FOUNDATIONS OF THE GREAT DEEP 



29 



every high station turns out to be much smaller than the 

 theoretical value given for the same point by the standard 

 spheroid. That relation may be otherwise expressed: for con- 

 tinental stations well above sealevel the "Bouguer anomaly" is 

 characteristically and strongly negative. 



There is analogous procedure for using measurements of 

 gravity at sea. In this case let us assume that the ocean, only 

 about 40 per cent as dense as continental rock, represents defi- 

 ciency of attracting matter as compared with that under a 

 rocky plain with surface at sealevel. We imagine such rock to 

 replace the water and calculate the gravitational attraction at 

 each station after the replacement. See Figure 13. The differ- 



TOPOGRAPHY 



y+ + + + +\ 



POSITIVE 



NEGATIVE 



+ ++ + +++ + +,+ + + +++ + + +\ 



Az 



BOUGUER ANOMALY 



FIGURE 13. DIAGRAM ILLUSTRATING THE ASSUMPTIONS ON WHICH CALCULA- 

 TION OF THE BOUGUER ANOMALY OF GRAVITY IS BASED. 



ence between the calculated value of gravity and the theoretical 

 or standard-spheroid value is the Bouguer anomaly for the sea 

 station. If the former is much larger than the latter, then we 

 can be sure that our initial assumption is false; that we must 

 seek elsewhere for explanation of the depth of the ocean. 



Professor Vening Meinesz measured gravity at series of 

 stations across both Atlantic and Pacific stations and then 



