292 IKIDI JOK NAN.SKN. M.-N. Kl. 



Ijv sLicli j^ical masses of rock al)o\'e sca-levcl. Accoflinj^^ to Airv, the 

 natural explanation was that the excessive mass of the hij^h mountains 

 was conipeiisatcfl for l)\' a r(lati\cly low density of the masses of the crust 

 below llie nionnlain system. This mi^dit seem to inrlicate that the ri^'^id 

 continental crust is, as it were, floatini^' on a plastic, rlcnser substratum, 

 and, as also assumed by Jleim, tlie lighter "floatinj.(" lithosphère seeins 

 to be thicker under llie mountain rejufion, being- there submerg^ed deeper 

 into the flenser substratum or magma. Later observations seem to have 

 borne out in essentials the correctness of Airy's views. 



A series of penrlulum o])servations marie during the J'ram- expedition 

 in 1893 to 1 8(j6 ])y ("aptain .S. Scott-I lansen (»n the floating ice over the 

 deep North I'olar .Sea, seemed to ])ro\e that similar conditions also prevail 

 under the Ocean. P)y the computation of tliese observati(jns Prof. C). \i. 

 Schiotz [tqoi] f(iund the gravity over the North Polar Sea to be normal, 

 which pro\es that the deficiency of density of the deep layer of water, 

 3800 metres thick, ])eing not much more than one third of that oi rock, 

 must be compensated for 1)\- an excess of density of the lithosphère under 

 the Ocean, \\hich must be appreciablv denser than the continental crust. 

 It is also in remarkabh* good harmonv with this result that a deficiency 

 of gravity was observed over the deep sea near the efige of the continental 

 shelf, corresponding to the excess of gravity found on land near the coast- 

 lines of the continents. 



In addition to the dynamic reasons for this peculiar distribution of 

 the variations in gravity near the edge of the continental shelf, I think 

 that it (/Ught also to be taken into consideration that by the deposition 

 of sediments, the continental shelves and the slope towards the deep Ocean 

 floor will gradually sink towards their level of ecjuilibrium; but owing to 

 the rigidity of the crust, the Ocean floor outside the slope will also bs 

 bent down to some extent, although no appreciable addition of sediments 

 has been deposited on it. A deficiency of mass will consequently arise 

 in this boundary region of the Ocean. 



The important discoverv that the gravity is normal over the Ocean, 

 has been confirmed bv numerous observations by Heckcr [1903, 1908, 

 19 10] over the deep Ocean in various regions of the earth. Hecker's 

 determinations of gravity were made onboard ship by simultaneous meas- 

 urements of atmospheric pressure by means of boiling-point thermometers 

 and the mercury barometer, a method first suggested bv H. ^Mohn of 

 Christiania. 



Although the accuracy of this method is much less than that of deter- 

 minations with the pendulum, still Hecker's observations over various 

 parts of the Ocean agree on the whole so well that, in connection with the 

 pendulum observations on the Fram-Expedition, the}- may be considered 

 to prove that the gravity is, on the whole, approximately normal over the 

 deep Oceans. The deficiency of mass of the Ocean water must be com- 



