1^2 THE PHYSICAL GEOGRAPHY OF THE SEA. 



very subtile mass is in a state of unstable equilibrium, and in per- 

 petual commotion by reason of various and incessant disturbing 

 causes — when we reflect farther upon the recent discoveries of 

 Schwabe and of Sabine concerning the spots on the sun and the 

 magnetic elements of the earth, which show that if the sun or its 

 spots be not the great fountain of magnetism, there is at least rea- 

 son to suspect a close alliance between solar and terrestrial mag- 

 netism ; that certain well-known meteorological phenomena, as 

 the aurora, come also within the category of magnetic phenom- 

 ena; that the magnetic poles of the earth and the poles of maxi- 

 mum cold are at the same spot ; that the thermal equator is not 

 parallel to or coincident with either the terrestrial or with that 

 which the direct solar ray would indicate, but that it follows, and 

 in its double curvatures conforms to the magnetic equator ; more- 

 over, when we reflect upon Barlow's theory and Fox's observa- 

 tions, which go to show that the direction of metallic veins of the 

 northern hemisphere, which generally lie northeast and south west- 

 wardly, must have been influenced by the direction of the mag- 

 netic meridians of the earth or air — when, I say, we reflect upon 

 magnetism in all its aspects, we may well inquire whether such a 

 mass of highly magnetic gas as that which surrounds our planet 

 does not intervene, by reason of its magnetism, in influencing the 

 (circulation of the atmosphere and the course of the winds. 



348. This magnetic sea, as the atmosphere may be called, is con- 

 The needle in its di- tinually agitated ; it is disturbed in its movements 

 l!;'i?omIterSuTre*ad! ^J various influcnccs which prevent it from adjust- 

 iEelS ufdeSricai i^^g itsclf to any permanent magnetic or other dy- 

 laine^'hou" for tiS ^amical status ; and its para-magnetic properties are 

 maxima and minima, j^j-jown to Vary with cvcry changc of pressure or 

 of temperature. The experiments of Faraday show that the mag- 

 netic force of the air changes with temperature; that it is least 

 near the equator, and greatest at the poles of maximum cold ; that 

 it varies with the seasons, and changes night and day ; nay, the 

 atmosphere has regular variations in its electrical conditions ex- 

 })ressed daily at stated hours of maximum and mininum tension. 

 Coincident with this, and in all parts of the world, but especially 

 ill sub-tropical latitudes, the barometer also has its maxima and 

 minima readings for the day. So also, and at the same hours, the 

 needle attains the maxima and minima of its diurnal variations. 



