TERRESTRIAL MAGNETISM.—MURPHY. 291 
ART. XI.—TERRESTRIAL MAGNETISM.—By Martin MurRpuy, 
C. E. 
I merely wish to make a few remarks on certain points which 
have occurred to me in reading what Arago, Hansteen, Humboldt, 
Sabine, Faraday, Thomson and others, have said on the subject 
of terrestrial magnetism. The magnetic power of our globe is 
manifested on the terrestrial surface in three cases of phenomena, 
one of which exhibits itself in the varying intensity of the 
force, and the two others in the varying direction of the inclina- 
tion and in the horizontal deviation from the terrestrial meridian 
of the spot. Their combined action may therefore be graphi- 
cally represented by three systems of lines or those of equal 
force, equal inclination and equal declination. The distances 
apart and relative positions of these moving, oscilating and 
advancing curves, do not always rcmain the same. 
If we take a steel bar which has been magnetized, its centres 
of power are located near each extremity, while near the middle 
is a neutral ground, over which the influence of neither end 
predominates. If fine iron filings be sprinkled around the 
magnet, they will form into curved lines emenating from each 
other, and tending towards an union. 
These centres are called poles. The magnetism in one is 
opposite in kind and nearly equal in degree to that in the other ; 
there is a mutual attraction between these opposite magnetisims 
and this tendency to rush across neutral ground, and by com- 
bining yield up every distinctive feature of the magnet is 
successsully opposed by the hardness of the steel bar. 
These lines are called the lines of magnetic force, and the area 
over which their influence is felt is known as the magnetic field. 
If a compass needle suspended by a silk thread, and free to move 
in any plane, be brought into the field, it will assume a direction 
parallel to the lines of force, as at e e’ e” e”’ 
The strength of the field and hence the force that tends to 
give the needle steadiness and direction, varies greatly at 
different points ; at e it is powerful, at e”’ it is feeble. 
