166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
0.001 of that which it is at 1 kilometer distance. I have represented 
in the diagram some of Doctor Zenneck’s curves. The dotted line 
shows the decrease of amplitude by distance alone, and the firm lines 
that due to distance and terrestrial absorption in various cases. We 
are able to see from them the large effect due to travel over large dis- 
tances of very dry soil. Thus, for instance, if the absorption is such 
as to cut down the amplitude in the ratio of 1:0.367 at 1,000 kilo- 
meters, then at a distance of 3,000 kilometers the amplitude of a wave 
of 1,000 feet in length would be cut down in the ratio of 3,000 to 1 by 
distance alone, but in the ratio of 60,000 to 1 by distance and terres- 
trial absorption combined. 
An important matter is the question of the influence of wave 
length on this absorption. It can be shown from theory that an 
increase of wave length reduces the energy dissipation by the earth. 
Thus in certain cases increasing the wave length from 1,000 to 10,000 
feet increases the range of effective communication one hundred 
‘times. The absorption is also determined by the decrement of the 
wave train being greater the larger the decrement. 
One practical deduction to be made from this investigation is that 
the reduction in wave amplitude which takes place when the wave 
moves over very dry soil is as much due to small dielectric constant 
of the material as to high resistivity. We see also that the wave 
front is very far from being vertical when the waves travel overland, 
and hence it is an advantage in that case for the receiving antenna to 
slope away from the direction in which the waves are traveling or 
from the radiant point. Lastly, it points to the advantage of a long 
wave for overland working. Generally speaking, then, we find that 
electric wave telegraphy is conducted with much greater ease over 
sea than over dry land, the reason being that the dielectric constant 
is large and the conductivity of sea water is sufficient to prevent 
much penetration of the electric wave in the sea, and therefore there 
is not much dissipation of its energy by absorption due to the surface 
over which it travels. We have here an instance of economy in 
nature. Over sandy deserts, where we can, if need be, put up tele- 
graph posts and wires, radiotelegraphy has had some natural diffi- 
culties placed in its way, but on sea, where connection between mov- 
ing stations is the important matter, and telegraph posts are impos- 
sible, special facilities seem to have been afforded us for conducting it. 
The next point in connection with the antenna to be noticed is the 
means adopted of setting up the oscillations in it. The universal cus- 
tom at present is to excite oscillations in a reservoir circuit con- 
sisting of a condenser and an inductance by means of the spark or 
arc. If the spark method is used, then the condenser is one of 
relatively large capacity and the inductance is kept small. If the 
capacity is measured in electrostatic units, and the inductance in 
electromagnetic units, the ratio of capacity to inductance may be 
