TERRESTRIAL MAGNETISM. 105 



magnetic force in the higher strata of the atmosphere should 

 not be looked for solely in the increased distance from the 

 source of the force, the solid terrestrial globe, but may 

 with n less probability be sought in the exceedingly rarefied 

 state of the air at those elevations, whereby the quantity of 

 oxygen contained in a cubic foot of atmospheric air is much 

 less than at the surface of the earth. It appears to me that 

 we should not be justified in extending this supposition 

 further, than to say that the diminishing paramagnetic 

 property of the oxygen with increased rarefaction of the 

 air, at increased elevations, may be a concurrent modifying 

 cause of the phenomenon in question. Modifications of 

 temperature and of density, by the action of ascending 

 aerial currents, will, again, modify the measure of this con- 

 current action ;( 116 ) and disturbing influences thus assuming 

 a variable and local character, will act in the atmosphere as 

 do the different kinds of rock at the surface of the earth. 

 At every cheering step in advance towards the better know- 

 ledge of the constituents of the gaseous envelope of our 

 planet, and their physical properties, we become aware of 

 fresh liabilities to error amidst the varying concurrent action 

 of forces, and are admonished thereby of the need of still 

 greater caution in arriving at conclusions. 



The intensity of the earth's magnetic force measured at 

 determinate points of the earth's surface, has (like all the 

 other phenomena of terrestrial magnetism) its hourly, and 

 also its secular variations. The former were distinctly re- 

 cognised in Parry's third voyage, by that distinguished 

 navigator, and by Lieutenant Poster, in 1825, at Port 

 Bowen. In the middle latitudes the increase of the inten- 

 sity of the magnetic force from the morning to the evening 



