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is nearly north; but it generally deviates somewhat from the 
astronomical meridian. This deviation is termed the declination 
or variation of the compass. The existence of the declination 
was not unknown in remotest times, although its discovery is 
sometimes erroneously attributed to Christopher Columbus. To 
this great man we are, however, indebted for our knowledge of 
‘the variation of the variation,” since he was the first who 
noticed its change as he altered his geographic position in his 
great yoyage of discovery across the Atlantic. _ : 
The first and most important fact in terrestrial magnetism, 
viz., the declination or horizontal lie of the freely suspended 
magnet, being established, we may take an unmagnetised piece of 
iron, similar in weight and form to our magnet, and balancing it 
on an axis passed through its centre of gravity, allow it to rest 
on the extremities of this axis. We shall find that the unmag- 
netised needle will remain at rest in whatever position we choose 
to place it, since we have taken care to suspend it by its centre 
of gravity. But if we now substitute the magnetised instead of 
the unmagnetised needle, and place it in the plane of the hori- 
zontal magnet, we shall perceive at once that at whatever angle 
we place it on its supports it will invariably take up a definite 
position with respect to the horizon, the marked end, which 
points W. of N., dipping down until it stands in this country at 
anangle of about 70° to the horizon. The earth, therefore, not 
only tends to bring the magnetic needle into a certain azimuthal 
plane, but it also forces it to take a fixed position in that plane. 
The direction of the magnet is thus wholly determined by the 
earth’s magnetic force. 
Our next care will, therefore, naturally be to discover, if 
possible, what is the intensity of this terréstrial force which acts 
upon the needle. This might be determined by finding the re- 
sistance it is capable of overcoming, or the weight it will balance, 
the weight being attached to a thread wrapped round the axis 
of the needle. But the intensity of the earth’s pull is more accu- 
rately found by a method similar to that which has been used 
with such success in observing the force of gravity at different 
pvin's of the surface of the globe, in view of ascertaining the 
amount of itscompression. A magnetic needle is suspended by 
a thread, from which all torsion has been removed, and then an 
oscillatory movement at right angles to the plane of minimum 
dip is imparted to the needle in such a manner as to leave the 
point of suspension at rest. The square of the time of a single 
oscillation is a sure measure of the intensity of the force pro- 
ducing the vibration, which in this case is the product of the 
magnetism of the needle by the horizontal component of the 
earth’s magnetism. The factor due to the magnetic strength of 
the needle can be eliminated at once if the power of our needle 
is known, and the horizontal component of the terrestrial mag- 
netism divided §by the cosine of the dip of the needle will then 
give the required totalintensity. But if the power of the magnet 
is unknown, and on account of slight but continual changes, it 
is always safest to consider it as doubtful within certain limits, 
the quotient of the earth’s horizontal force by the magnet’s 
power can easily be found by measuring the deflection of a free 
magnet produced by the attraction of the vibration needle at 
given distances The result of these experiments is to place in 
evidence that the intensity of the earth’s magnetism follows laws 
as constant as those of its directive force. 
Having thus made ourselves acquainted with the three essential 
elements of the magnetism of our globe, viz , the dip and decli- 
nation, which determine the direction, and the third, which ex- 
presses the intensity of the a‘tracting force, our next step in the 
study of the earth’s magnetism, as a whole, is to secure the most 
trustworthy observations of these three elements at as many 
different stations as possible. The instruments used must be of 
the most delicate description, as the differences to be measured 
are often excessively minute. For this purpose the needles are 
suspended by the slenderest thread of unspun silk, or the 
smoothest axis rests on knife edges of polished agate. The care 
with which the observations have to be taken may be judged of 
from the fact that, to obtain the time of a single vibration of the 
needle, twelve sets of 100 or 200 vibrations are taken, and each 
estimated to the twentieth of a second ; or, again, for a single 
observation of the dip, the needle, which is balanced by the 
maker with scrupulous care, is so far suspected that readings are 
taken of each end, the needle is turned on its Ys, the whole in- 
strument is reversed, and finally the poles are altered, and each 
of these readings repeated at least twice before the observer has 
satisfied himself that all necessary caution has been taken to 
secure a perfect observation. An apparatus of greater delicacy 
NATURE 
than those in general use has lately been invented by Dr. Joule, — 
: 
of Manchester, which we may hope will furnish results of still 
greater accuracy than those already obtained, 
’Twas not until towards the middle of the sixteenth century 
that accurate determination of any of the magnetic elements were — 
attempted ; but since that time the declination has continued to 
be observed with some regularity, and before the end of the 
seventeenth century Halley had already made two long voyages 
to observe this element at different parts of the globe. 
