Nov. 17, 1881] 
NATURE 
67 
combined with the earth’s magnetic force, will cause such a needle 
to rest. This is usually done by loading a steel needle at one 
end and then magnetising it with its poles so arranged that the 
extra weight of the heavier end shall balance the downward pull 
of the magnetic force on the other end. In this case the needle 
when magnetised will remain at rest in a horizontal direction, 
when suspended on a point on which it can turn freely in a 
horizontal plane. 
A magnetic needle suspended in this way has been called a 
declination needle. Such a needle is employed in the mariner’s 
compass, in our galvanometers for measuring currents of elec- 
tricity, and in magnetic observatories for determining the 
declination or what is sometimes called the variation of the 
macnetic needle. This needle determines the position of the 
vertical plane in which lies the direction of the earth’s magnetic 
force; this is called the plane of the magnetic meridian. The 
plane of the magnetic meridian is usually different from the 
vertical plane through the north and south poles, which is called 
the geographical meridian, and the angle between these two planes 
is the declination or variation of the magnetic needle. 
Jf such a magnetic needle as I have just described be sup- 
ported on horizontal knife edges instead of being supported on 
a point, the needle when magnetised may remain at rest balanced 
in a horizontal direction, one end being pulled downwards by 
the earth’s vertical magnetic force, and the other by the force of 
gravity. Any change in the intensity of the vertical magnetic 
force of the earth will be shown by an up or down motion 
of the marked end of the needle. Such an instrument, called 
a balance magnetometer, is specially adapted for showing any 
changes in the vertical magnetic force of the earth, and the 
changes or disturbances of the earth’s vertical mignetic force are 
determined by means of such a balance magnetometer. We have 
then our declination or variation needle to determine the vertical 
plane, called the magnetic meridian, and we have our balance 
magnetometer to determine any changes which may take place 
in the vertical magnetic force of the earth. 
By the declination needle we can not only determine the plane 
of the magnetic needle, but by making the needle oscillate to 
and fro horizontally and counting the number of oscillations in 
a given time we can determine the horizontal pull upon the 
poles of the needle, z.¢. the intensity cf the earth’s horizontal 
magnetic force upon the needle, just as by the swing of a simple 
pendulum in a vertical plane under the action of the force of 
gravity we can determine the pull of the force of gravity upon 
the bob of the pendulum. Ly a similar method and by a pro- 
perly suspended needle either the vertical force or the total 
magnetic force of the earth may be determined. 
In order then to determine the direction of the earth’s mag- 
netic force we may make use of a declination needle to give us 
the vertical plane, and place the dipping needle in such a position 
that it will oscillate in that plane; when it comes to rest it will 
point in the direction of the total magnetic force, ze. in the 
direction through the room of Faraday’s lines of magnetic force. 
In order to determine the magnitude of that force the hori- 
zontal force may be found by finding the number of oscillations 
of the declination needle in the way that I have already ex- 
plained, and these three determinations will give us the direction 
and magnitude of the earth’s total magnetic force. 
Another method of making the required determinations is to 
take a coil of copper wire, which is wound on a circular frame 
in such a way as to be capable of spinning on a diameter of the 
circular frame. 
Faraday showed that on turning such a coil in a magietic 
field a current of electricity is induced in the coil, and the strength 
of this current is proportional to the number of lines of force 
cut by the coil, We may describe such an arrangement as a 
magneto-electric machine, in which the magnet employed is the 
earth itself. 
By means of this instrument we may determine either the 
horizontal or the vertical magnetic force -f the earth. By placing 
the axis vertical and spinning the coil at a given rate we may 
determine the horizontal force, and by placing the axis horizontal 
in the magnetic meridian and spinning the coil at the same rate 
we may determine the vertical force, the currents produced in 
the two cases being in the same ratio as the numbers of the lines 
of force cut in the two positions. 
The greater the angle at which the axis of rotation is inclined 
to the direction of the lines of force the greater will be the 
number of them included in the revolving circle, and the greater 
the induced current produced in the coil. 
