employed in the Observatory . 209 
dian at the same place ; and the magnetic meridian is un¬ 
derstood to mean the vertical plane passing through the 
direction of the horizontal needle at any place. The places 
on the earth’s surface where the magnetic and true meri¬ 
dians coincide have, consequently, no magnetic declina¬ 
tion ; and the lines encircling the earth on which these 
places are situated are called the “lines of no declination.” 
The next element is the Inclination : it is measured by 
the angle which a needle, well balanced, and afterwards 
rendered magnetic, forms with the horizon when it has 
the power of free motion in the plane of the magnetic 
meridian, and is supported on a horizontal axis at right 
angles to its length. As a general rule, it increases from 
the equator to the poles ; the south end of the needle 
inclining downwards in south latitude until it arrives at 
the magnetic pole, intending thereby the point on the 
earth’s surface in which all force exerted by the earth in 
a horizontal direction vanishes, and the needle becomes 
vertical. In north latitude the north end is acted upon in 
a similar manner. The line passing round the globe 
near the terrestrial equator, where the inclination is 
nothing, or where the magnetic needle preserves its 
horizontal position, is called the magnetic equator, and 
is an irregular line of double flexure, cutting the ter¬ 
restrial equator in four points. 
The determination of the last element—the Intensity— 
is one of the most important practical problems in the 
science, and next demands our attention. The usual 
method is, by vibrating a magnetised needle in the 
meridian. If its magnetism be similarly and equally dis¬ 
tributed in its north and south poles, and its axis of 
suspension passes through its centre of gravity, it is 
manifest that, when it is made to vibrate by turning it 
from its position in the meridian, the earth s magnetism 
will act equally on each arm of the needle, and that both 
vol. i. ko. hi. r 
