1 84 Mr. Henry Wilde on 



at Cape Comorin, where the declination was i6°W. in the 

 year 1601, and is now i°E. 



The observations of deep-sea temperatures made during 

 recent years have brought out the important fact that, at 

 great depths, the temperature of the ocean beds is little 

 above the freezing point of water. Prestwich and others 

 have inferred that this low temperature of ocean depths is 

 competent to produce a greater thickness of the earth's 

 crust under the oceans than under the land. The large 

 amount of iron which enters into the composition of the 

 earth's crust is well known from the analysis of volcanic 

 ejections from all parts of the globe, while at extreme 

 depths this element exists in the metallic state, as at 

 Ovefak, off the coast of Greenland, where it is found 

 diffused in the basaltic rocks and in separate masses. We 

 have, therefore, through the low temperature and increased 

 thickness of the ferruginous ocean beds, the precise con- 

 ditions required for producing the differences in the 

 magnetic elements which have been shewn on the mapped 

 globe when the ocean areas were covered with iron. 



Now that the great influence which the land areas 

 exercise in retarding the translatory motion of the lines of 

 the declination has been shewn, which is distinct from the 

 magnetism of local geological formations, the same 

 influence in determining the form and position of the 

 declination lines on the terrestrial surface becomes very 

 apparent on the charts. An important negative feature of 

 this influence is the symmetry and simplicity of the declina- 

 tion lines in the southern hemisphere, where the ocean 

 completely encircles the globe in latitude 6o°, as compared 

 with the devious lines of the declination on the great land 

 areas in the same latitude of the northern hemisphere. 



The dominant influence of longitudinal coast lines is 

 well seen in the bend of the zero and other declination lines 

 towards the north pole of the earth's axis in their westerly 



