Evaporation—Distribution of Vapour—Dew—Hoar-Frost—Fog—Region of Clouds—
Forms of Clouds—Rain—Distribution of Rain—Quantity—Number of rainy
Days in different Latitudes—Rainless Districts—Snow Crystals—Line of
perpetual Snow—Limit of Winter Snow on the Plains—Sleet—Hail—
Minuteness of the ultimate Particles of Matter—Their Densities and
Forms—Their Action on Light—Colour of Bodies—Colour of the Atmosphere—
Its Absorption and Reflection of Light—Mirage—Fog Images—Coronæ and
Halos—The Rainbow—Iris in Dewdrops—The Polarization of the Atmosphere—
Atmospheric Electricity—Its Variations—Electricity of Fogs and Rain—
Inductive Action of the Earth—Lightning—Thunder—Distribution of
Thunder-Storms—Back Stroke—St. Elmo’s Fire—Phosphorescence—Aurora—
Magnetism—Terrestrial Magnetism—The Dip—Magnetic Poles and Equator—
Magnetic Intensity—Dynamic Equator—Declination—Magnetic Meridian—Lines
of equal Variation—Horary Variations—Line of Alternate Horary
Phenomena—Magnetic Storms—Coincidence of the Lines of equal Magnetic
Intensity with Mountain Chains—Diamagnetism.
MOISTURE is evaporated in an invisible form from every part of the land
and water, and at all temperatures, even from snow. Mr. Darwin mentions
that the snow once entirely disappeared from the volcano of Aconcagua,
in Chile, which is 23,300 feet high, from evaporation under a cloudless
sky and an excessively dry air. The vapour rises and mixes with the
atmosphere; and as its pressure and density diminish with the height
above the surface of the earth, in consequence of gravitation, there is
absolutely less moisture in the higher than in the lower regions of the
air.
Seven-tenths of the atmosphere rests on the ocean; therefore the sea has
the greatest influence in modifying climates and supplying the air with
moisture. The evaporation is greatest between the tropics, from the
excess of heat and the preponderance of the ocean, and its average
quantity decreases from thence to the poles. Over the open sea, in all
latitudes, the air is saturated with moisture; and in that over the
coasts the quantity is very great, but it diminishes from the coasts to
the interior of the continents. In the interior of the United States of
North America, in the deserts of Asia, and in the interior of New
Holland, the air is continually dry. There is scarcely any evaporation
in the deserts of Africa, and the extreme heat, increased by the
reverberation of the sand, opposes aqueous precipitations, so this land
is doomed to perpetual sterility. The air over the steppes of Siberia is
likewise nearly deprived of moisture. The greatest degree of dryness on
record is that observed by M. Erman between the valleys of the Irtish
and Obi, after a continued south-west wind and a temperature of 74° 7ʹ
of Fahrenheit.
Throughout all the countries in the northern hemisphere where
observations have been made on the variations of atmospheric moisture,
it appears that the air contains less vapour in January than in any
other month of the year, yet at that time there is the greatest
dampness; while in July the air is driest, and yet, on account of the
heat, evaporation is the greatest: the reason is, that the heat in July
dissolves the moisture and increases its elasticity or tension so much
that it becomes insensible, whereas the cold of winter condenses it and
renders it apparent.
The quantity of atmospheric moisture varies also with the hours of the
day and night. In early morning the evaporation accumulates near the
surface of the ground from the resistance of the air above it, but as
the sun rises above the horizon the warm air descends and carries the
vapour with it; so that the quantity near the ground is diminished till
evening, when, on account of the lowness of the temperature, the
ascending currents cease, and the air becomes loaded with vapour, and
deposits its excess in the shape of dew or hoar-frost. For in the night
the earth radiates part of the heat it received during the day through
the atmosphere into space, and the temperature of the bodies on its
surface sinks below that of the air; and by abstracting part of the heat
which holds the humidity of the air in solution a deposition takes
place. If the radiation be great, the dew is frozen and becomes
hoar-frost, which is the ice of dew. Cloudy weather is unfavourable for
the formation of dew by preventing the free radiation of heat, and
actual contact is necessary for its formation, as it is never suspended
in the air like fog. Dew falls in calm serene nights, but not on all
substances indifferently; it wets them in proportion to their power of
radiation, leaving those dry that radiate feebly or not at all. Dew is
most abundant on coasts; in the interior of continents there is very
little, except near lakes or rivers. When dew is congealed into
hoar-frost it forms beautiful crystals, and the cold which produces it
is very hurtful to vegetation, but a slight covering preserves plants
from its effects.
