144 The Sky 
land to a cape on Tahiti to observe one. The point of land is 
still called Venus Point. 
THE EARTH 
When you see a ship disappear below the horizon you ac- 
cept this as evidence that the earth is round as geography 
books tell us. A few facts may not be out of place. The earth 
is a sphere 7,927 miles in diameter, with the poles somewhat 
flattened so that points at the poles are about 13 miles nearer 
to the center than points on the equator. The total area of the 
earth is 196,950,000 square miles, only 57,510,000 Square : 
miles of which is land; the rest, 139,440,000 square miles be- 
ing covered by the oceans. The irregularities of sea and land 
topography are not sufficient to make the earth a very rough 
sphere. In fact, in proportion, the earth is: smoother than a 
billiard ball. In other respects it is more like a golf ball— 
with a tough outer skin and a liquid center. The liquid 
center, known to geophysicists as the Dahm core, is sup- 
posed to be composed of molten iron. We are certain that the 
center of the earth is liquid because of the way earthquake | 
waves that travel through it behave. There are two kinds 
= bi. 
os eeeiee ds, 
of earthquake Waves: compressional and transverse waves. — 
At the source of an earthquake both kinds are produced, and 
nag 
both may be received at near-by seismological observatories. — 
However, at great distances, where the waves have to pene-_ 
trate deep layers of the earth, seismological stations are able 
to pick up only the compressional waves. Now it is known 
from laboratory experiments as well as from theory that. 
transverse waves are unable to pass through a liquid, so that 
their absence is interpreted as being evidence of the liquid ~ 
state of the earth’s interior. Further astronomical evidence in- 7 
