The various receiving detectors previously described are arranged to work with dis-continuous waves, producing a separate current impulse from each group or train of waves. In continuous wave systems there are of course no separate groups, and for this reason these detectors are of no use, and a different arrangement is required. The oscillatory currents set up in the aerial are allowed to charge up a condenser, and this condenser is automatically disconnected from the aerial and connected to the operator’s telephones at regular intervals of about 1/1000 second. Each time the condenser is connected to the telephones it is discharged, and a click is produced. These clicks continue only as long as the waves are striking the aerial, and as the transmitting operator interrupts the waves at long or short intervals the clicks are split up into groups of corresponding length. Continuous waves have certain advantages over dis-continuous waves, particularly in the matter of sharp tuning, but these advantages are outweighed to a large extent by weak points in the transmitting apparatus. The dynamos used to produce the high-frequency currents in the Goldschmidt system are very expensive to construct and troublesome to keep in order; while in the Poulsen system the arc is difficult to keep going for long periods, and it is liable to fluctuations which greatly affect its working power. Although all the commercial Marconi installations make use of dis-continuous waves exclusively, Mr. Marconi is still carrying out experiments with continuous waves. There are many points in wireless telegraphy yet to be explained satisfactorily. Our knowledge of the electric ether waves is still limited, and we do not know for certain how these waves travel from place to place, or exactly what happens to them on their journeys. For this reason we are unable to give a satisfactory explanation of the curious fact that, generally speaking, it is easier to signal over long distances at night than during the day. Still more peculiar is the fact that it is easier to signal in a north and south direction than in an east and west direction. There are also remarkable variations in the strength of the signals at certain times, particularly about sunset and sunrise. Every station has a certain normal range which does not vary much as a rule, but at odd times astonishing “freak” distances are covered, stations having for a short time ranges far beyond their usual limits. These and other problems are being attacked by many investigators, and no doubt before very long they will be solved. Wireless telegraphy already has reached remarkable perfection, but it is still a young science, and we may confidently expect developments which will enable us to send messages with all speed across vast gulfs of distance at present unconquered.