Mr. Bell sums up the defects still existing in his system as follows:--1st. The receiver of the messages must have a good musical ear, in order to distinguish the value of sounds. 2nd. Since the signals can only take place when the transmitted currents are in the same direction, two wires must be employed in order to exchange messages on each side. He surmounted the first difficulty by providing the receiver with an instrument which he called the vibrating contact breaker, and which registered automatically the sounds produced. This contact breaker was placed in the circuit of a local battery, which could work a Morse instrument under certain conditions. When the sounds emitted by the instrument did not correspond with those for which it had been tuned, the contact breaker had no effect on the telegraphic instrument: it only acted when the sounds were those which were to be interpreted, and its action necessarily corresponded to the length of the sounds. Mr. Bell adds that he applied the system to electro-chemical telegraphs; but we need not dwell on this part of the invention, since, as we have said, it is no longer his special study. _System of M. Lacour of Copenhagen._--M. Lacour’s system was patented on the 2nd September, 1874, but his experiments were commenced on the 5th June of the same year. Since M. Lacour believed that the vibrations would be imperceptible on long lines, his first attempts were made on a somewhat short line; but in November 1874 fresh experiments were made between Fredericia and Copenhagen on a line 225 miles in length, and it was ascertained that vibratory effects could be easily transmitted, even under the influence of a rather weak battery. [Illustration: FIG. 59.] In M. Lacour’s system, the sending instrument is a simple tuning-fork, placed in a horizontal position, and one of its arms reacts on a contact breaker, which can produce precisely the same number of discharges of currents as there are vibrations of the tuning-fork. If a Morse manipulator is inserted in the circuit, it is evident that if it is worked so as to produce the dots and dashes of the Morse alphabet, the same signals will be reproduced at the opposite station, and the signals will be manifested by long and short sounds, if an electro-magnetic receiver is connected with the circuit. This sender is shown fig. 59. Fig. 60 represents M. Lacour’s receiver. It consists of a tuning-fork F made of soft iron, not of steel like the sending tuning-fork, and each of its branches is inserted in the bobbin of an electro-magnetic coil C C; two distinct electro-magnets M M react close to the extremities of the fork, in such a way that the polarities developed on the two branches of the fork under the influence of the coils C C should be of contrary signs to those of the electro-magnets M M. [Illustration: FIG. 60.]