274 REVIEWS 



observations are the first systematic determinations of the force of 

 gravity over the great ocean depths. "The observations show that 

 the gravity may be regarded as normal over the polar basin ; and as it 

 is not probable that this is a peculiarity of the Polar Sea, we are led to 

 the assumption that the force of gravity is normal all over the great 

 oceans. The increased attraction observed on oceanic islands must 

 therefore only be due to the local attraction of the heaped-up masses at 

 the bottom of the ocean that form the islands" (p. 63). This deter- 

 mination of the normal character of the force of gravity over the 

 ■ocean depths, if its theoretical extension to all the ocean basins be 

 justified, must be regarded as a contribution of the first order. The 

 determination in the polar basin was made possible by the relative 

 fixity of the vessel in the ice. The tremors which more or less con- 

 stantly affect the polar ice sheets may perhaps have slightly influenced 

 the results but probably in no serious way. Regarding the theoreti- 

 cal extension, it is however to be noted that these polar observations 

 were nowhere made at a great distance from the edge of the conti- 

 nental plateau, and that the extent of the depression is undetermined, 

 and that, furthermore, the depth of the sea is somewhat less than the 

 average depth of the ocean. The polar basin is probably not, at most, 

 much greater in extent than the Mediterranean basin, and may be 

 much less. It would seem, therefore, that some reserve may be 

 prudently exercised in accepting the assumption that the observations 

 in the Polar Sea determine the force of gravity over the great ocean 

 depths in general. In view of the importance of determining this 

 beyond question, it may be suggested that attempts be made to make 

 pendulum observations in the calm belts of the tropics. This sugges- 

 tion is made on the assumption that the sea might there be found 

 sufficiently calm to permit observations of approximate accuracy. 



The discussion of the crust of the earth, which follows that of the 

 pendulum observations, is less satisfactory than it might have been, 

 owing to the limitation of the theoretical assumptions to a single line 

 of hypothesis. Apparently the results might be appreciably different, 

 if different hypotheses of the internal constitution of the earth had 

 been assumed. The discussion proceeds upon the conception that all 

 the differences in the density of the solid portion of the earth are con- 

 fined to its superficial portion. This is doubtless in accord with 

 present majority views based on deductions derived from prevalent 

 theories as to the origin and early state of the earth, but it is none the 



