In his celebrated 1888 experiment on standing waves, Hertz found the velocity of transmission along a wire line to depend on wavelength and to differ from that for wireless transmission, a result that was in contradiction to theory. Hertz called on others to repeat the experiments and verify or refute his results. The call was heard by two groups of scientists. In Dublin, George Francis Fitzgerald and associates repeated and elaborated Hertz's experimental discoveries. For wire transmission, their results were in good agreement with those of Hertz. On the other hand Édouard Sarasin and Lucien de la Rive of Geneva obtained the results required by theory. Hertz looked for an explanation of his own results in the ambient conditions of his apparatus. He corresponded with both Fitzgerald and with the Genevan scientists. These letters are an important historical source in reconstructing the circumstances of Hertz's experiment.