When the atmosphere is so saturated with the vapour of water that it is
precipitated in the air itself, a fog is the result, which consists of
small globular particles of water. When dew is formed the earth is
colder than the air in contact with it; but the case is exactly the
contrary when fogs take place, the moist soil being warmer than the air.
In countries where the soil is moist and warm, and the air damp and
cold, thick and frequent fogs arise, as in England, where the coasts are
washed by a sea of elevated temperature, and the excess of the heat of
the Gulf-stream above the cold moist air is the cause of the perpetual
fogs in Newfoundland.
Superior to all these phenomena, and at a considerable height above the
earth, the air is very dry, because, under ordinary circumstances, the
vapour ascends in a highly elastic and invisible state till it reaches a
stratum of air of lower temperature, and then it is condensed into
clouds. The region of clouds is a zone at a height varying from one to
four miles above the surface of the earth, which is saturated with
moisture. From friction and other causes the currents of air in the
lower parts of that zone run horizontally on each other; and as they
generally differ in moisture, temperature, and velocity, the colder
condense the invisible vapour in the warmer, and make it apparent in the
form of a cloud, which differs in no respect from a fog, except that one
floats high in the air, while the other rests on the ground.
At moderate heights clouds consist of vapour, but at great elevations,
where the cold is severe, they are an assemblage of minute crystals of
ice. They assume three primary characters, from whence four subordinate
forms are derived. The cirrus, or cat’s-tail of sailors, is the highest;
it sometimes resembles a white brush, at other times it consists of
horizontal bands of slender silvery filaments. To these all Kämtz’s
measurements assign a height of 19,500 feet, which is confirmed by their
appearance being the same when seen from the tops of mountains or from
the plains; consequently, they must consist of minute particles of ice
or flakes of snow floating in the higher regions of the zone of clouds.
The cirri for the most part arrange themselves in parallel bands which
converge to opposite points in the horizon by the effects of
perspective, and as they travel in their longitudinal direction they
appear to be stationary. In the middle and higher latitudes of the
northern hemisphere they tend from south-west to north-east, and at the
equator from south to north. It is supposed that their parallel form
arises from their being conductors between two foci of electricity, but,
whatever the cause of this arrangement may be, it is very extensive.
Among these clouds, which occasionally appear like fleecy cotton or
wool, halos and parhelia are formed, which often precede a change of
weather, announcing rain in summer, in winter frost and snow.
Cumuli, or summer-clouds, are rounded forms resting on a straight band
in the horizon, and resemble mountains covered with snow. They are
formed by ascending currents drawing the vapours into the higher regions
of the atmosphere; sometimes they rise and cover the whole sky, and in
the evening they frequently become more numerous and of deeper tint,
presaging storm or rain.
The stratus is the third of the primary characters of clouds: it is a
horizontal band, which forms at sunset and vanishes at sunrise. The
subordinate varieties of clouds are combinations of these three
principal classes.[139] The winds, the great agents in all atmospheric
changes, carry the vapour to a distance, where it is often condensed on
the tops of mountains into clouds which seem to be stationary, but which
in reality are only maintained by a constant condensation of fresh
vapour, which is carried off, as soon as formed, by the wind, and
becomes invisible on entering warmer air.
When two masses of air of different temperature meet, the colder, by
abstracting the heat which holds the moisture in solution, causes the
particles to coalesce and form drops of water, which fall in the shape
of rain by their gravitation. And when two strata of different
temperature moving rapidly in contrary directions come into contact, a
heavy fall of rain takes place; and as the quantity of aqueous vapour is
most abundant in tropical regions, the drops are larger and the rain
heavier than elsewhere.
Since heat is the cause of evaporation, rain is very unequally
distributed, and with it decreases from the equator to the poles. From
the island of Otaheite [Tahiti], in the Pacific, to Uleaborg, in
Finland, the annual quantity of rain that falls decreases from 150
inches to 13. It is, however, more abundant in the New World than in the
Old; 115 inches fall annually in tropical America, while in the Old
World the annual fall is only 76 inches; so also in the temperate zone
of the United States the annual quantity is 37 inches, while in the Old
Continent it is but 31-3/4 inches.
