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Wireless Telegraphy 7 into the ether, behave in a manner precisely similar to a stone flung into a mill-pond. As the stone pro- duces a series of ripples or waves which travel across the surface of the pond in regular circles until they either strike the banks or become weakened to such a degree as to become invisible, apparently exhausting themselves, so do the electric waves radiate through the ether, the point at which the electric force is dis- charged corresponding with the spot at which the stone strikes the water. Moreover, as the water ripples grow weaker as they travel, so do the electro- magnetic waves, the result being that a delicate instru- ment is required to detect and to trap them. The arrest of these electro-magnetic waves occupied the attention of many notable scientists. Two, French and British respectively, attacked the problem, but independently and unknown to each other, and both achieved success. The Frenchman was Monsieur Edouard Branly, and the Britisher Sir Oliver Lodge. What is more remarkable, the devices which each contrived were broadly alike, at least in principle. They took a small length of tube resembling that in which the mercury is contained for a thermometer. At each end of this tube a small plug was inserted. The wire leading from the “ capacity ” aerial, or antennæ, to quote its different terms, which was placed upon a mast, and upon which the electro- magnetic waves impinged and were collected during their journey through the ether, was carried to one plug. From the other plug a second wire extended to an instrument. This arrangement left a short gap within the tube. Now the electro-magnetic wave is too weak to jump this gap, so a means of bridging the