When the circuits are altogether metallic, there is much less risk of confusion, and M. Zetzche declares that sounds proceeding from other wires are in this case little heard, and then only momentarily, so that it is much more easy to hear with this arrangement than with the one in ordinary use. ‘It is not,’ he says, ‘the resistances of the wire, but rather the diversions of the current near the posts, which interfere with telephonic correspondence on long lines above ground. This was proved by the following experiments:--I connected the telegraphic line from Dresden to Chemnitz with a line from Chemnitz to Leipzig (54 miles), which made a circuit of 103 miles, going to earth at its two extremities. There was no communication between Dresden and Leipzig, but Leipzig and Dresden could communicate with ease, in spite of the greater extent of line. I broke the connection with earth, first at Leipzig, then simultaneously at Leipzig and Dresden, and I observed the following effects. When insulation took place at Leipzig only, the telephone could be heard at the stations of Dresden, Riesa, and Wurzen; when the line was insulated at both ends, the communication was good between the two latter stations, but it was observed that at the intermediate station the words spoken at Wurzen were more distinctly heard than the words spoken at Riesa were heard at Wurzen. Since the distance from Wurzen to Leipzig is little more than half that from Riesa to Dresden, there are consequently nearly twice as many posts on the latter line, which carry the currents to earth, and hence I conclude that these diversions of current explain the possibility of conversing on an insulated line, and also why sounds are more distinctly heard at the Riesa station in consequence of the greater intensity of current still remaining on the line.’ Some vibrations also result from the action of currents of air on telegraphic wires, which produce the humming sound so well known on some lines, and these may also react on the telephone; but they are in this case generally mechanically transmitted, and they may be distinguished from the others, if the sounds which ensue are heard after the telephone is excluded from the circuit by a break with a short circuit and after the communication to earth established behind the telephone has been broken.