Page:Popular Science Monthly Volume 60.djvu/433

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nitude required. Professor Lebedew's measurements are, however, in close agreement with the amounts as calculated from the theory; he finds that the pressure per square meter is 0.4 milligram for absolutely black bodies, and double as much for perfect reflectors. This experimental verification of one consequence of Maxwell's wonderful electro-magnetic theory suggests reference to another which is gradually passing out of the experimental stage and becoming of practical utility. It is only a few years since Principal Lodge astonished the British Association by showing that the electric waves in connection with which Hertz's name is famous, could be propagated to a distance of half a mile or so; yet already the wireless telegraphy so initiated has become a recognized portion of the equipment of ships in all the important navies of the world. In the mercantile marine, too, it is making way, so much so that Lloyd's, in the course of the 5'ear, contracted to have it fitted in a number of their signal stations round the British coasts. Almost every country can show workers who are engaged, with more or less success, in perfecting its appliances, among them being Popoff in Russia, Slaby in Germany, Guarini in Belgium, Ducretet in France, and Marconi in England. The last-named, by using waves a thousand feet long, succeeded in detecting electro-magnetic radiation which had traveled, 1,800 miles from the source that produced it, and he is looking forward to the early establishment of a commercial system of sending messages across the Atlantic by wireless telegraphy, and later to opening up communication with the Cape. But while the performance of such feats in long-distance transmission is certainly a legitimate object of ambition, it must not be forgotten that very much remains to be done in perfecting wireless telegraphy for comparatively short distances. If ever it is to become of real commercial importance, means must be found not only to enable two parties to communicate with each other without fear of their message being overheard, but also to prevent a third party from making communication impossible altogether by the simple device of working his own apparatus and thus rendering the signals unintelligible. Even before these refinements comes the necessity of ensuring certain transmission of simple signals over moderate distances. The imperfections of present methods are sufficiently illustrated by the experiences of the Ophir and her consorts, and by the difficulty which the Admiralty have found in obtaining coherers that can be trusted to respond satisfactorily over their test-distance of 70 miles—a difficulty which has induced them to take the manufacture of these delicate pieces of apparatus virtually into their own hands.

Of all the physical forces which it is the business of science to investigate, none bulked more largely in the public eye than electricity. The idea of distributing current for various purposes from a central station to a large surrounding area made distinct progress during the year.