302 HERMANN VON HELMHOLTZ 



has shown, which were originally in stable equilibrium, but 

 with alterations of temperature pass gradually into unstable 

 equilibrium. For instance, cloudy and dry air lying upon or 

 by the side of each other may be of such temperatures that 

 they are of exactly equal weight at a moderate height in the 

 atmosphere ; in this case, in the lower half of the atmosphere, 

 where the pressure is greater, the foggy air will become denser, 

 and sink to the ground, while in the upper half of the atmo- 

 sphere the same foggy air becomes more attenuated with lower 

 pressure than the dry, grows lighter, and rises. The originally 

 stable equilibrium will then (since with prolonged action of the 

 sun the lower layer becomes warmer and moister, while the 

 upper loses heat by its radiation into space) gradually pass into 

 unstable equilibrium. If the equilibrium is interrupted at any 

 point, so that pressure becomes less owing to the lighter 

 ascending foggy air, the lower air will rise, and be drawn into 

 the ascending current, while round it, where the equilibrium 

 was still stable, it will become even more so from the evacuation 

 of the moist air and sinkage of its upper surface ; the rise would 

 continue until the whole of the lower layer had mounted up. 

 In discussing thunderstorms, Helmholtz indicates the store of 

 negative electricity with which the earth is permanently charged, 

 as the source of the electric discharges. In conclusion, he lays 

 stress on the difficulty of predicting the weather, and the 

 circulation of air in the atmosphere. 



In general it is to be remarked that we can only calculate 

 beforehand, and understand in all observable details, those 

 natural processes in which small errors in the formulation of 

 the premises involve only small errors in the final results. As 

 soon as unstable equilibrium comes into play, this condition is no 

 longer fulfilled. Thus chance still exists on our mental horizon ; 

 but in reality it is only an expression for the complexity of our 

 knowledge and the clumsiness of our methods of combination. 

 A mind endowed with exact knowledge of the facts, whose 

 thinking operations were accomplished so rapidly and precisely 

 as to precede events, would discover in the wildest caprices of 

 the storm, no less than in the motions of the stars, the harmonious 

 ruling of eternal laws, that we can only guess at and assume. 



On November 4, 1875, a heavy blow fell on Helmholtz and 

 his family. Robert von Mohl, who had come a few days 



