142 PHYSICS. 
with regard to the horizon. One extremity or pole will be found to point 
nearly in the direction of the north pole of the heavens or earth, and the 
other pole towards the south. The extremities of the magnet are hence 
called the north and south poles, from their pointing in this manner. Some 
authors, however, call that the north pole of the magnet, which points to 
the south pole of the earth, and the one pointing to the north pole of the — 
earth, the south pole. 
The magnetic meridian of a place, or the line of direction of the magnetic 
needle, will generally be found 1o deviate somewhat from the true meridian. 
The angular value of this deviation is called the variation or declination of 
the compass. It is termed east or west as the north pole of the compass 
deviates east or west from the meridian. In fig. 9, be represents the 
astronomical meridian of a place, and sn the magnetic meridian. The 
variation here is west. This variation differs not only in different places, 
but in the same place at different times. At the present time it is western. 
in all Europe, and in northern Germany amounts to about 18°. The. 
variation at New York city was found by Professor Renwick in 1837 to 
amount to 5° 28’ west. In some localities there is no variation, or the 
magnetic and astronomical meridians coincide. 
An instrument for indicating the magnetic meridian, as also the variation 
of the needle, is called a compass ( pl. 20, fig. 8). The pivot on which the 
needle is suspended is erected in the centre of a horizontal circle, whose 
circumference is graduated to 360°. The north and south line passes 
through 0° and 180°. To determine the variation a telescope is attached 
to the side of the compass box, with its axis parallel to the north and south 
line. By sighting the telescope in the astronomical meridian, the deviation 
of the needle from the north and south line of the compass will indicate the 
variation. 
When a magnetic needle is suspended carefully by its centre of gravity, 
its position assumed in the magnetic meridian is not parallel to the horizon, 
but inclined to it. This position is called the inclination or dip of the 
needle, and varies in different latitudes. In the northern hemisphere it is 
the north pole that is depressed, the south pole dipping in the southern. A 
needle constructed to show the amount of this inclination is called a dipping 
needle. Here (fig. 10) the needle is placed on a horizontal axis in the 
centre of a graduated vertical circle. By placing the plane of the circle in 
that of the magnetic meridian, the inclination of the needle as read off on 
the graduated circle will show the dip. This inclination varies at different 
times for the same place, and is greater as we approach the poles of the 
earth. Near the terrestrial equator this needle will be horizontal, and an 
irregular curve connecting those places near the equator where the needle 
is horizontal, is called the magnetic equator. This curve encompasses the 
earth, at no point being more than fourteen degrees from the terrestrial 
equator. On each hemisphere, and near the true poles, there is one point 
where the dipping needle stands vertically : these two points are called the 
magnetic poles of the earth. 
The greatest separation (14°S.) of the magnetic from the true equator 
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