., s. 289 5 8. 309 ; s. 333. 

 22 3 