The dip, whose discovery is due to Norman, was first observed 
in 1576, but it does not seem to have attracted much attention 
until two centuries later. 
The determination of the intensity, by means of the vibrations 
of a magnet, was first suggested in the last century by Graham, 
and the first maps of the isodynamics, or curves of equal inten- 
sity, are the fruits of the labour of General Sir Edward Sabine, 
who is now devoting the declining years of his life to the pub- 
lication of the results of his life-long study of terrestrial 
magnetism. 
From the above observations of the three magnetic elements, 
taken at different positions on the surface of the globe, the first 
general conclusion we are able to draw is one of no little im- 
portance. For, starting from any point of the earth, and follow- 
ing the direction of the horizontal needle, we are invariably led 
to one or other of two points, situated respectively in the 
Northern and Southern hemispheres. The entire globe is, there- 
fore, traversed from N. to S. by a system of magnetic lines, all 
meeting in the same two points, resembling in this respect our 
geographical meridians and poles, and therefore termed the 
magnetic meridians and the magnetic poles of the earth. Our 
second conclusion is of scarcely inferior importance to the first. 
For if, instead of following the direction of the horizontal needle, 
we carefully observe the dip, and travel along the line, where 
we find the inclination invariable, we shall always be led, not 
up to a magnetic pole, but in a more or less circular path around 
the pole. These curves of equal dip, generally called isoclinals, 
bear a close resemblance to our geographic parallels of latitude ; 
and as the geographic latitude varies from zero at the equator to 
go” at the poles, so in like manner the dipping needle, which is 
horizontal at the magnetic equator, gradually increases its incli- 
nation until it becomes vertical at the magnetic poles, 
From these angles of position of the dipping needle we can 
conclude at once that the horizontal component of the earth’s 
magnetism must be zero at the poles, and probably maximum 
at the magnetic equator, where the terrestrial force is wholly 
horizontal. 
We may, therefore, describe the magnetic poles in the words 
of the Astronomer Royal, ‘‘as the common points for the con- 
vergence of magnetic meridians, for the verticality of the dip, 
and for the evanescence of the horizontal force.” 
But there are other points on the earth’s surface which merit 
our most special attention. I will not call them poles, as they 
have little in common with the two poles of which we have just 
been speaking, but I will describe them as points of maximum 
intensity. The isodynamics, or lines of equal intensity, are not 
found to follow such simple laws of distribution as the meridians 
and the lines of equal dip and horizontal force, though these 
latter are far from being arranged with the same regularity as 
the meridians and parallels of latitude of a geographic globe. 
None of the magnetic curves are perfect circles, the poles are not 
coincident with the geographic poles, nor are they opposite to 
each other, one being situated north of Baffin’s Bay, and the 
other in South Victoria, but still there isa general approach to 
regularity in the magnetic lines, if we except the isodynamics, 
and the law of variation of the dip was found to be fairly repre- 
sented by the formula, tan. 3 = 2. tan. Z (the magnetic latitude), 
a law discovered by Krafft in 1809. The greatest departure from 
the general regularity of the curves we have been mostly con- 
sidering, is the indication of a second pole in the Southern 
Hemisphere from the peculiar distribution of the lines of equal 
horizontal force. But in the case of the isodynamics we find 
three well-marked points of maximum intensity, one N.W. of 
fudson’s Bay, another in Siberia, and the third not far from 
the South magnetic pole in Victoria. Besides these there are 
also two maxima of small intensity, one situated slightly north, 
and the other at about 15° S. latitude. We are sul, however, 
able to trace a rough approximation to a law in the change of the 
intensity, the value at the principal maximum being about 
double what is found to be on the curve of minimum intensity. 
The distinction between points of maximum intensity and the 