Between the tropics the rains follow the sun: when he is north of the
equator the rains prevail in the northern tropic; and when he is south
of that line, in the southern: hence, one half of the year is extremely
wet and the other half extremely dry; the change taking place near the
equinoxes. Nevertheless, in countries situate between the 5th and 10th
parallels of latitude, north and south, there are two rainy seasons, and
two dry; one occurs when the sun passes the zenith in his progress to
the nearest tropic, and the other at his return, but in the latter the
rains are less violent and of shorter duration. Although the quantity of
water which falls between the tropics in a month is greater than that of
a whole year in Europe, yet the number of rainy days increases with the
latitude, so that there are fewest where the quantity is greatest.
Neither does it fall continually during the rainy season between the
tropics, for the sky is generally clear at sunrise, it becomes cloudy at
ten in the morning, at noon the rain begins to fall, and, after pouring
for four or five hours, the clouds vanish at sunset, and not a drop
falls in the night, so that a day of uninterrupted rain is very rare.
At sea, within the region of the trade-winds, it seldom rains, but in
the narrow zone between them known as the _Variables_, in both the great
oceans, it rains almost continually, attended by violent thunder and
lightning.
Throughout the whole region where the monsoons prevail, it is not the
sun, directly, but the winds, that regulate the periodical rains. That
region extends from the eastern coasts of Africa and Madagascar across
the Indian Ocean to the northern districts of Australia, and from the
tropic of Capricorn to the face of the Himalaya, the interior of China,
and even to Corea, inclusive. In these countries the western coasts are
watered during the south-west monsoon, which prevails from April to
October; and the eastern coasts are watered during the north-east
monsoon, which blows from October to April. For example, the south-west
wind condenses the vapour on the summit of the Ghauts, and violent rains
fall daily on the coast of Malabar, while on the Coromandel coast the
sky is serene. Exactly the contrary takes place during the north-east
monsoon; it rains on the coast of Coromandel, while there is fair
weather on the Malabar coast, and the table-land of the Deccan partakes
of both. In the southern hemisphere the rainy season corresponds with
the south-west monsoon, and the dry with the south-eastern.
Between the tropics it rains rarely during the night, and for months
together not a drop falls; while in the temperate zone it often rains in
the night, and rain falls at all seasons, though more abundantly in some
than in others. It seldom rains in summer throughout the north of
Africa, Madeira, the southern parts of Spain and Portugal, Sicily,
southern Italy, all Greece, and the north-western part of Asia; but it
falls copiously during the other seasons, especially in winter;
consequently, that extensive region is called the province of winter
rains.
The province of autumnal rains includes all Europe south of the
Carpathians, western France, the delta of the Rhine, northern and
western Scandinavia, and the British isles; throughout these countries
more rain falls in autumn than in the other three seasons.
The province of summer rains comprises the eastern parts of France, the
Netherlands (with the exception of the delta of the Rhine), the north of
Switzerland, all Germany north of the Alps, the Carpathian mountains,
Denmark, southern Scandinavia, all central Europe, and the countries
beyond the Ural Mountains to the interior of Siberia, where showers are
very rare in winter. In some places it rains almost perpetually, as in
the island of Sitka, on the north-eastern coast of North America, where
the year has sometimes passed with only 40 days of fair weather.
In the southern hemisphere, in Chile and the south-western part of
America, winter is the rainy season, while on the eastern side of the
Cordilleras the rains occur in summer. In Tierra del Fuego and the
extreme point of the continent the two provinces meet, the periodical
precipitation disappears, and it snows and rains throughout the year in
torrents. At Cape Horn the quantity of rain which fell in 41 days
measured nearly 154 inches. This excessive fall of rain occurs along the
whole western shores of Patagonia, from the Straits of Magellan to Cape
Tres Montes—a circumstance favoured by the high and rugged coasts, and
the incessant westerly winds, which carry the vapour exhaled from the
ocean to be precipitated here in the form of rain.
South Africa and Australia resemble each other in their rainy seasons,
which in both countries take place in the winter months.
