THE earth's magnetism BAUEE, 203 



ization parallel to a diameter so that the magnetic poles would be 

 diametrically opposite to each other. Wliile the conclusion reached 

 by him that " the whole Globe of the Earth is one great Magnet hav- 

 ing four Magnetical Poles, or Points of Attraction, near each Pole of 

 the Equator Two," has, in a certain sense, been found to be incorrect 

 nevertheless, this view appears to have been the first definite recog- 

 nition of the heterogeneity or complexity of the earth's magnetic 

 condition. 



The increased knowledge gained from magnetic surveys since 

 Halley's time has taught that the more carefully a country has been 

 explored, i. e., the nearer together the points at which the magnetic 

 elements have been determined, the greater is the number of irregu- 

 larities usually shown by the so-called isomagnetic lines; indeed, re- 

 gions have been found where no system of lines can adequately and 

 correctly represent the pi-evailing magnetic conditions. We have 

 learned that the regularities in the distribution of the earth's mag- 

 netism, far from being normal features, as was once thought, are, 

 instead, the abnormal ones, and that the irregularities are the normal 

 and to-be-expected phenomena. 



The magnetic forces, as measured at any given point on the earth's 

 surface, appear, according to various analyses, tt) be the resultant 

 effects of (1) a general or terrestrial magnetic field due to the general 

 magnetic condition of the whole earth; (2) a general terrestrial dis- 

 turbing cause which distorts at the place of observation the general 

 magnetic condition of the earth; (3) a disturbing effect continental 

 in extent; (4) a regional disturbance effect due to low-lying mag- 

 netized substances; and (5) a local disturbance due to the magnetized 

 masses in the immediate vicinity. 



No formula has as yet been established which will represent the 

 observational facts within the error of observation, in fact not even 

 with sufficient accuracy for the practical purposes of the surveyor 

 and of the mariner. 



THE EARTH'S MAGNETIC POLES. 



We have noticed that Halley, as the result of his study of the 

 observations of the magnetic declination, as far as they had become 

 know^n up to 1683, reached the conclusion that the earth had " four 

 Magnetical Poles or Points of Attraction." Some confusion has 

 arisen as to the precise meaning which Halley attached to his " poles." 

 Owing to his alternative term — " Points of Attraction " — certain 

 eminent writers have sought to identify Halley's supposed four mag- 

 netic poles with the four foci of maximum total magnetic force, 

 whose existence appeared to be indicated when, near the middle 

 of the nineteenth century, it became possible to construct a chart of 

 the lines of equal magnetic force. By this incorrect inference these 



