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 Their path is straight and any obstacle in that path suffices to stop them. These are consequences of the extreme shortness of the waves transmitted. In wireless telegraphy the waves are perhaps a thousand million times as long as the waves of light; they can only be generated or received by special electric machinery and they are not stopped by obstacles in their path. Hence the sender can communicate freely with a station furnished with a receiver adapted to receive his signals just as could be done by the heliograph but with much more certainty and with less risk of the message being intercepted, and these signals can be made to lap round the convex surface of the globe and will doubtless before long be made to reach the Antipodes.

It is not necessary to dwell on the services of wireless telegraphy in the war. Without its assistance aeroplanes could not have produced the profound changes in tactics on land and sea that they did. Indeed it is doubtful whether but for it the labour and cost of bringing heavy artillery into the field would have repaid 39