December 13, 1907] 



SCIENCE 



815 



services to the Academy of Natural Sci- 

 ences he was unanimously elected its pres- 

 ident in 1881, he having served the institu- 

 tion as chairman of the board of curators 

 continuously for forty years. Both posi- 

 tions he held at the time of his death. That 

 the value of Dr. Leidy's contributions to 

 science have not been over-estimated by his 

 personal friends and admirers is shown by 

 the honors conferred on him by the learned 

 institutions both at home and abroad, and 

 by the marked courtesy and attention paid 

 to him by the most distinguished savants 

 on the occasion of his visits abroad. 

 Among the honors conferred upon Dr. 

 Leidy may be mentioned the LL.D. of Har- 

 vard, the medals of the Royal Microscopical 

 and Geological Societies of London, the 

 Cuvier medal of the Academy of Sciences 

 of Paris, membership in all the most im- 

 portant learned societies in this country 

 and in those of England, France, Germany, 

 Eussia, Italy, Norway, Sweden, Hungary, 

 Denmark, Spain, Portugal and Brazil. 

 Surely it was a fitting tribute to one so 

 honored at home and abroad as Joseph 

 Leidy that his personality should be em- 

 bodied in enduring stone in his native city, 

 even though his works were an imperishable 

 monument to his memory. 



Henry G. Chapman 

 Jeffeeson Medical College 



THE ARC AND THE SPARK IN RADIO- 

 TELEGRAPHY ' 



The discovery by Heinrich Hertz be- 

 tween 1887 and 1889 of experimental 

 means for the production of electric waves 

 and Branley's discovery that the conduct- 

 ivity of metallic particles is affected by 

 electric waves form the foundation on 

 which, in 1896, Signor Marconi built up 

 his system of wireless telegraphy. 



'Evening discourse before the British Associa- 

 tion for the Advancement of Science, Leicester, 

 1907. 



Many of the early investigators certainly 

 had glimpses of a future system of being 

 able to transmit messages without connect- 

 ing wires, for as early as 1892 Sir William 

 Crookes predicted in the Fortnightly Re- 

 view the possibility of telegraphy without 

 wires, posts, cables, or any of our costly 

 appliances, and said, granting a few rea- 

 sonable postulates, the whole thing comes 

 well within the realms of possible fulfil- 

 ment. 



Two years later Sir Oliver Lodge gave 

 his memorable lecture on the work of 

 Hertz, and carried the matter a step nearer 

 the practical stage. 



There will not be time to dwell to-night 

 on the early history of the art and its de- 

 velopment. It will be necessary, however, 

 to explain some of the fundamental prop- 

 erties of signaling by means of Hertzian 

 waves in order to be able to bring out 

 clearly the relative advantages and disad- 

 vantages of the two rival methods now in 

 practical use for producing Hertzian waves 

 for wireless telegraphy. 



The fundamental part of the transmit- 

 ting apparatus may be said to consist of 

 a long conductor, generally placed ver- 

 tically, in which an alternating or oscil- 

 lating current is set up by some suitable 

 means. Such a conductor radiates energy 

 in the form of Hertzian waves at right 

 angles to itself into space, in very much 

 the same way that an ordinary candle sends 

 out light in all directions. This radiation, 

 though it is strictly in the nature of light, 

 is invisible to our eyes, as the frequency is 

 too low. 



If we set up any other conductor ap- 

 proximately parallel to the first, there will 

 be produced in this second conductor alter- 

 nating or oscillating currents having the 

 same frequency as those in the first con- 

 ductor, and which can be detected by suit- 

 able instruments. 



The simplest and one of the earliest 



