168 TEERESTRIAL PHENOMENA. 



gates itself gradually, in short intervals of time, in every 

 direction over the surface of the Earth. ( 143 ) In the first 

 case, the simultaneity of the phsenomena may serve, like 

 occupations of Jupiter's satellites, or like fire signals and 

 shooting stars, to determine within certain limits geogra- 

 phical differences of longitude. We recognise with wonder 

 and admiration, that the movements of two small magnetic 

 needles, even if suspended at depths beneath the surface 

 of the Earth, should measure the distance which divides 

 them from each other; that they should tell us how far 

 Kasan is situated east of Gottingen, or of the banks of the 

 Seine. There are parts of the Earth where the mariner, who 

 has been enveloped for many days in fog, seeing neither 

 Sun nor stars, and having no means of determining time, 

 may know with certainty, by an observation of the magnetic 

 Inclination, whether he is to the north or south of the port 

 which he desires to enter. ( 144 ) 



When the sudden interruption or disturbance of the 

 horary movement of the needle announces the presence of a 

 magnetic storm, we are unhappily still unable to determine 

 the seat of the perturbing cause, whether it be in the 

 crust of the earth, or in the upper regions of the atmo-r 

 sphere. If we regard the earth as an actual magnet, we 

 know from the profound investigator of a general theory of 

 terrestrial magnetism, Friedrich Gauss, that to each portion 

 of the globe one-eighth of a cubic metre in volume, we must 

 assign an average amount of magnetism equal to that con- 

 tained in a magnetic bar of 1 Ib. weight. ( 145 ) If iron and 

 nickel, and probably cobalt (but not chrome, ( 146 ) as was 

 long believed), are the only substances which become per- 

 manently magnetic, and by a certain coercive force retain 



