330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
linked with the question of a common composition of the three planets. 
The planet Mercury in its present state has too high a mean density 
to fit this common composition, but there is a possibility that the 
present Mercury through its proximity to the sun and consequent 
temperature may be appreciably denser than the primitive Mercury. 
The question of the composition of the outer core continues to be 
controversial, and as yet it has not been possible to devise a crucial 
experimental or theoretical test to discriminate between the old and 
the new theories. 
Although the regions E and F of the earth may not be composed 
predominantly of iron and nickel, several arguments all show that 
these two metals must predominate in the inner core. The course of 
the P velocity variations further suggests that there is some progres- 
sive change of composition inside the inner core, perhaps due to the 
presence of some additional denser materials. 
EARTHQUAKES AND NUCLEAR EXPLOSIONS 
Atom and hydrogen bombs resemble earthquakes in that they can 
send seismic waves down into the earth’s interior. Although, as 
pointed out earlier in this article, the available energy in them is less 
than that in the waves produced by large natural earthquakes, there 
is a great compensating advantage to seismic research in that the 
source and location of artificial explosions can be known in advance. 
In spite of the knowledge so far gained from natural earthquakes, 
we are much troubled in our inferences from this data because of our 
initial total ignorance of conditions at the source. On the other hand, 
a nuclear explosion can in effect be regarded as a “controlled earth- 
quake” in which the time and place of origin can be precisely known, 
so that there is the possibility of inferring the internal structure of the 
earth much more accurately. For this reason, seismologists have been 
greatly interested in the possibility of using nuclear explosions to 
help them in their studies. Chemical explosions have already been 
employed with much success in unraveling the structure of the earth’s 
crust. But larger sources are needed to send waves deep into the 
interior. 
The very first atom bomb, exploded in New Mexico in 1945, was 
seismically recorded. A noteworthy feature of that explosion is that, 
whereas the origin time recorded at the source was uncertain by 15 
seconds, the origin time is known to within 2 seconds from seismic 
records. 
There was no mistake at the source in recording the Bikini under- 
water explosion of July 1946. The origin time of that explosion was 
officially released to two decimal places of a second! The resulting 
seismic waves were recorded at eight observatories in the United 
States at epicentral distances as great as 5,000 miles. Even though so 
