The Express.
London, England, 1852-09-04 00:00:00

corn spending to the present epoch. Until these comb;
nations and calculations are performed, it would be oh.
viouely premature to speak of numerical values by whieh
the magnetic forces at one part of the globe may be cow
pared with those of another, or with forces of other den
scriptions ; and for the same reasnn it I. &tearable to
abstain fur the present from notices of the peogrephicel
positions which particular lines, or, as some may deem
them, critical points in the magnetic resultants m occupy
on the earth's surface at the present epoch. Such notices
could only be as yet provisional and liable to the nmend-
menta which more exact and extended calculation must be
expected to produce. But thus much may be safely stated
in reference to the general character of the three systeins of
lints which have been spoken of, that when derived afresh
and exclusively from the observati,nie of the last
few years, they do most fully confirm the gene-
ral conclusions derived from the observations of earlier
I date, which were submitted to the British Aesoriation in
the report on the " Variations of the Intensity of the Mag-
netic Force at different points of the earth's surface," which
preceded the recommendations of MM. The magnetic
phenomena, or as it is now cuetomery to cull them, the
three magnetic elements, appear to be everywhere and in
both hemispheres the resultants of a duplicate system of
magnetic forces, of which one at least undeigoes a conti-
nuous and progressive translation in geographical space,
the motion b iug from West to East in the Northern hew
miephere, and Irons Eat to West in the Southern. It is
to this motion that the secular change in all localities is
chiefly if not entirely due, affecting systematically, and ac-
cording to their relative positions on the globe, the confi-
gurations and geographical positions of the magnetic lines,
and producing conformable changes in the dir.etion and
amount of the magnetic elements in every part of the globe.
The comparison of the earlier recorded observations with
those of the present epoch gives reason to believe that,
viewed in its generality, the motion of the system of forces
which produces the secular change has been uniform, or
nearly so, in the last two or three centuries. Under fa-
vourable conditions the regularity of this movement can
be traced down to comparatively very minute fractions of
time. By the results of careful observations continued for
several years at the observatory of St. Helena—where, in
common with the greater part of the dist. iet of the South
Atlantic, the secular change of the declination exceeds
eight minutes in the year, and from its magnitude, there-
fore, may be advantageously studied—every forteight of
the year is found to have its precise aliquot portion of the
annual amount of the secular change at the station. This
phenomenon of secular change is uudeubtediy ou, of the
most remarkable features of the magnetic system, and can-
not with propriety be overlooked, as too frequently it has
been, by those who would connect the phenomena of ter-
restrial magnetism generally, mediately or immediately with
climatic circumstances, relations of land and sea, or other
eau.es to which we are assuredly in no degree entitled to
ascribe secular variation, and who reason therefore as if
the magnetic phenomena of the earth were persistent
instead of being, u they are, subject to a con. inual and
progressive change. It may confidently be affirmed that
the secular magnetic variation has no analogy with, or re-
semblance to, soy other physical phenomenon with which
we are acquainted. We appear at present to be without
any clue to guide us to its physical cause., but the way is
preparing for a future secure derivation of its laws to be
obtained by a repetition, after a sufficient interval, of
the steps which we arc now taking to determine the
, elements corresponding to a definite c poets. The perio-
disal variations in the terrestrial magnetic force, which
I have before adverted to as distinguished from its secular
change, see small in comparison with the force itself, but
they are highly deserving of attention on account of the
probability that, by suitable methods of investigation, they
may be made to reveal the sources to which they owe their
origin, and the agency by which they are produced. They
formed accordingly the subject of a distinct recommenda-
tion from the British As .ociation, which met with an equally
favourable reception. To investigate these variations by
suitable instruments and methods, to separate each from
the othere, and to seek its period, its epochs of maximum
and minimum, the laws of its progression, and its mean
numerical value or amount, constituted the chief purposes
fur which magnetic observatories were established for
limited periods at certain stations in her Maj. sty's do-
minions, selected in the view that by a combinetiun of the
results obtained at them a general theory of each at least
of the principal periodical variations might be derived, and
tests be thus supplied whereby the truth of physical theories
propounded fur their explanation might be examined. We
are just beginning to profit by the collocation and study
of the great body of facts which have been collected. Va-
riations corresponding in period to the earth's meantime
around the sun, and to its rotation around its own axis, have
been ascertained to exist, and their numerical values ap-
proximately determined in each of the three elements, the
declination, inclination, and magnetic force. We unitise
sitatingly refer these variations to the sun as their primary
source, since we find that in whatever part of the globe
the phenomena are observed, the solstices and equinoxes
are the critical epochs of the variation whoa, period
is a year, whilst the diurnal variation follows, in all
I meridians, nearly the same law of local solar hours. To
these unquestionable evidences of solar influence in the
magneto affections of the earth, we have now to add the
recently ascertained fact, that the magnetic storms or dis-
turbance' which, in the absence of more correct know-
ledge, were supposed to be wholly irregular in their in-
currence, are strictly periodical phenomena, conforming,
with systematic regularity, to laws in which the influence
of local attar hours is distinctly traced. But, whilst we
recognise the sun as the primary cause of varietions whom
periods attest the MUM from whence they derive their
origin, the mode or modes in which the edicts are pro-
duced constitute a question which has been, and may still
be, open to a variety of opinions—the direct action of the
sun, as being itself a magnet—its calorific agency, ores-
sinning thermo-electric and galvanic currents, or m alter-
nately exalting and depreining the magnetic condition of
substances neer the surface of the earth, or in one of the
constituents of its atmosphere—have been severally ad-
duced as hypotheses affording plausible explanations. Of
each and all such hypotheses the facts are the only true
criterion ; but it is right that we should bear in mind that
lathe present state of our knowledge, the evidence, which
may give a decided countenance to one hypothesis in pre-
ference to others, does not preclude their possible C.heltili•
!WC. The analysis of the collected materials, and the dis-
entanglement of the various effects which are comprehended
is them, are far from being yet complete. The correspon-
dence of the critical epochs of the annual variation of the
solstices and equinoxes rather than with the epochs of
maximum and minimum temperature—which at the ear-
fuls of the earth, in the subsoil beneath the surface, or in
the atmosphere above the surface, are separated by a wide
interval from the solstitial epochs—appear, to favour the
hypothesis of a direct action, as does also the re-
markable fact which has been established, that the mag-
netic force is greater, in both the northern and southern.
