Variation of the Magnetic Needle. 313 



85° north latitude, 116° east longitude. The plain lines 

 shew the isothermal lines of 5°, 10°, 20°, 30°, and 40° of 

 temperature ; the dotted line is the line of maximum tem- 

 perature. This is not an isothermal line, as it varies from 

 less than 80° to more thau 85° of mean temperature. These 

 lines are all from Johnston's Physical Atlas. 



As few persons are acquainted with the theory I propose 

 on the cause of terrestrial magnetism, it will be necessary to 

 explain it ; but I shall do so in as brief a manner as possible. 



As the trade-winds are caused by the cold and condensed 

 air from high latitudes pressing onward and raising the 

 warmer and lighter air in the tropical regions, the air thus 

 raised must flow back again in an upper current towards the 

 colder regions, to keep an equilibrium. Thus the air is in 

 constant circulation from the colder to the warmer regions 

 along the earth's surface, and from the warmer to the colder 

 above. 



It is well known that during evaporation electricity is 

 carried off, and the water from which the vapour has arisen 

 is left in a negative state ; it follows, therefore, that the 

 tropical regions must always be in a negative state, owing 

 to the vapour and its electricity being carried from thence 

 by the rising air ; and the vapour and its electricity carried 

 by the upper current to the polar regions must render those 

 parts positively electrified, and it is to the rush of electricity 

 from the positive to the negative parts of the earth that I 

 attribute the direction of the needle. 



The cause of the magnetic poles in this hemisphere I be- 

 lieve to be the vast quantities of ice blocked up both in 

 winter and summer above the two continents, thus keeping 

 those parts constantly colder thau any other parts in the 

 same latitudes ; consequently these centres or poles of cold 

 are the magnetic poles. For as the density of the air in- 

 creases with the degree of cold, it follows that there must 

 be more air flow from these coldest parts towards the warmer 

 regions than from any other district ; consequently there 

 must be a greater flow of the upper currents of air with its 

 vapour and electricity, from the warmer regions to these 

 coldest parts, than to any other. 



