404 ANN-UAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



The geological evidence which has been cited to show that iso- 

 static equilibrium can not well exist in detail may be regarded as 

 demonstrating a certain rigidity of the earth's crust, which is most 

 severely taxed when erosion has planed away the compensating 

 heights to the nearest possible approach to level plains. It is inter- 

 esting to note that, per contra, the isostatic balance is probably most 

 nearly complete in regions of most vigorous mountain growth, or 

 for the continents as a whole is most perfect at a time like the pres- 

 ent, when uplift is most general. If the disturbing process of ero- 

 sion could be eliminated from continental activities the uplifts and 

 subsidences w^ould establish perfect equilibrium or a close apjoroach 

 to it. Now erosion has no effect over those portions of the ocean 

 basins which are beyond the reach of the sediments that surround 

 the continents and which occupy nearly three-fourths of the surface 

 of the globe. These areas are depressed because they are heavy, 

 according to the hypothesis of isostasy, and should be depressed 

 more or less according to the density of the underlying masses. 

 The adjustment should be nearly or quite complete except where 

 disturbed by vulcanism or by other special stresses. It is, there- 

 fore, of great interest to determine the law of distribution of density 

 beneath the oceans in relation to the depth of the waters, apart from 

 the interest which lies in the comparison of oceanic gravitation with 

 that of continents. 



As it is impossible to observe a j^endulum on board ship measure- 

 ments of gravity in ocean areas were restricted to oceanic islands 

 until recently, when they were made possible on the water by a 

 method in which the pressure of the air as shown by a barometer 

 is compared with the pressure of the air as determined by the boiling 

 point of water. 



In measuring the air pressure with a barometer the air is balanced 

 by the column of mercury, which will be somewhat shorter at a 

 place where the intensity of gravity is high than at a point where 

 the intensity is less. If the air pressure be measured by observing 

 the boiling point of water, the result is independent of any influence 

 of gravity upon the apparatus. By using both methods at a station 

 the effect of gravity on the barometer at that station can be ascer- 

 tained, and by comparing the effects obtained at various stations 

 relative intensities are found. 



This method, which was originally invented by the German physi- 

 cist Mohn, was adapted to oceanic work by Dr. E. O. Hecker, who 

 devised an elaborate apparatus for the purpose. It consists of five 

 mercurial barometers which are hung in a metal plate swung on 

 gimbals and which are so illuminated that the movements of the 

 upper surface of the mercury are registered on a iDhotographic film. 

 The record is a wavy line, since the barometers are constantly agi- 



