Prof. W. A. Norton on Terrestrial Magnetism. 



229 



(21) cannot be made to extend to the cold poles except by sup- 

 posing n there equal to unity. 



If we consult a chart of isothermal lines, we find the greatest 

 variation of temperature to be set down for about longitude 90° 

 W. ; and latitude 45° ; and that this occurs at a lower latitude 

 than the maximum variations on other meridians : which nearly 

 accords with the theoretical conclusion obtained above. 



It is to be observed that all the considerable changes of total 

 intensity in high latitudes, are almost entirely attributable to chan- 

 es in the vertical intensity. In latitude 50° on this continent, 

 the entire suppression of the horizontal intensity would not di- 

 minish the total intensity more than 0*07. On consulting Mahl- 

 mann's table of temperatures, it may be seen that the change in 

 the variation of temperature in the direction towards the cold 

 pole, as we go north beyond 40°, is much less than from the equa- 

 tor to 40° ; and that the changes of temperature in this direction 

 accord very well with the changes of magnetic intensity in high 

 latitudes upon this continent, as exhibited by Sabine's chart. 



GENERAL RESULTS. 



The following points appear to have been conclusively estab- 

 lished by the foregoing discussion. 



1. All the magnetic elements of any place on the earth, may 

 be deduced from" the thermal elements of the same ; and all the 

 great features of the distribution of the earth's magnetism may 

 be theoretically derived from certain prominent features in the 



distribution of its heat. 

 2- Of the magnetic elements, the horizontal intensity is nearly 



mean 



aeit's thermometer ; the vertical intensity is nearly proportional 

 t0 the difference between the mean temperatures at two points 

 situated at equal distances north and south of the place, in a di- 



tow*** ._ ,. , . . .t 1 i:~r> . onrl in orpnpmL 



perpendicular 



the direction of the needle is nearly at right angles to the is 

 thermal line, while the precise course of the inflected h 



isogeo- 

 line to 



g and putting the dif- 



~~m n ik perpenuicuiar may ue uouuwu 



J^la for the temperature, by different iatin 



terential equal to zero. . , . . ., ,. „ nf 



3. As a consequence, the laws of the terrestrial distribution of 

 th e physical principles of magnetism and heat must be the same, 

 or nearly the same ; and these principles themselves must have 

 towards one another the most intimate physical relations. 



4 - The principle of terrestrial magnetism, in so tar as the ptie- 

 fomena of the magnetic needle are concerned, must be confined 

 t0 the earth's surface, or to a comparatively thin stratum of the 

 *ass of the earth. 



