MAGNETISM AND ELECTRICITY. 143 
takes place about 28° west of Paris; at 120° west of Paris it reaches the 
true equator, turns then again towards the south, and at 160° west of Paris 
attains a south deviation of 3° 57’. At 174° west longitude, both equators 
intersect, the magnetic crossing into the northern hemisphere, but returning 
again to intersect the terrestrial equator at 18° east of Paris. At 62° east 
of Paris, the magnetic equator has a north latitude of 11° 47’; at 150° east, 
of 7° 44’; and at 180° east of Paris, of 8° 57’. 
From the preceding phenomena it follows that the earth itself acts as a 
magnet, or possesses magnetism, although it is impossible to say exactly in 
what this magnetism consists, or where the centres of the magnetic influence 
lie. The total action exerted by the earth upon a magnetic needle is, 
however, not attractive, but simply directive. Thus a magnetized delicate 
needle laid carefully upon water, does not move towards the north, but only 
takes up a position in the magnetic meridian. This is because the distance 
between the poles of the needle is so infinitely short, compared with the 
distance of the needle from the magnetic pole, that while one pole of the 
needle is attracted, the other is repelled by precisely the same amount. As 
a consequence, the needle cannot advance, but must take up a position, the 
resultant of these two equal and opposite forces. This condition is illus- 
trated by fig. 11. 
Every declination needle oscillates continually, if unimpeded, describing 
arcs of variable extent. These oscillations are termed the daily variation 
of the compass, being greater some days than others. The limits are from 
half a degree to five or six minutes. The variations of the dipping needie 
are less conspicuous. For further information as to the several variations 
and occasional phenomena of the magnetic needle, we refer our readers to 
the section on meteorology. 
The different durations of oscillation of a magnetic needle, before 
coming to rest, in different places, show a difference in the intensity of the 
magnetic force of the earth. The quicker the oscillation of the needle, the 
greater is the intensity of the terrestrial magnetism, the intensities of 
terrestrial magnetism being as the square of the number of oscillations 
made in an equal period of time. The intensity compass of Gambay is 
intended for experiments on the oscillations of the declination needle. It 
is shown in section by fig. 12, and consists of a circular box of wood, 
covered above by a glass plate, and containing two opposite apertures in 
the sides. The telescope, /, serves to observe through the apertures the 
oscillations of the needle, suspended from the thread, which passes through 
the upright column. 
The laws of magnetism have been recently investigated by Messrs. 
Gauss and Wilhelm Weber. In his experiments on the action of magnets, 
Weber made use, among other apparatus, of a common compass, a magnet 
for deflecting the needle of the former, and a scale. In one series of 
experiments, the scale is laid perpendicularly to the magnetic meridian 
(pl. 22, fig. 50) as also the magnet, ns, lying upon the extremity of the 
scale. In the second series, the scale was laid in the direction of the 
magnetic meridian, and the deflecting magnet at right angles to it 
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