68 
magnetic changes of declination and horizontal force at Green- 
wich Observatory during thirty-five years from 1841 to 1876 by 
a comparison ef the observations of those elements. The results 
of his observations are shown on a large diagram which has 
been enlarged from his curves, and they show what a close 
relationship exists between solar storms and terrestial magnetic 
changes. There are not only daily and yearly periods of the 
variations of the different magnetic elements, but there also 
seems to be in the horizontal intensity a period of twenty-five 
or twenty-six days, which is the time of rotation of the sun on 
his axis. Other recent investigations have shown that these regular 
magnetic changes depend not only on the sun, but that they are 
also in part due to the action of the moon, and these portions 
depend upon the length of the lunar day and on the position of 
the moon with regard to the earth. Just as there are regular 
earth currents whose direction depends upon the sun, which we may 
call the solar earth currents, so there are lunar earth currents 
which go through their changes under the action of the mcon, 
and it has been shown that the effects are produced not imme- 
diately under the moon, but there is a lagging behind in the 
case of the lunar earth currents, just as in the case of the solar 
earth currents, In the case of the lunar earth currents we can- 
not attribute the production of the electricity either to heat or 
to thermo-electric currents from one part to another of the earth’s 
crust, and we must therefore look for some other source. May 
we not find it in the fact that the moon causes tides in the solid 
crust of the earth, just as she causes tides in the oceans? The 
earth’s crust is made up of elastic materials and materials capable 
of yielding and altering their form to a considerable amount 
with the change in the direction of the pull of the moon uron 
them. This crust also contains magnetic substances in abund- 
ance which alter their form under the moon’s attraction, and 
so from the changes of position of masses of magnetic matter 
changes are produced in the magnetism of the earth which must 
give rise to induced currents of electricity or earth currents. Let 
us imagine a conductor of electricity outside the earth, stretching 
from the North Pole to the equator and fixed in space, with the 
earth, a niagnetic body, revolving beneath it from west to east ; 
then it follows, from Faraday’s laws of induced currents, that 
the revolution of the earth on its axis would cause a current in 
the fixed conductor in a direction from the pole to the equator. 
If the conductor moved over the surface of the earth from west 
to east, and the earth did not revolve, or revolved at a slower rate, 
then the current in the conductor would be from the equator to 
the pole. The current depends upon the relative motion of the 
earth and the wire. If then we have an insulated wire running 
north and south, the tides in the earth’s crust, of which I have 
spoken, will be equivalent to a lagging behind of magnetic 
matter, and so we may expect in that wire a current of electricity 
whose general direction would be from the equator to the pole, 
The position of the wire with reference to the magnetic pole of 
the earth would modify the direction of these earth currents, and 
it is quite conccivable that the position of Engl nd with regard 
to the magnetic pole might cause these regular earth currents to be 
greatest in the south-west and north-east direction. The lagging 
of the lunar earth currents behind the position of the moon 
would also be accounted for by the lagging of the tides tehind 
the moon, If this is a true cause for some portion at least of 
the lunar earth currents, then the same reasoning applied to the 
sun may in a smaller degree apply to the case of the regular 
solar diurnal earth currents, and may help to account for the 
lagging behind of the effects due to the sun, so that the fact 
that the greatest solar effect happens about 2.30 p.m. may not 
be entirely due to the fact that that is the hottest part of the 
day, but may also in part depend upon the tides. 
We have now to consider those more sudden changes of the 
suspended magnets, which are distinguished by the name mag- 
netic disturbances. In 1874 Dr. Lloyd said of them :—‘‘ The 
duration and the magnitude of these oscillations are as yet outside 
the domain of law, and probablydepend upon so many operating 
causes that, like the gusts and lulls of the wind in an atmo- 
spheric storm, they will long baffle all attempts to refer them to 
their actuating forces, or even to reduce them to order.” 
