254 DETERMINATION OF THE ELEMENTS OF TERRESTRIAL 



Captain Scott, of the Discovery, the British vessel whicli took 

 part in this expedition, besides establishing a record ror farthest 

 south, latitude 80' 17', reached a point on the line between the 

 south magnetic pole and the south pole in November 1"03. The 

 secular changes in the angle of dip are much less than those in 

 the declination. At London the dip reached a maximum of 74° 

 42' in the yeax 1720, since which time it has continually 

 decreased, being 67° 9' in 1900. Daily and annual changes in 

 the amount of dip are also noticed. 



CJtavges in the Intensity. 

 As with the other elements, so the values of the Intensity 

 varies from place to place, and we find that the points where 

 the intensity is greatest are not coincident with the magnetic 

 poles, but that there are two northern " foci of greater magnetic 

 intensity " and two southern. The secular changes in total 

 intensity are small. At London it was 4791 gauss in 1S48 

 and "4736 in 1880, since when it has been increasing. The daily 

 and annual changes are very small. 



Terrestrial Magnetism and Geology. 

 When a complete survey of the three magnetic elements — 

 declination, dip, and intensity — is made over a considerable area, 

 most interesting: geological results are often obtained. The first 

 such survey was that of the British Isles, .made in 1886-1838. 

 This survey was repeated in 1857-1862 ?ind again on a great 

 scale in 1884-1892, observations Ijeing made at 882 stations. 

 The results of this survey were published by Sir Arthur Rucker 

 in 1896 and one section of his report deals with the " Relation 

 between the Magnetic and Geological Constitution of Great 

 Britain and Ireland." In this report it is shown that where 

 large masses of basalt occurred the north pole of the needle tended 

 to move towards them even from a distance of over 50 miles. 

 Hence, if local attraction is found where no magnetic rocks 

 appear on the surface, it is probably due to concealed masses of 



