FOUNDATIONS OF THE GREAT DEEP 25 



represented by perfectly still water, whether the water be that 

 of the ocean or that of a narrow canal supposed to be cut in 

 any direction through continent or island and continuous with 

 mean sealevel. Such a sealevel figure of the earth bears the 

 technical name geoid. It is a wavy, highly complicated figure. 

 On the other hand, "figure of the earth" is often used to mean 

 a shape that can be defined in comparatively simple terms and 

 yet gives a close approximation to the geoid. A century of in- 

 tensive study by government and privately interested geode- 

 sists has proved that one figure satisfying the requirement is an 

 ellipsoid, which is the three-dimensional shape made by the 

 complete revolution of a perfect ellipse on its minor axis. 

 Actually the minor axis of the ellipse is taken to be identical 

 with the polar axis of the earth, while the end of the major 

 axis of the generating ellipse traces in imagined space a circle 

 that closely corresponds with mean sealevel at the equator. In 

 principle we recognize the "oblate spheroid" of our school- 

 days. As a matter of fact, however, the gravitational field of the 

 earth is such that a still more exact mathematical figure must 

 differ slightly from a perfect ellipsoid; hence the geodesists 

 have devised another formula to represent more accurately the 

 "figure of fluid equilibrium" or "standard spheroid." The 

 standard spheroid expresses, then, the best approximation to 

 the actually wavy surface of the geoid or world-circling sea- 

 level. 



Now, while working out the geoidal figure, geodesists have 

 discovered some fundamental facts about the earth's outer 

 shells, including the sub-oceanic crust, and have confirmed the 

 discoveries by measurement of the force of gravity at many 

 points in the geoidal level. 



The first of two ways of thus adding to knowledge depends 

 on measurement of the degree of waviness for the geoidal sur- 

 face. With exquisitely accurate instruments it is proving pos- 



