300 DISCOVERY CH. 



in October, 1913, during a heavy gale. The wireless 

 telegraphic call for help was first received by the Car- 

 mania, which immediately made use of her wireless 

 installation to spread it far and wide, with the result 

 that ten steamships hurried to the burning vessel and 

 were able to save all the people more than five hundred 

 who had remained on board. 



The maximum distance at which a light can be seen 

 depends upon the brightness of the light and the sensitive- 

 ness of the eye. A light of twelve candle-power is 

 visible at a distance of five miles, and for it to be seen at 

 a distance of 100 miles it would have to be nearly five 

 thousand candle-power. In wireless telegraphy the 

 transmitting station sends out electric waves precisely 

 similar to the waves which produce the sensation of 

 sight, but they are invisible and have to be detected by 

 sensitive receiving instruments instead of the eye. 

 These detectors have reached such a degree of perfection 

 that they are of the same order of sensitiveness as the 

 human eye. 



Expressed in terms of energy, we may say that the 

 light which can just be seen by the eye, the sound which 

 can just be heard by the ear, and the electric wave 

 which will just affect a receiver, is of the same intensity. 

 If, therefore, energy could be radiated at the same rate, 

 either as waves of sound in the air, or as waves of light, 

 or as the longer electric waves, we should be able to 

 detect them at approximately the same distance by 

 means of our ears, or our eyes, or a wireless telegraphy 

 receiver. When we speak, our voice is heard and under- 

 stood by those who listen to us ; and if we could speak 

 with the same energy as that of the transmitting appara- 

 tus of a wireless telegraphy station, the ear would be 

 able to detect the sound at about the same distance as 