The annual amount of rain at the equator is 95 inches, which falls in 78
or 80 days, giving an average of 1·14 inch daily; while at St.
Petersburg the annual amount is 17 inches, which falls in 169 days, the
average being little more than the tenth of an inch daily.
The quantity of rain decreases in ascending from the plains to
table-lands, especially if these be edged by mountains, because they
precipitate the vapour before it arrives at the high plains. On the
contrary, the quantity increases in ascending from plains to the tops or
slopes of rugged mountains, on account of partial currents of air which
condense the moisture into clouds.
The quantity of rain decreases on receding from the coasts into the
interior of continents, because more vapour rises from the sea than from
the land. The vapour from the Gulf-stream produces a greater quantity of
rain and fog in the southern counties of England and Ireland than that
which falls in the other parts of the islands.
The number of rainy days depends upon the direction of the wind. In
Europe, if the wind always blew from the north-east, it would never
rain, because it blows over a great extent of continent; whereas it
would never cease raining were the wind always to blow from the
south-west, because it would come loaded with vapour from the Atlantic.
Hence, the greatest quantity of rain falls on the west coasts of Great
Britain and Ireland, the coast of Scandinavia, the eastern Alps, and the
centre of Portugal; in the two last it depends partly on the height and
serrated form of the mountains. In western Europe it rains on twice as
many days as in the eastern part; in Ireland there are three times as
many rainy days as in Italy or Spain. In fact, on the western side of
Ireland it rains on 208 days out of the 365. In England, France, and the
north of Germany, there are from 152 to 155 rainy days in the year; the
number decreases towards the interior of the continent, so that in
Siberia it only rains on 60 days in the year.
There are enormous tracts of land on which rain never falls, and others
where it rains at long intervals and in small quantities. The most
extensive rainless district stretches from the borders of Morocco
eastward through the desert of Africa, the low coasts of Arabia, Persia,
and the desert province of Meekran, in Beloochistan, occupying a space
of 80 degrees of longitude and 17 of latitude. The desert of Gobi, on
the table-land of Tibet, and part of Mongolia, form another rainless
province in the great continent; while, in the New World, the rainless
districts are—the table-land of Mexico, part of Guatemala and
California, and the western declivity of the Andes of Peru, towards the
Pacific; in all occupying a surface equal to 5,500,000 square miles. The
whole of the moisture is intercepted by the Andes of Peru; so that rain
only occurs on the coast once or twice in a century—to the great terror
of the inhabitants when it does fall. [The absence of rain is here
compensated for by copious dews and mists, termed “llovisnas.”] South
Africa, and Australia beyond the tropics, suffer from droughts, which
are periodical in Australia; they recur in the countries of the eastern
coasts in a period of 12 years, and continue 3 years. The Pampas of
South America are also subject to droughts, though they do not appear to
be periodical, nor do they continue more than a season.[140]
When the temperature of the air is near the freezing-point or below it,
snow falls instead of rain; but the colder the air the less moisture
does it contain, consequently the less snow falls, which is the reason
of the comparatively small quantity on the high plains of the Himalaya
and Andes. Snow sometimes assumes the form of grains; but is generally
in regular crystals of great beauty, varying in form according to the
degree of cold. Captain Scoresby, whose voyages in the Polar Seas
afforded him constant opportunities of studying them, of which he so
diligently availed himself, mentions five principal kinds of snow
crystals, each of which had many varieties, in all amounting to 96. M.
Kämtz, however, is of opinion that there are several hundred.
Snow never falls between the tropics except on the tops of very high
mountains. The mean elevation of the line of perpetual snow above the
level of the sea in these hot regions is about 15,207 feet, from whence
it decreases on both sides, and at last grazes the surface of the earth
at the arctic and antarctic circles, subject however to various
flexures. In the Andes, near Quito, the lowest level has an elevation of
15,795 feet, which is higher than the top of Mont Blanc; from thence it
varies very irregularly, both to the north and south. In 18° of N. lat.
