4 i2 HERMANN VON HELMHOLTZ 



assumption to give a full explanation of the laws that govern 

 the alterations and motion of the ether. The summing-up of 

 the laws of electrodynamics in the principle of least action 

 presents a system of action and reaction that is complete in 

 itself, and requires no farther complement than the introduction 

 of the hypothesis of incompressibility. Helmholtz succeeded 

 in showing this by adding to the expression which he had 

 previously given for the electro-kinetic potential, another which, 

 for every motion of an incompressible fluid, does not alter the 

 value of that potential. He finds that if ether that is not freely 

 mobile is at rest, the pondero-motive forces derived partly from 

 electrical tensions, partly from a magnetic origin, which cannot 

 be referred back to a potential but evoke recurrent lines of 

 force, are only present in the ether when the flow of energy 

 in unit time increases or diminishes ; that, further, they are not 

 annulled by the incompressibility of the ether, and must throw 

 the ether itself into motion. On the other hand, in fully mobile 

 ether, cyclical forces which cannot be held in equilibrium by the 

 pressure must call forth instantaneous streaming motions of the 

 ether, which attain every degree of velocity, and may augment 

 until the induced forces cancel the pondero-motive forces. 



On August 6 began the momentous journey to America. 

 Frau von Helmholtz could not shake off a certain feeling of 

 anxiety. She wrote to her daughter on July 28 : 



' I hope the journey will do your father good. He looks so 

 pale and tired now . . . and I dread any extra exertion for him. 

 He is getting more and more like his bust, as though Hildebrand 

 had suspected what was coming, and had fixed it in his ex- 

 pression and attitude. To me it is inexpressibly sad to realize 

 that this signifies the coming of old age, the idea which I have 

 so long kept at a distance.' 



Arrived at Bremerhaven the Helmholtzes embarked on the SS. 

 Lahn. ' Fate/ writes Felix Klein to the author, 'ordained that I 

 should be with Helmholtz not only on the outward voyage upon 

 the Lahn, from Augusts to 17, but also on the return upon the 

 Saale. On the voyage out Helmholtz was escorted by a regular 

 staff of physicists, Dr. Lummer in particular. Kindly as Helm- 

 holtz endeavoured to answer lay questions, I found it very 

 difficult to get into scientific relations with him. Nor was there 

 much opportunity, as we had frequent bad weather. One 



