28 THE FLOOR OF THE OCEAN 



depth of 200 or 300 feet, where the effects of wave oscillation 

 are generally small. The remarkable gravimeter for work at 

 sea was invented by Dr. F. A. Vening Meinesz of the Nether- 

 lands Geodetic Commission, who with the instrument has cir- 

 cumnavigated the earth and has occupied more than 500 sta- 

 tions at sea. His feat of invention and his physical and moral 

 endurance in securing highly accurate measurements where, 

 twenty years ago, these seemed humanly impossible, are among 

 the wonders of science. 



To compare the intensities of gravity on lands and seas, it is 

 necessary to reduce the value found at each station to a com- 

 mon level. The level chosen is naturally the geoid or sealevel. 

 The reductions of immediate interest are of two principal 

 kinds. 



The first kind is called the Bouguer reduction, because sug- 

 gested by Pierre Bouguer, a staff member of the famous French 

 party which, in the latter part of the eighteenth century, spent 

 ten years in measuring the length of a degree of latitude in 

 Peru. Bouguer's line of thought may be followed. Let it be 

 assumed, first, that all the rock of a continent above sealevel 

 represents excess of matter over that required to keep the con- 

 tinental region in hydrostatic balance with a plain having its 

 surface at sealevel. Let us measure gravity on the high land of 

 the continent and reduce the observed value to sealevel, on the 

 assumption that the rock above sealevel is replaced by air, of 

 negligible mass, as suggested by the section of Figure 13. If this 

 last value is much smaller than that expected from the stand- 

 ard spheroid, and if the same result accrues at many other 

 observing stations, then we can be sure that much or all of the 

 continental rock above sealevel is not in excess. In other words, 

 we shall have found that the initial assumption was false and 

 that the projection of the continent must be explained in some 

 other way. As a matter of fact, the reduced value of gravity at 