Thus placing the axis in different positions we get currents of 
different strengths, and may readily see that we get the greatest 
current when the axis is at right-angles to the direction of the 
lines of force, 7.e, to the l'ne of the dip 
We may further make use of such a coil to find the direction 
of the lines of force, for if we place the axis parallel to the lines 
of force, the currents in opposite halves of the coil will balance 
one another, because each line of force is cut twice by the coil, 
and so no current is produced in the external circuit through the 
galvanometer. 
If then we place the coil so as to get no current when we 
rotate it, then the direction of the axis of the coil is the direction 
of the dippi: g needle, z.e. of the magnetic lines of force. 
We will suppose now that for some point of time, say June 
Ist at 12 o’clock midday, the three magnetic elements, ze, the 
declination, the horizontal force, and the vertical force, have 
been determined, we have now to consider the changes or dis- 
turbances produced in these magnetic elements, and the connec- 
tion of these changes with other phenomena, and especially the 
connection between auroras, earth currents, and the larger and 
more irregular magnetic disturbances. 
I have already drawn attention to the declination needle and 
the balance magnetometer for measuring the changes of declina- 
tion and of the vertical force. 
For measurement of the changes in the horizontal force a 
special instrument is employed, called a bifilar magnetometer, in 
which a magnet is suspended by two threads, which are so placed 
that by their torsion acting against the magnetic force of the 
earth, the magnet is kept at rest in a horizontal position in a 
direction at right angles to the magnetic meridian. 
This completes the li t of instruments for our magnetic obser- 
vatory. 
Any change or disturbance of the horizontal force pulls this 
magnet round more or less in the horizontal plane, and its 
change of position is observed as in the other instruments, The 
results I have to bring before you this evening have been derived 
from the photographic registrations of similar instruments in 
different parts of the world, so that the motio. of the needle bas 
recorded its own tale on the prepared paper which is wrapped 
on a cylinder driven by clockwork, and so placed as to receive 
the spot of light reflected by the moving needle. 
First, there are regular daily and yearly changes, showing that 
the sun produces regular changes in the three magnetic elements 
which de; end on the time of the day and the season of the year, 
so that the change of position and apparent motion of the sun 
with respect to the place of observation produce regular mag- 
netic changes. These regular daily changes are accompanied by 
and have very generally been supposed to be due to electric 
currents or electric waves traversing the earth’s crust, and a 
discussion by Dr. Lloyd of the observations made by Mr. Barlow 
in 1847 of currents on telegraph wires showed a very close 
relationship between the two-hourly changes of the declination 
needle and the changes of intensity and direction of earth cur- 
rents on telegraph lines. 
Both Dr. Lamont and Dr. Lloyd conclude from their com- 
parisons of earth currents and magnetic changes that the changes 
of the declination needle cannot be due to the direct action of 
the electric current traversing the earth’s crust, but that these 
currents or waves, extending toa considerable depth, alter by in- 
duction the magnetism of the earth itself, and this change of 
magnetism causes the observed changes in the declination needle. 
Thus the magnetic changes are the indirect effects of (not the 
earth current in its immediate neighbourhood, but of) a change 
in the magnetism of the earth itself, which may be due to an 
electric wave extending over a considerable area of the earth’s 
surface, 
The point towards which the total earth current is directed 
follows the sun and seems to lag two or three hours behind, but 
not the same distance behind at different places. 
These earth currents have been ascribed to different causes : 
thus Dr. Lamont regards them as the results of electric force 
emanating from the sun; De Saussure regards them as developed 
by evaporation, the vapour being positively charged, and the 
water being negative; Dr. Lloyd regards them as effects of 
solar heat ; whilst M. de la Rive ascribes them to chemical 
actions going on in the interior of the solid crust of the earth, 
the electricity being transported into the atmosphere by eva- 
poration. 
Mr, Ellis of the Greenwich Observatory has shown the inti- 
| mate relation between solar action and the regular diurnal 