it descends to 14,772 feet on the mountains of Mexico, while on the
south it rises to 18,000 feet in some parts of the western Cordillera of
the Bolivian Andes, owing to the extensive radiation from the subjacent
plains and valleys. The line is at an altitude of 17,000 feet on the
western Cordillera, whence it sinks to 13,800 feet at Copiapo, to 12,780
near Valparaiso; it is only 8300 in the southern end of the Chilian
Andes, and 3390 in the Straits of Magellan. In lat. 31° N. the snow-line
is at an elevation of 12,981 feet on the southern side of the Himalaya,
and at 16,620 feet on the northern side, while Captain Gerard gives from
18,000 to 19,000 as its altitude on the mountains in the middle of the
plain of Tartary. On Mont Blanc the line is at the height of 8500 feet,
so that mountain is snow-clad for 7000 feet below its summit. In the
Pyrenees it is 8184 feet, and at the island of Mageroe it is at 2160
feet above the Polar Ocean.
In the southern hemisphere, snow never falls on the low lands at the
level of the sea north of the 48th parallel of latitude, on account of
the predominance of water, whereas in the northern hemisphere it falls
on the plains much nearer the equator, on account of the excess of land,
but its limit is a curved line, on account of the alternations of land
and water. In the western part of the great continent, the southern
limit of the fall of snow on the low lands nearly coincides with the
30th parallel of north latitude, so that it includes all Europe. In the
American continent it follows nearly the same line, extending through
the southern parts of the United States. In China, snow falls at the
level of the sea as far south as Canton; on the north-western coast of
America, on the contrary, it does not fall at that level till about the
48th degree of N. lat.—these are the two extremes. Although Europe lies
within the region of snow, the quantity that falls is very different in
different places, increasing greatly from south to north. On an average,
it snows only one day and a-half at Rome in the year, while at
Petersburg there are 171 snowy days, but in that city the quantity of
rain is to that of snow as 1000 to 384.
Sleet, which is formed of small particles of rounded hail, falls in
squally weather in spring and autumn. True hail, when large, is
pear-shaped, and consists of a nucleus of frozen snow coated with ice,
and sometimes with alternate layers of snow and ice. Hailstones have
often fallen as large as pigeons’ and even hens’ eggs. The masses and
blocks of ice of great size, which have not unfrequently fallen, appear
to have been formed of hailstones of large size frozen together. It
appears to be formed in the high cold regions of the atmosphere, by the
sudden condensation of vapour during the contention of opposing winds,
and is intimately connected with electricity, since its fall is
generally accompanied with thunder and lightning. Hail-showers are of
short duration, exceedingly partial, and extend over a country in long
narrow bands; one which took place on the 13th of July, 1788, began in
the morning in the south of France, and reached Holland in a few hours,
destroying a narrow line of country in its passage.
Local circumstances, no doubt, have a great influence on its formation;
it occurs more frequently in countries at a little distance from
mountains than in those close to them or farther off, and at all hours,
but most frequently at the hotter time of the day. In the interior of
Europe one half of the hail-storms take place in summer. Hail is very
rare on the tropical plains, and often altogether unknown, though it
frequently falls at heights of 1700 or 1800 feet above them. If the air
is very cold throughout the greater part of the stratum through which
hail falls, it is probably increased in size during its descent; and, on
the contrary, large drops of rain which precede a thunder-storm are
supposed to be hail melted in its passage through low warm air.
LIGHT.
We know nothing of the size of the ultimate particles of matter, except
that they must be inconceivably small, since organized beings possessing
life and exercising all its functions have been discovered so minute
that a million of them would occupy less space than a grain of sand.
The air is only visible when in mass; the smallest globule of steam
tells no more of its atoms than the ocean; the minutest grain of sand
magnified appears like the fragment of a rock—no mechanical division can
arrive at the indivisible. Although the ultimate atoms are beyond the
power of vision, chemical compounds show that the divisibility of matter
has a limit, and that the particles have different densities; moreover,
the cleavage of crystalline substances gives reason to believe that they
have different forms.[141] Thus, the reasoning power of man has come to
the aid of his imperfect sense of vision, so that what were before
imaginary things are now real beings with definite weights, and uniting
by fixed laws. Though nothing had been known of their size, their
effects were evident in the perceptions of sweet and sour, salt and
bitter, and in the endless varieties of aroma in the food we eat and the
liquors we drink. Moreover, their different densities are evident, as
they arise by their buoyancy in the perfume of the rose, or sink by
their weight in the heavy odour of mignonette. Every substance on earth
is merely a temporary compound of the ultimate atoms, sooner or later to
be resolved into its pristine elements, which are again to be combined
in other forms, and according to other laws; so that literally there is
nothing new under the sun, for there is no evidence of new matter being
added to the earth, nor of that which exists being annihilated. Fire,
which seems utterly to destroy, only resolves bodies into their
elementary parts, to become what they were before, the support of animal
or vegetable life, or to form new mineral compounds. It is to the action
of these particles on the light of the sun that nature owes all its
colours.
