RADIOTELEGRAPHY—FLEMING. 161 
the case of an electric wave propagated over a terrestrial surface. 
If the surface is a very good conductor the wave hardly penetrates 
into it, but glides over the surface. If it is a poor conductor the 
wave penetrates into it to a greater extent, and the worse the con- 
ductivity the deeper the penetration. 
The materials of which the earth’s crust is composed, with some 
exceptions, owe their electric conductivity chiefly to the presence of 
water in them. They are called electrolytic conductors. Substances 
like marble and slate when free from iron oxide are fairly good insu- 
lators. Dry sand or hard dry rocks are poor conductors, but wet sand 
and moist earth are fairly good conductors. Sea water, owing to the 
salt in it, is a much better conductor than fresh water. The follow- 
ing table gives some figures, which, however, are only approximate, 
for the specific resistance of various terrestrial materials in ohms per 
meter cube. It will be seen that dry sand or soils are of very high 
specific resistance, and damp or wet sand or clay fairly low. 
TABLE I1.— Approximate conductivity and dielectric constant of various terres- 
trial materials, 
Waterinle Specific resistence in ohms | Dielectric constant. 
per meter cube. AGT ile 
SGM, W/GHSIRe Gsanps Goonsoccondnanandndgonannpdobasee i 80 
Mreshiwateneearee eecn cece cee ee ea eeeaae 100 to 1,000 80 
Moisit#eamtht y excne ccs eee ase oon oscic'e weminetisns 10 to 1,000 5 to 15 
DIVeCAnt HAAS SG see iee wo hacen simecoee monase ss cqemeae ve 10,000 and upward 2to6 
Wietisain die meeeeseee ce omen eee ea eens enince Eee 1 to 1,000 9 
DryATiVerisanG whee = cease ~ algae aeccceice oceans very large 2to3 
Wieticlavccsctotee cee FE SUOCO RSE BOGOEE OE EEE AGaEDS 10 to 100 oe 
Dr yg Cl ayers ee acentoa ce eraeicis winine cin leis eee wisianialeiowe 10,000 and upward 2tod 
Slatesasacccee cose serene cree sae Ana oaemecerekaae 10,000 to 100,000 aes 
Marblemcteass ctaceccaneetrs saesesesee sceeenees 5,000,000 6 
IMELGUTY ese ceiccwcisb ease oecidsemnnauioeece sce cece -000001 Infinity 
If our earth’s surface had a conductivity equal say to that of 
copper, then the electric radiation from an antenna would glide over 
the surface without penetration. In the case of the actual earth there 
is, however, considerable penetration of the wave into the surface, and 
therefore absorption of energy by it. 
Brylinski and also Zenneck have calculated the depth to which 
electric waves of such frequency as are used in radiotelegraphy pene- 
trate into the sea or terrestrial strata of various conductivities. For 
mathematical reasons it is customary to define it by stating the 
depth in meters or centimeters at which the wave amplitude is 
reduced to 1/e=0.367 of its amplitude at the surface. I have repre- 
sented in a diagram some of Zenneck’s results calculated for waves 
of 1,000 feet in length, and for terrestrial surface materials of 
various kinds, conductivities, and dielectric constants (see fig. 2). 
You will see that in the case of sea water an electric wave traveling 
over it penetrates only to the depth of a meter or two, whereas in 
