xi ACROSS THE BORDER 301 



that at which a wireless receiver is affected. The energy 

 of the vibrating air acting upon the drum of the ear in the 

 case of the faintest audible sound is about the same as 

 that falling upon the retina of the eye from the faintest 

 visible star. The energy of the loudest sound which 

 can be distinguished (at the point when the ear cannot 

 decide which of two tones is the louder) has been esti- 

 mated to be about as much as that involved in the 

 growth of a single blade of grass in June. 



" Communication by wireless telegraphy is very 

 wonderful," once remarked a lady to Mr. Marconi. " It 

 is not half so wonderful as our conversing together now," 

 modestly replied the inventor ; and he was right. The 

 production of sounds by the organs of speech, their 

 transmission by waves in the air, and their appreciation 

 by the organs of hearing, involve infinitely more com- 

 plicated actions than are concerned in producing electric 

 waves at one station and detecting them hundreds or 

 thousands of miles away. 



It is not possible here to refer to the further develop- 

 ments of wireless telegraphy. Already the messages 

 sent out from the Eiffel Tower and other high-power 

 stations can be read with instruments which can be 

 carried in the pocket and connected to an iron railing 

 or gate ; and human speech has been conveyed by wire- 

 less telephony from Arlington (Virginia) to Mare Island 

 (California) a distance of 2500 miles ! With such 

 remarkable achievements to command our admiration, 

 who will have the temerity to assign limits to what will 

 be accomplished by science in the future ? 



Next to wireless telegraphy, the discoveries of science 

 which seem the most wonderful to most people are 

 those of Rontgen rays and radium. When in 1895 the 

 announcement was made that Prof. Rontgen, then of 



