IGY IN RETROSPECT—ROBERTS 283 
tus, permitted widespread detailed surveys of the earth’s gravity 
field, supported by gravity surveys made in the search for petro- 
leum. Thus are disclosed the effects of irregular earth-mass distribu- 
tion, not only of mountains and ocean deeps that we can see 
but also of hidden ore bodies and structural irregularities of the 
earth’s rocks. The new gravimeters are free of some of the limita- 
tions on use of the pendulum and have even been refined to cope with 
the accelerations of ship motion. Thus we may make gravity sur- 
veys of the watery three-fourths of the earth. It is believed, more- 
over, that airborne gravimeters will soon be an actuality. 
The gravimetry program, aside from general survey coverage and 
the accomplishment of several important Antarctic profiles, dealt 
with the problem of earth tides, in the measurement of which gravity 
observations play a leading part. Gravimeters are sufficiently sen- 
sitive to indicate not only the changes in the lunar and solar tide- 
producing forces but also the small changes in distance involved in 
the rise and fall of the earth’s crust. Such motion at Washington 
is nearly 6 inches in amplitude. Thus the gravity work contributed 
to our knowledge of the elastic constants of the earth and its crust, 
as well as of world mass distribution. 
The gravitational force, which man has learned to measure with 
exquisite precision—one part in a million for absolute determinations, 
and a hundred times better for relative measurements—remains a 
scientific mystery, its true nature hidden somewhere outside man’s 
conceptual capacity. 
Seismological work also was stimulated by the unusual opportuni- 
ties to place seismograph recorders in neglected parts of the world, 
particularly the polar regions. In the Arctic and sub-Arctic many 
gaps in coverage were filled, mostly by Soviet scientists. Antarctica, 
an aseismic continent except for one or two minor shocks a year, was 
nevertheless notable for its valuable readings on a broad range of 
far-southern quakes and for its clues to the seismicity of a vast region. 
Numerous readings of earth waves from Japanese shocks, agitating 
the seismographs after traveling the longest all-oceanic wave paths 
on record, helped in the determination of travel velocities through 
oceanic crustal formations. 
A specialized application of seismology, in which reflected waves 
from small explosions on the surface are used to discover subsurface 
structure, disclosed the ice depths in Antarctica and Greenland, and 
indicated the continental structure of Antarctica. Similar explora- 
tion of the Andean massifs in South America showed that the under- 
lying crust is unexpectedly thin, contrary to the normal expectation 
of a deep root structure. 
Seismologists began the intensive development of seismographs 
sensitive to ground waves of ultralong period—waves which have 
