SCIENCE IN 1901. 425 



nitude required. Professor Lebedew's measurements are, however, in 

 close agreement with the amounts as calculated from tlie theory; he 

 finds that the pressure per square meter is 0.4 milligram for abso- 

 lutely 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 Asso- 

 ciation 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 rec- 

 ognized 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 Eus- 

 sia, 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 trav- 

 eled, 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 open- 

 ing uj) 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 neces- 

 sity of ensuring certain transmission of simple signals over moderate 

 distances. The imperfections of present methods are sufficiently illus- 

 trated by the experiences of the Ophir and her consorts, and by the diffi- 

 culty 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 inves- 

 tigate, none bulked more largely in the public e3'e 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. 



