178 TERRESTRIAL PHENOMENA. 



mometers sunk in the Earth, which are now accurately 

 observed at many places, show how slowly the heat of the 

 Sun penetrates even to the small depth of a few feet. 

 Moreover, the thermic condition of the surface of the sea, 

 which covers two -thirds of the planet, is but little favourable 

 to an explanation assuming an immediate influence, and not 

 an induced action proceeding from the gaseous and aqueous 

 strata of the atmosphere. 



In the present state of our knowlede, no satisfactory 

 reply can be given to questions respecting the ultimate 

 physical causes of phseriomena so complex. On the other 

 hand, that part of the subject which, in the threefold mani- 

 festations of the Earth's magnetic force, presents relations 

 admitting of measurememt in regard to space and time, and 

 which leads us to discern, amidst constant and apparently 

 irregular change, the order and dominion of laws, has 

 recently made the most brilliant progress in the determina- 

 tion of mean numerical values. Erom Toronto, in Canada, to 

 the Cape of Good Hope and Van Diemen's Land, and from 

 Paris to Pekin since 1S28, the globe has been covered by 

 magnetic observatories, in which every movement or mani- 

 festation, regular and irregular, of the Earth's Magnetic 

 force, is watched by uninterrupted and simultaneous obsei- 

 vation. A variation of 4 /, of the magnetic intensity 

 is measured. At certain epochs observations are taken 

 at intervals of two minutes and a half, and are continued 

 during twenty-four consecutive hours. A great English 

 astronomer and physical philosopher, has computed ( 165 ) that 

 the mass of observations to be discussed amount in three 

 years to 1958000. Never before has an effort so grand, 

 and so worthy of admiration, been made to investigate the 



