96 Our North Land. 



Magnetism the following observations bearing upon the magnetic 

 work which Mr. Stupart is to perform. They will no doubt be 

 interestinsr and instructive to the reader : — 



The question is best opened by making two propositions, viz. : 



1. If a rigid rod be suspended freely by a string without torsion, 

 passing through its centre of gravity, its position will ordinarily be 

 horizontal, whatever be the vertical plane in which it is situated. 



2. If, however, a bar of magnetized steel be thus suspended, this 

 horizontal ity of position and indifference as to the azimuth of the 

 vertical plane in which it hangs no longer obtains. On the con- 

 trary, it is found that when the bar comes to rest, its direction 

 makes a certain angle with a horizontal line in the vertical plane 

 containing the bar and the suspending string ; this vertical plane 

 makes a certain angle with the meridian of observation. 



Now the causes producing these effects is called terrestrial mag- 

 netism. 



The angle which the horizontal needle of a compass makes 

 with the meridian of the place of observation is called the declination 

 (or by some the variation), and is said to be so many degrees east or 

 west, according as the north pole of the needle deviates to the east or 

 west of the meridian. 



The vertical plane passing through the magnetic axis of the 

 needle is called the magnetic meridian. 



The angle which the vertical plane needle makes with a hori- 

 zontal line in its plane of motion (supposed to coincide with the 

 magnetic meridian) is called the dip or inclination, and is said to 

 be north or south according as the north or south pole of the 

 needle is below the horizon. 



The horizontal needle furnishes the means of determining the 

 intensity — or isochronous oscillations of the horizontal needle when 

 displaced from its position of equilibrium on either side of the mean 

 position, — of the horizontal component ; and the direction and plane 

 of action being known, the resultant force, or intensity, is known in 

 magnitude and direction. 



To show at once the value of these magnetic observations to the 

 navigator, it is only necessary to state that the declination or varia- 



