78 Intelligence and Miscellaneous Articles. 



the velocity in free air. The number thus obtained, 331*2 metres, 

 is a little higher than that of Eegnault, but is near that obtained 

 more than sixty years ago by the members of the Bureau des 

 Longitudes. 



By numerous experiments made with different instruments (fog- 

 horn, organ-pipes, &c), we have ascertained that the height of the 

 sound has no influence on the velocity of propagation of the wave. 



We have found, in fact, for the time of propagation of the wave 

 along the first path 2h : — 



sec. 



ut_! Bombardon 37'30 



ut_, Mute -42 



ut x Flute -38 



ut 2 Flute -40 



fa 2 Double Bass -37 



fa 3 Horn '38 



Si 3 j? Cornet *38 



Sij> Cornet -41 



The small differences observed seem to be due merely to differ- 

 ences in the intensity of the wave in the various cases. 



The movement of the wave is independent of the vibrations 

 which it carries along. — Comptes Rendus, April 3, 1888. 



ON THE ELECTRIFICATION OF METAL PLATES BY IRRADIATION 

 WITH ELECTRICAL LIGHT. BY WILLIAM HALLWACHS. 



At the close of an article recently published * I had mentioned 

 that, to judge from preliminary experiments, metal plates become 

 electrostatically charged when irradiated with the electrical 

 light. It could be seen from the outset that we were not dealing 

 here with charges at potentials that could be measured by a gold- 

 leaf electroscope. In order to ascertain whether a charge measur- 

 able with sensitive electrometers could not be obtained, the final 

 experiments w r ere made as follows :— The circular metal plate, 

 8 centim. in diameter, to be illuminated was suspended by an 

 insulated wire in the centre of a cylindrical sheet-iron vessel, 

 50 centim. in length and 37 centim. in diameter, which lay with 

 its axis horizontal. The surface of the metal plate was coated with 

 rust, but one portion of the surface was brightly polished and was 

 put to earth. The wire by which the plate was suspended passed 

 through, but insulated from, an earth-connected brass tube, to a 

 HankeFs electrometer, which was chosen for these experiments on 

 account of its small capacity. In the lid of the metal cylinder was 

 a circular aperture of about 8 centim. diameter, which trans- 

 mitted the rays of the arc lamp to the plate hanging inside. This 

 aperture was covered by a piece of wire gauze, in order to 

 prevent any appreciable induction of the lamp on the plate to be 



* Wiedemann's Annalen, vol. xxxiii. p. 301. 



