— Fan. 9, 1873] 
NATURE 
193 
observation. At page 434 is a rough statement of the results of 
his researches, the heights being given in Paris lines. 
Lat. Barometer 
° mercury at O° C. 
° 5 ‘ ; 337'0 
‘ular = 7 . 337°5 
20 7 2 : A 338'5,; 
30 F Py! be - 339°0 
40 c : . 338'0 
ees : ' 337°0 
60 2 “ “ 3 335'5 
65 : : ~ 333'0 
+; 7Ome\ ¢ é E < 3340 
i 75 ° . : 335°5 
3 The expedition might contribute to the examination of this 
law, not only by giving special attention to the barometer obser- 
vations at about the critical latitudes 0°, 30°, 65°, 70°, but also 
by comparing any barometers with which long series of observa- 
tions have been made at any port they may touch at, with the 
ship’s standard barometer. , J 
It appears probable from Schouw’s paper, that certain meri- 
dians are meridians of high pressure and others of low pressure. 
For comparison of barometer and measures of heights, it 
appears that the aneroid barometer constructed by Goldschmid 
of Zurich, would be very useful. 
It is very desirable that the state of the barometer and ther- 
mometer should be read at least every two hours, 
(To be continued, ) 
TERRESTRIAL MAGNETISM* 
II; 
‘THE problem was attacked later on by General Sabine ina 
success. The earth, as we are all well aware, moves round the sun 
in an elliptic orbit, the nearest approach of the two bodies occur- 
ring at about the time of the winter solstice ; if, therefore, there 
be an annual inequality, it will probably attain its maximum 
when the earth is in perihelion, and its minimum at aphelion, 
since the magnetic force is known to vary inversely as the square 
of the distance. The year was, therefore, divided by Sabine into 
two equal parts, and the mean of all the observations taken 
during the six winter months compared with the mean for the 
six summer months. The records of the three British observa- 
tories of Hobarton, Toronto, and Kew all agree in showing 
that the magnetic intensity of the earth is greater in winter than 
in summer. This was very satisfactory ; but the same calcula- 
tions have since been made for other magnetic stations, where 
monthly determinations of the three elements are carried on with- 
out interruption, and some of the results are far from confirming 
the above conclusion ; for we find that observatories as near as 
Kew and Greenwich are in direct opposition on this point. A 
more extensive series of comparisons will finally show how far 
this disagreement depends on the accidental nature of the ob- 
serving stations ; but at present the preponderance of the eyi- 
dence is decidedly in favour of a semi-annual inequality, _ 
A simular investigation of the effect of the moon’s action on 
terrestrial magnetism requires a series of observations made at 
much less distant intervals than the monthly ones, which suffice 
4 for the study of the annual variation. This new question pre- 
sents itself to our view under a twofold aspect. The effect of the 
moon may be studied either in its independent action, or as it 
acts conjointly with the sun; in the former case we must group 
the observations with respect merely to the position of the moon 
| in its orbit, and, as this is an ellipse with the earth in the focus, 
the force, varying inversely as the square of the distance, will 
have its maximum disturbing influence at perigee and its mini- 
mum at apogee. The range also of the inequality will depend 
on the eccentricity of the orbit, and the period of variation will 
coincide with the siderial, or more strictly the anomalistic, month 
of a little over twenty-seven days. 
Butif we consider the moon as acted upon by the sun, receiv- 
ing its magnetic power, as it does its light and heat, from the 
central body of our system, or merely having its own inherent 
magnetism modified by solar action, then we must choose as our 
* Continued from p. 173. 
a 
much more definite manner, and with much greater chance of 
unit the lunation, or synodic month of 29°5 days, observing the 
changes that take place as the noon approsches to or recedes 
from thesun. A careful sifting of the Greenwich ob-ervations 
led Mr. Airy to a belief in the existence of a menstrual inequality 
of the declination, attaining its maximum on the fifth day of the 
moon’s age, and of a semi-menstrual inequality of the horizontal 
force whose maximum occurs on the second day. The solar 
effect on the moon's magnetic power would, therefore, appear to 
be cumulative, and not to be fully developed till several days sub- 
sequent to the conjunction of the two bodies. 
