194 
winter months. Other observers, such as Lamont of Munich, 
Col. Beaufoy, &c., may be cited in confirmation of the existence 
of this apparent connection between the vegetative force and that 
of magnetism, a connection which may perhaps serve to throw 
some light on the nature of magnetic action. The horizontal 
force follows a law similar to that of the declination, varying in 
its daily range with the seasons, and attaining its maximum value 
in summer. 
Another peculiar semi-annual inequality in the diurnal varia- 
tion has been detected by Mr. Chambers, the times of opposition 
being the equinoxes. This inequality is found to exist in the 
observations taken at seven stations—five in the northern, and 
two in the southern hemisphere. It only lasts from 6 A.M. to 
6 P.M., reaching its maximum at 9 A.M. from January to June, 
and at 3 P.M. from July to December, always passing through 
the mean value at noon, 
If now we turn from the consideration of the effect of the 
earth’s rotation on the direct solar magnetism to examine its in- 
fluence on that of our satellite, we are again led to expect a 
positive result, but on very different grounds from those we have 
just been reviewing. ‘The heat sent to us by the moon, even 
when full, is so insignificant, that it is requisite to collect the 
rays in some enormous mirror, such as that of the Earl of Rosse, 
or to bring them to a focus on a very sensitive thermometer, in 
order to make it sensible. It would be absurd then to look for 
any effect that the rotation might produce in the variation of the 
temperature ; but it is very reasonable to expect that the’ altera- 
tion of distance due to the rotation will not be equally insensible. 
We are not separated from our satellite by more than 240,000 
miles, and as the diameter of the earth is nearly 8,000, the rota- 
tion may alter the distance of the moon from a station on the 
earth’s surface by about one-thirtieth of the whole distance, and 
the resulting change of the attracting force must be very con- 
siderable. An examination of the Greenwich magnetic observa- 
tions, arranged according to lunar hours, has led Mr. Airy to the 
conclusion that no doubt can be entertained as to the existence of 
a luno semi-diurnal inequality, though he has failed to detect any 
luno diurnal inequality. He also found so close an agreement 
between the values of the luno semi-diurnal variation in the 
years of greater and also of smaller solar curves, that he suggests 
the two following ‘‘conjectural reasons for this remarkable 
association in the time-law of changes of solar and lunar effect. 
One is that the moon’s magnetic action is reaily produced by the 
sun’s magnetic action ; and a failure in the sun’s magnetic power 
will make itself sensible, both in its direct effect on our magnets, 
and in its indirect effect through the intermediation of the moon’s 
excited magnetism. The other is, that, assuming both actions, 
solar and lunar, to act on our magnets indirectly by exciting 
magnetic powers in the earth, which alone or principally are felt 
by the magnets, the earth itself may have gone through different 
stages of magnetic excitability, increasing or diminishing its 
competency to receive both the solar and lunar action.” The 
ratio of the moon’s disturbing action on the horizontal force 
is to that of the sun as I to 20. 
We have just been considering the irregularities in the magnetic 
action of the sun and moon, which arise from the orbital motions 
of the earth and its satellite, and from the rotation of our globe, 
but there are still other variations depending on much more com- 
plex causes that remain yet to be examined. A very important 
inequality has been detected in the daily range by several ob- 
servers, and of late years by Mr. Chambers of the Colaba Obser- 
vatory. It is a change that takes place in the amplitude of the 
range, not from season to season, but from year to year, and 
which completes its cycle in ten or eleven years. Other periodical 
inequalities of the daily range have been more than suspected, 
as that of twenty-two years, noticed by Hansteen ; and some 
of these may possibly be found to have a connection with such 
phenomena as the revolution of the moon’s nodes. _ It will suffice 
to have mentioned these ; but we must not so lightly pass over 
the decennial period, which is identical with the cycle of those 
great but irregular disturbances of which we must now say a few 
words. 