hemispheres, in the months of December, January, and
February, when the sue is usenet to the earth, than in
those of May, June, and July, when he is moat distant
from it ; whereas, if the effects were due to temperature,
the two hemispheres should be oppositely instead of simi-
larly affected in each of the two periods referred to. Bell
there are doubtless minor periodical irregular variations yet
to be made out by suitable analytical processes, which, by
their possible accordance with the epochs of maximum sad
minimum temperature, may support in • more limited
sense, not as a sole, but as a co-ordinate, cause, the hype.
thesis of calorific agency, so generally received, and so ably
advocated of late in connexion with the discovery, by our
great chemist and philosopher, of the magnetic properties
of oxygen, and of the manner in which they are mo-
dified and affected by differences of temperature. It
may, indeed, be difficult to suppose that the magnetic
phenomena which we measure at the surface of the globe
should not be in any degree influenced by the variations
in the magnetic meditates of the oxygen of the atmo-
sphere, in different seasons, and at different hours of the
day and night; but whether that influence be sensible or
sok whether it be appreciable by our instruments or in-
appreciable by them, is a question which yet remains
for solution, by the more minute sifting of the accumulated
facts which are now undergoing examiaation in so many
quarters. To justify the anticipation that conclusions of
the most striking character, and wholly unforeseen, may
yet be derivable from the materials in our possession, we
need only to recal the experience of the last few menthe,
which have brought to our knowledge the existence of
what mayfisoesibly prove the most instructive, MS it ter-
tahily, at rst sight, the least explicable of all the periodi-
cal magnetic variations with which we have become am
guide:dad. I refer to the concurrent testimony which ob-
servations at ports of the globe the most distant from each
other bear to the existence of a periodical variation or in-
equality, affecting slam the magnitude of the diurnal varia-
tions, and the magnitude and frequency of the disturb-
ances or storms. The cycle or period of this inequality
appears to extend to about tan of our years ; the maximum
and minimum of the magnitudes affected by it being ups-
rated by an interval of about five years, and the differences
being much too great, sad resting on an induction far too
extensive, to admit of uncertainty es to the facts them-
selves. The existence of a well marked magnetic period,
which has certainly no counterpart in thermic conditions,
appears to render still more doubtful the suppmed eon-
nexion between the magnetic and caloric influences el
the sun. It is not a little remarkable that this periodical
magnetic variation is found to be identical, in period sad
in epochs of maxima and minima, with the periodical
variation in the frequency and magnitude of the solar
spots which Mr. Schwabe has established by twenty-six
years of unremitting labour. From a comical connexion
of this nature, supposing it to bo finally estehliehed, it
would follow that the decennial period, which we measure
by our magnetic instruments, is in fact a solar period,
manifested to us also by the alternately increasing and
decreasing frequency and magnitude of obecurations on the
surface of the miler disc. May we not have in these phe-
nomena the indication of a cycle or period of secular change
in the magnetism of the sun affecting visibly his gaseous at-
mosphere or photosphere, and sensibly modifying the mag-
netic iafluence which he exercises on the surface of our
earth.

Col. Sabine next detailed the various steps which have
been made in advance towards an efficient method of de-
termining the figure and dimensions of the globe, and pro-
ceeded as fellows : The recent researches of -Cap-
tain Bcecbey, which have given a new and un-
expected view of the tidal movements of the
ocean, show how much yet remains to be learnt respecting
the tides even for the practical purposes of n•vL
ption. The fasts derived a few years since from the be-
rometical observations at St. Hal.na, showing the existence
of a lunar atmospheric tide, have been corroborated is
the last year by • similes conclusion, drawn by Captain
Elliot, of the Madras Engineers, from the barometieal
observations at Singapore. The influence of the moon', at-
traction on the atmosphere produces, as might be expected,
a somewhat greater elect on the barometer at Singapore,
in lat. 1 deg. 19 secs., than at St. Helena, in lat. 15 det
51 secs. The barometer at the equator appears to KIM