SECT. 4] OTHER ELECTROMAGNETIC RADIATION 471 



and the attenuation are summarized in Table II. Note the curious result that, 

 at 1 c/s, the electromagnetic and acoustic velocities are practically identical in 

 sea-water! This table also gives the thickness of sea-water through which an 

 electromagnetic wave will be attenuated 40 dB, this being a reasonably practical 

 limit of penetration. 



Table II 



Propagation of Electromagnetic Waves 

 through Sea-Water 



Frequency Penetration Velocity 



distance ( — 40 dB) in sea- water, 



m/s 



1 c/s 1160 m 1.8x103 



100 c/s 116 m 18x103 



1 mc/s 1 m 2 X 106 



104 mc/s 4 mm 2.7 x lO^ 



4. The Effect of the Sea-Surface on Electromagnetic Propagation 



It was the intention of the preceding section to demonstrate that the at- 

 tenuation of electromagnetic waves in sea-water is sufficiently high that 

 penetration beyond 100 m even at low audiofrequencies is practically im- 

 possible. In spite of this assertion a seemingly contradictory experiment can 



AIR 



DIPOLE 

 Fig. 1 . vSurface-wave propagation from a submerged dipole. 



be performed : if a horizontal electric dipole is submerged below the sea- 

 surface and excited with audiofrequencies, it will be found to radiate an 

 electromagnetic field at considerably greater distances than is shown in Table II. 

 This apparently paradoxical propagation arises from the effect of the sea- 

 surface. The path of an electromagnetic wave between submerged source and 

 receiver is not a straight line through the medium but rather vertically to the 

 sea-surface and then horizontally along the interface. This form of propagation 

 is known as a "surface wave." Fig. 1 shows schematically the radiation path 



