296 HERMANN VON HELMHOLTZ 



elated with the production of dielectric or magnetic polarization 

 in the ether, and in the ponderable medium. Since on this 

 assumption all electric currents are to be regarded as closed, 

 the disparity between the different theories of electro-dynamics 

 (which give identical results for closed currents) disappears, 

 inasmuch as the experiments can be shown to tally with the facts. 



At the beginning of 1873, Helmholtz was tempted by an 

 invitation from Knapp to give a series of public lectures in 

 America ; but after deliberate reflection he replied on January 5, 

 1873:- 



' I get very weary of the Berlin rush, so that at the end of 

 the term the one and only thing I wish is to see no living soul, 

 and to collect my thoughts in some quiet spot. America 

 would be exactly the opposite to all this. And as regards 

 lectures, I am convinced that although I can put scientific 

 matters before people who understand them, in a dry technical 

 fashion, I have not sufficient command of language to do so 

 in a way that will rivet the attention of a large audience 

 who are not professionally trained. Then the preparation in 

 a foreign language costs me double the time, and even if I had 

 the assistance of an Englishman it would only be patchwork. 

 There are still many things that I want to do for science, and 

 I must not lose too much time. Indeed I begin to think that 

 I shall never see America in this life.' 



He had in fact at this time, in addition to his great electrical 

 researches, begun some investigations in Aero-dynamics, the 

 first results of which, under the title ' On a Theorem referring 

 to the Geometrically Similar Motions of Fluid Bodies, with 

 applications to the Problem of guiding Air-balloons ', was pre- 

 sented to the Academy on June 26, 1873. 



The amount of resistance opposed by air or water to a body 

 of complicated form that is moving through it, comes essentially 

 into consideration when it is a question of constructing a ship 

 or balloon, which is to be propelled by any kind of motor 

 apparatus. Since the resistance of the water or air to the oars, 

 paddles, or screws gives the propelling force, while the same 

 resistance against the body of the ship or balloon gives the 

 force of resistance to be overcome, the velocity of progress 

 which can be attained must depend on the ratio of these two 

 forces Yet it was seldom enough that mathematical analysis 



