162 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
the case of very dry soil it would penetrate much deeper. Owing to 
the conductivity of the soil, this movement of lines of maenetic force 
through it sets up currents of electricity which expend their energy in 
heat. This energy must come from the original store imparted to 
the sending antenna, and therefore the wave is robbed of its energy as 
it travels over the surface. 
It should be clearly understood that when a wireless telegraph 
antenna is in operation it sends out into the surrounding space a 
nearly hemispherical electrical wave, which spreads out in all direc- 
tions. There are five causes which weaken the wave as it travels 
outward: 
(1) The distribution of the energy continually over a larger and 
larger area. The wave amplitude diminishes inversely as the distance, 
and the wave energy inversely as the square of the distance. This 
is proved theoretically from first principles by Hertz’s equations, and 
tres. a 
100 Metre K- j 
40 » <80 
1 10 100 ©1000 10,000 100,000 1,000,000 
Sea Fresh Damp Dry Soil 
Water. | Water. Soil. ee 
Specific Resistance in Ohms per dletre Cube. 
Fig. 2.—Depth of penetration of waves 1,000 feet in length (Doctor 
Zenneck). 
has been confirmed experimentally by the experiments of Messrs. 
Duddell and Taylor and of Professor Tissot. 
(2) There is a certain absorption of energy due to the ionization 
of the atmosphere by daylight and to other causes, but this is only 
detectable over long distances and for the present moment we shall 
neglect it. We include, however, under this head, obstructions due 
to special atmospheric conditions, electrical or material. 
(3) There is a diminution, due to earth curvature, which is ope 
tive only over long distances. 
(4) There is some reduction of intensity which results from ke 
stacles—such as hills, trees—especially from cliffs of ironstone or 
conductive rocks, due to distortion of the electric field. 
(5) Lastly, there is the weakening due to the dissipation of energy 
by the penetration of the waves into the surface over which they 
travel. 
