PROFESSOR IN BERLIN 301 



and of the sympathetically vibrating atoms, those equations for 

 the velocity of transmission, and absorption constants, which 

 Ketteler had already deduced from his observations. In the 

 case of weak light absorption, as in solutions of dyes with 

 anomalous dispersion, these agree well enough with the ob- 

 served facts. With a stronger degree of absorption the theory 

 corresponds with the phenomena in the vicinity of the maximum 

 of absorption ; as is found by observation, the curve of refraction 

 reaches its maximum before the maximum of absorption, its 

 minimum after the absorption-maximum, and falls continuously 

 from the former to the latter. With colours that are far from 

 the absorption-maximum, we must, however, have recourse to 

 new theories as to the structure of the ether in the body. 

 Lastly, Helmholtz shows that the extension of the theory to 

 media with a larger number of absorption-bands presents no 

 insuperable difficulties, provided different kinds of sympa- 

 thetically vibrating ponderable masses are presupposed. 



It was about this time that Helmholtz took up the preliminary 

 studies for his meteorological work, coming forward with a 

 popular lecture on 'Whirlwinds and Thunderstorms', which he 

 delivered in the year 1875 in Hamburg. After describing the 

 mechanical conditions from which it follows that the constant 

 alternation of the state of our weather depends (as Dove had long 

 ago shown in detail) upon the displacement of cool, dry polar 

 winds by warm, moist equatorial winds, and vice versa, he goes 

 on to investigate the motions of the air by which the regularity 

 of tropical weather is interrupted, such as the hurricane or 

 cyclone. 



We learn from one of Helmholtz's letters that it was the 

 accident of his observing the formation of cloud and storm from 

 the top of the Rigi that drew his attention to these natural 

 phenomena, and led him to the wonderful experiments in which 

 a vertical tube filled with air is formed in the centre of a circu- 

 lating mass of water, in the exact shape in which a waterspout is 

 usually represented. The storms too develop in vortex form, 

 and at the centre of such a vortex there is generally a space 

 where there is little motion of the air. While the storm travels 

 in the direction of the earth's rotation, the side which it presents 

 to the equator invariably blows a west wind ; if dry and moist 

 air come together, great masses of air may accumulate, as Reye 