The accurate study of magnetic storms was nearly impossible 
before photography was calied to the aid of the observer ; but 
now that every movement of the needle is faithfully recorded by 
the ever watchful light of the gas jet, a continuous curve shows 
at a glance the nature, extent, and duration of even the slightest 
disturbance. The arrangement of these self-recording magnets 
is extremely simple and equally effective. To each magnet, 
whose movements we desire to study, is attached a small mirror, 
NA TURE : 
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Ps 
oe 
, ie ae 
Fan. 9, 1873 — 
and the rays from a gas jet falling on the mirror are sent by it to 
a cylinder covered with sensitised paper. A lens brings the rays — 
toa focus on the cylinder, and this focus traces on the paper 
every movement of the magnet. A second mirror fixed im- 
mediately underneath the first, but having no connection with 
the magnet, sends the rays of the gas jet always in thesame ~ 
direction, and thus traces a base line frofm which the variations of _ 
the magnetic curve can be measured with the greatest exactness. 
A clock turns the cylinder through a complete revolution in 
twenty-four hours, and the light being cut off for a few minutes 
every two hours, breaks are thus made in the curye, which serve 
as an excellent time scale. The magnetic curves, traced in this 
manner, are in general and lightly irregular lines, which reach their __ 
highest point towards 2 P.M., and are more or less curved at all 
hours of the day. Scarcelya day passes without some apparently 
accidental departure from the ordinary bend of the line, but these 
disturbances are often only of short duration. There are, how- 
ever, occasions on which the magnets seem to be subject to the 
action of a disturbing force far exceeding in intensity any of those 
we haye been hitherto considering, and subject itself to no appay 
rent laws, but causing the needle not unfrequently to oscillate 
through several degrees of arc on either side of its mean position. 
It will be interesting to know what account can be given of this 
disturbing power, which assumes such Protean shapes, at one 
time raising a storm that dies away as gradually as it commenced, 
and at another bursting forth in an instant in all its fury ; now 
continuing its disturbing action for days together, and then im- 
parting but a single momentary impulse ; affecting sometimes 
one element, and then another, and sometimes all together ; and 
finally appearing not unfrequently at the same hour on several 
successive days. ‘ 
The coincidence of these disturbances with the passing of 
earth currents, so perfectly recorded on the Greenwich curves ; 
their never-failing appearance at all auroral displays; their 
simultaneous occurrence at places the most remote from each 
other ; and lastly the agreement of their period of variation of 
intensity, as well as their maxima and minima with the decennial 
period, and the maxima and minima of sun-spot development; all _ 
these facts will be most powerful aids towards the solution of our 
difficulty, Neither is it unreasonable to expect that some light 
may be thrown upon the question, if we examine with careful 
attention the not impossible connection of magnetic storms with 
solar outbursts, or. with volcanic eruptions and violent earth< 
quakes, with the variations of the wind, or even with the showers 
of falling meteors. Much of interest has already been ascer- 
tained in connection with these several points, but I will not tax 
too severely your indulgent patience by entering at present into 
these details. 
I must, however, before concluding, allude for one moment to 
those researches of De La Rue, Stewart, and Loewy on solar 
physics, in which they have made a first step towards establishing 
a connection between the period of solar spots and the relative 
position of the planets. If this can be maintained ; if the solar 
disturbances are in any way due to the combined action and re- 
action of the planets, and these again are found to be coincident 
with the great perturbations of terrestrial magnetism, shall we 
not be inclined to attribute a wider range to the magnetic force 
than is in general assigned to it? May not that, which has lon 
been allowed to rank among the most extensively diffused o 
nature’s agents, find a home in each individual member of the — 
solar system, causing them to act and react upon each other as ‘ 
well by their magnetic energy as by their force of gravity? The 
perfect solution of such a problem would well repay many a 
year of persevering observation and of assiduous study, and well 
will those be rewarded by whose labours the general cause of 
terrestrial magnetism ceases to be one of the unsolved mysteries 
of cosmical physics. ‘ ; 
SCIENTIFIC SERIALS 
No. 3 of the Bulletin de 1’ Academie Impériale des Sciences de 
St. Petersburg, t. xvii., contains seven anatomical papers by Dr, 
Wenzel Griiber—six on various abnormal muscular forms, and 
the seventh being an account of the formation of supernumerary 
wrist-bones.—An appreciative paper on Sir Roderick Murchison _ 
is communicated by G. Helmensen. He refers to Murchison’s 
visits to Russia between 1840 and 1845 to study the paleozoic 
formations, In a résumé of results, he mentions, among others, 
the discovery, n post-pliocene strata in the lower course ot the 
ee. eyes |. 
