March, Igzo.] CURRENT TOPICS. 419

travels from one point to the other, not in a straight line, but along a series of curved paths. If the two free ends of two wires attached to a receiving instrument are inserted into the earth at some distance from each other, and not far from the earth current just mentioned, they will have a difference of potential and a current will flow through the receiving instru- ment. If it is a telephone current which flows through the earth, and if the receiving instrument is a telephone, then in the latter may be heard the message picked up from the earth. To magnify the sensitiveness of the receiver the French introduced the “ Low Frequency Amplifier,” three valves in cascade, with total magni- fication of 125, by which telephone conversations could be overheard at a distance of several thousand yards. The Huns cap- tured one of the valves, saw for what it was used, and at once applied it themselves. There ensued a war of wits, each side en- deavoring to overhear the messages of the other side and to keep its own messages from being intercepted. By the “ Fuller- phone ” messages could be sent by the Morse code without dan- ger of being picked up.

By picking up current from the earth signal messages were sent from one part of the Allied line to another. The secondary of a “ Bower Buzzer ” (weight 6 pounds and fifty times as strong as a telephone buzzer) was earthed at both ends. The receiving ampli- fier was likewise earthed at two points about 200 yards apart. The nature of the soil determined the distance over which sig- nals could be sent. In the muddy soil of Flanders the range was 2000 yards under conditions for which the range on the Somme was doubled. The reason was that in the latter region, which has 6 feet of earth with chalk beneath, the electric currents were con- fined chiefly to a surface layer.

From August to November, 1918, the Royal Engineers erected 326 standard steel bridges and 213 bridges of heavy timber or salved material in the British sector.

G. F.S.

Destruction of Whales by Depth Bombs. (La Nature, Decem- ber 27, IgIg.)-To the eye of an observer in an aircraft a sub- marine and a whale had very much the same appearance provided both were some distance below the surface of the water. This possibility of confusion resulted disastrously to whales in three cases in the North Sea and in one case west of Ireland. In three cases the wounded animal came to the surface. In the fourth case the crew of the dirigible were so certain of their con- clusions that they announced by wireless that they had destroyed a submarine, and the news appeared in print. To their discom- fiture the mangled body of a whale came to the surface somewhat later in the presence of a torpedo boat.

G. F. S.