Go INTRODUCTION. 



III. PHYSICAL OBSERVATIONS. 



One of the most interesting physical inqui- 

 ries relates to an object, which is not only im- 

 portant with a view to the knowledge of our globe, 

 but the more accurate investigation of which may, 

 under certain circumstances, become useful to 

 navigation. This is the remarkable property of the 

 magnetic needle, in the northern half of our globe, 

 to incline its northern end, and, in the southern 

 hemisphere, its southern end. As this inclination 

 increases with the distance from the equator, en- 

 deavours have long since been made to measure it 

 in different parts of the earth by means of an in- 

 strument called the dipping needle. Ill calculated 

 as the mobility of a ship appears to be for such 

 observations, yet, on the other hand, the advantage 

 possessed by the navigator of visiting the remotest 

 parts of the earth is so great, in respect to this 

 subject, that even imperfect observations are a 

 gain to science. The unsteadiness of the ship is, 

 how ever, not the only obstacle to obtaining a true 

 knowledge of the dip. A much more important 

 obstacle is the difficulty of making dipping-needles, 

 which, in a not magnetic state, shall remain in 

 exact equilibrium, however turned, be moved by 

 no power but that of magnetism, and be sensible 

 to the smallest gradations of that power. The 

 latter has been attempted to be effected by making 

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