Certain facts relating to these disturbances have long been 
known. From the series of observations started by Gauss in 
1834, and made every five minutes at the same times at a varicty 
of places, at first in Europe and afterwards in various parts of 
the world, the disturbing power was found to increase in 
northern latitudes ; also it was made out that the appearance of 
a disturbance in several places occurred at the same time, but 
NATURE 
[Moo. 17, 1881 
ae were great differences in the results at different 
places. 
In Europe the agreement was very good, and also in America ; 
but the agreement between: Europe and America was not so 
satisfactory. 
The force seemed to originate in a certain point in the interior 
of the earth, and the direction of the disturbing force seemed to 
be constant: yet sometimes there were great differences in the 
deviations at places not far apart, and from the result of his 
observations Weber was led to believe that there was a centre 
of disturbances which was somewhere in the neighbourhood of 
St. Petersburg. 
However sudden and unconnected single’ disturbances may 
seem to be, they still follow certain laws in their occurrence ; 
Sabine found that they had daily and yearly variations from 
their mean values, and that they have an eleven-year period, 
which agreed with the eleven-year period of the appearance of 
spots upon the sun. 
Disturbances are more frequent in summer than in winter, 
and this applies to each hemisphere ; and it has been confirmed 
by various observers that they are also subject to the influence 
of the moon. Lamont says of these disturbances, ‘* Their cause 
is a force which is subject to certain laws but which does not 
act constantly ; the mean direction and frequency have yet to be 
discovered.” 
Observations have shown that the magnetic disturbances and 
electric currents on the earth are so nearly related to one another 
that people naturally look upon the electric currents, either in 
the crust of the earth or in the atmosphere outside it, as the 
cause of the magnetic disturbances. These currents in the earth 
have usually been attributed to changes of temperature, because 
they also are found to be in some way governed by the sun. 
Now let us come to more recent observations of magnetic 
disturbances with the improved methods of recording observa- 
tions by photography which are now available. For some years 
past photographic records have been taken of the magnetic 
elements, but the curves have been laid aside, and very little use 
has been made of them; so much so that some three or four 
years ago a circular letter from Mr. Ellery, Director of the 
Melbourne Observatory, was sent round to those scientific men 
who were supposed to be interested in the matter to know 
whether it was advisable to continue the photographic records of 
magnetic changes at Melbourne, which isthe most southern station, 
and the only station in the southern hemisphere except Mauritius, 
where such observations are taken. Mr. Ellery did not for one 
moment suppose that they were of no value, but as no use 
was made of them he wished to know whether the money ex- 
pended might nct be better applied to another purpose. This 
matter has been taken up by the Kew Committee, of which Dr. 
De La Rue is the chairman, and a recommendation was made 
that the directors of all observatories which possess instruments 
of the Kew pattern should be invited to send to Kew their 
photographic records, or careful tracings of them, for a given 
period, so that a comparison might be made of the results, 
The period chosen was the month of March, 1879, and records 
for the whole month have been sent from Lisbon, Coimbra, 
Stonyhurst, Vienna, St. Petersburg, and Bombay, in the northern 
hemisphere, and from Melbourne and the Mauritius in the 
southern hemisphere. 
A preliminary account of a comparison of the declination 
curves from the European stations was brought before the 
British Association last year at Swansea, and this evening I have 
to bring before you some further points which come out of these 
comparisons. Let us take the disturbances on March 15-16, 1879, 
which will illustrate some of the points which I wish to bring 
out prominently. 
Not only do magnetic changes occur at the same time at 
different stations, but there is a great similarity between them. 
It must be remembered that at northern stations the horizontal 
force is smaller in proportion to the whole force than it is at 
stations nearer to the equator, so that the same disturbance will 
produce less effect on the herizontal force or on the declination 
needle in latitudes near the equator. 
Also the needles at different stations are by no means in the 
same state of sensibility, and even at the same station they 
change with time, so that they are not always equally sensitive, 
and when they lose their magnetism they have to be re-mag- 
netised, 
We see that soon after 10 a.m. G.T. on March 15, 1879, 
there is a disturbance wave showing first a diminution and then an 