No examination seems to have been as yet made to test the 
existence of a monthly variation due to the independent action of 
the moon, asthe sole disturbing force. 
The sun’s rotation on his axis presents another not improbable 
cause of periodic magnetic disturbance. For if the sun acts as 
a large magnet directly upon the earth, and the poles of the sun’s 
axis of rotation are not coincident with its magnetic poles, the 
rotation will present the solar magnetic poles alternately to the 
earth, and these acting singly, the result must bea synodic in- 
equality, dependent on the period of the sun’s rotation. The 
absence of any such irregularity is adduced, by a recent author 
on terrestrial magnetism, as a proof that the variations of the 
earth’s magnetic force are due solely to the indirect action of the 
sun ; but Prof. Hornstein has just succeeded in detecting in the 
magnetic records of Prague and Vienna an inequality in very 
close accord with the synodic period of the rotation of the solar 
spots. The magnetic period of 26 days 8 hours would give, as 
the true time of the sun’s rotation, 24d. 13h. 12m., whereas 
Sporer, from the most accurate observations of spots near the 
sun’s equator, found the time to be 24d. 12h. 59m. It becomes, 
therefore, probable that the sun has a direct magnetic action upon 
the earth, but this need not in the least interfere with the proba- 
bility of its simultaneous indirect action by means of its thermal 
energy. 
Having been able to detect, in the manner just described, the 
inequalities arising from the orbital motions of the earth and 
moon, we are immediately tempted to suppose that the diurnal 
rotation of the earth must also exert a not inconsiderable effect 
~on the magnetism of any particular station on the earth’s surface, 
and possibly even affect terrestrial magnetism as a whole. It is 
well known that change of temperature has a very powerful in- 
fluence on magnetism, and therefore we should be astonished to 
find that the daily range of temperature induced no correspond- 
ing range in the earth’s magnetic elements. The freely-suspended 
magnet is the most delicate of thermometers, and consequently, 
unless we wish the diurnal variation of the earth’s magnetism to 
be completely veiled by the more extensive changes due to the 
varying heat of the magnet itself, we must take the greatest care 
to keep the suspended needle in a locality not directly affected 
by the daily altcrnations of temperature. Attending to this pre- 
caution, by building our magnetic chamber at a considerable 
depth below the surface of the ground, we still find that there 
exists a most decided daily range in the motion of the magne’, 
to which the most delicate thermometer is wholly insensible. 
This daily range was detected by Graham as early as 1724, anda 
momentary inspection of nearly any two days’ march of the sus- 
pended needle will suffice to make this point evident. The maxi- 
mum west declination, about 2 P.M., is constant throughout the 
year, whilst the principal minimum varies with the seasons, as 
do also the secondary maximumand minimum. Canton has ac- 
counted for the leading feature in this diurnal change by the fact 
that the solar heat lessens the magnetic power of that portion of 
the earth on which it directly falls, and thereby gives a prep n- 
derating influence to the opposite portion, whose strength re- 
mains undiminished ; the needle, therefore, moves towards the 
West in the morning, and only returns towards the East as the 
Western sun restores the balance of attracting forces. 
But there are other variations of the daily range besides those 
just mentioned, for not only do most of the inflections of the diur- 
nal curves alter their time with the progress of the sun in his 
orbit, but the amplitude of the range passes through a constant 
order of phases as each year advances. Dr. Lloyd discovered 
that the maximum range of declination in summer is greater than 
in winter, and Quetelet not only confirms this, but also finds that 
the range is greater at the equinoxes than at the solstices. It was 
whilst engaged upon this investigation that the Director of the 
Brussels Observatory made the curious discovery, that the 
magnetic energy varies in the same manner as the vegetable 
force, both attaining their maximum in April, and diminishing 
gradually until they reach their minimum of intensity in the 
