146 The Sky 
the structure of the earth hundreds of miles down where we’ 
have never been able to gain access. Present theories indi- 
cate that the central iron core is enclosed with a series of 
further layers of different compositions, the outermost layer 
being of course our familiar land and sea as we know them. 
DEPTH CHARGES 
By exploding quantities of TNT on the ocean floor, and re- 
cording the reflections and echoes of the waves with sub- 
merged instruments, some information has been obtained 
about the rocks which lie beneath the bottom of the ocean, 
but little is really known about the structure of the ocean 
bottom. Most studies, such as our knowledge of the oozes, 
extend only to the first few inches. Dr. Maurice Ewing of the 
Woods Hole Oceanographic Institution was about to obtain 
data on the depth of ocean sediments and the rocks that lie 
underneath them. The war has interrupted these investiga- 
tions temporarily. 
THE EARTH IN SPACE 
As it goes through space the earth executes a rather com- 
plicated motion. First of all it rotates upon its North-South 
axis once every day. The rotation causes the. sequence of 
night and day, the rising of the sun and moon, of the stars, 
their procession across the sky, and their setting. Once 
around in a day at first may seem slow, but if measured as 
linear velocity at a point near the equator it means over 1000 
miles per hour—a speed that has been maintained for mil- 
lions of years. As a matter of fact, the earth is slowing down 
a little because of the action of the friction of the tides in 
shallow seas such as the Bering Sea. In the year 2000, a day 
