Page:Popular Science Monthly Volume 64.djvu/164

160 irregular signs, which, however, sometimes spell out words in the Morse code. He noted that these disturbances are more frequent in summer and autumn than in winter and spring, and in the neighborhood of high mountains more than in the open sea. In settled weather, if present, they reach their maximum between 8 and 10 and frequently last during the whole of the night, with a minimum of disturbance between 9  and 1  Another important matter noted by Captain Jackson is the shorter distance at which signals can usually be received when any electrical disturbances are present in the atmosphere, compared with the distance at which they can be received when none are present. This reduction in signaling distance may vary from 20 to 70 per cent, of that obtainable in fine weather. It does not in any way decrease with the number of lightning flashes, l)ut rather the reverse, the loss in signaling distance generally preceding the first indications on the instrument of the approaching electrical disturbance. It is clear that these observations fit in very well with the theory outlined above, viz., that the atmosphere when impregnated with free electrons or negatively charged gaseous ions is more opaque to Hertzian waves than when they are absent. Captain Jackson gives an instance of ships whose normal signaling distance was 65 miles, failing to communicate at 22 miles when in the neighborhood of a region of electrical disturbance. These effects in the case of wireless telegraphy have their parallel in the disturbances caused to telegraphy with wires by earth currents and magnetic storms.

Another effect which he states reduces the usual maximum signaling distance is the presence of material particles held in suspension by the water spherules in moist atmosphere. The effect has been noticed in the Mediterranean Sea when the sirocco wind is blowing. This is a moist wind conveying dust and salt particles from the African coast. A considerable reduction in signaling distance is produced by its advent.

Another interesting observation due to Captain Jackson is the existence of certain zones of weak signals. Thus, for instance, two ships at a certain distance may be communicating well; if their distance increases, the signaling falls off, but is improved again at a still greater distance. He advances an ingenious theory to show that this fact may be due to the interference between two sets of waves sent out by the transmitter having different wave lengths.

Finally, in the paper referred to, he emphasizes the well-known fact that long distance signaling can only be accomplished by the aid of an aerial wire and a 'good earth.' Summing up his results, he concludes: (1) That intervening land of any kind reduces the practical signaling distance between two ships or stations, compared with th