When a sunbeam passes through a glass prism[142] an oblong image of the
sun is formed, consisting of colours in the following order—red, orange,
yellow, green, blue, indigo, and violet. Sir John Herschel discovered
lavender rays beyond the violet, and dark red rays exterior to the red,
which are not so easily brought into evidence as the rest.
Even the most transparent substances absorb light; air, water, the
purest crystal, stop some of the rays as they pass through them. A
portion of the light is also reflected from the surface of all bodies;
were it otherwise, they would be invisible. We should be unconscious of
the presence and form of material substances beyond our reach except by
the reflected rays,—
“The mist of light from whence they take their form
Hides what they are.”
As the same light does not come to all eyes, each person sees his own
rainbow, the same flower by different rays. White substances reflect all
the light, black substances absorb all but that which renders them
visible, while coloured bodies decompose the light, absorb some of the
colours, and reflect or transmit the rest. Thus, a violet absorbs all
but the violet rays, which it reflects; a red flower only reflects the
red and absorbs the rest; a yellow substance absorbs all but the yellow.
In the same manner transparent substances, whether solid or fluid,
absorb some colours and transmit others: thus, an emerald absorbs all
but the green, a ruby all but the red; whereas a diamond does not
decompose the light, but transmits every ray alike. Very few, however,
of the colours, whether transmitted or reflected, are pure, but the
substance takes its hue from the colour that predominates.
The atmosphere absorbs all the colours of the sun’s light except the
blue, which is its true colour. In countries where the air is pure, the
azure of the sky is deep; it is still more so at great elevations, where
the density of the air is less; and its colour is most beautiful as it
gradually softens the outlines of the mountains into extreme distance,
or blends the sea with the sky. The air reflects and scatters part of
the white solar beams, whence the brightness and cheerfulness of day;
that property, together with the refractive power of the aqueous vapour,
gives the roseate hue to the early morning, and the gold and scarlet
tints to the closing day. Were it not for the reflective power of the
air, the sun and moon would be like sharply-defined balls of fire in the
profoundly black vault of the heavens, and dark night would instantly
follow sunset. When the sun is 18 degrees below the horizon, the air, at
the height of 30 miles, is still dense enough to reflect his rays, and
divide the day from the night by the solar shades of twilight.
A considerable portion of the sun’s light is absorbed by the atmosphere:
the loss increases with the density and obliquity of incidence and the
density of the air. It is diminished 1300 times by the thickness of the
air in the horizon, which enables us to look at the sun when setting
without being dazzled.
Mirage, or the delusive appearance of water, so frequent in deserts, is
owing to the reflection of light between two strata of air of different
densities, occasioned by the radiation of heat from the arid soil. It is
very common on the extensive plains in Asia and Africa, and especially
in Upper Egypt; villages on small eminences above the plain appear as if
they were built on islands in the middle of a lake when the dry sandy
ground is heated by the mid-day sun. Sometimes objects appear double,
and occasionally several images appear above one another, some direct
and some inverted; this is particularly the case in high latitudes,
where the Icy Sea cools the stratum of air resting on it.[143]
In the polar regions, or on the tops of mountains, when the sun is in
the horizon the shadow of a person is sometimes thrown on an opposite
cloud or mist, the head being surrounded by concentric coloured rings or
circles, the number varying from one to five; Captain Scoresby saw four
of these rings, on one occasion, round the shadow of his head, as he
stood between the sun and a thick low fog: the first ring consisted of
concentric bands of white, yellow, red, and purple; the second consisted
of concentric bands of blue, green, yellow, red, and purple; the third
of green, white, yellowish white, red, and purple; and in the fourth
were greenish white, deeper on the edges. These appearances, called
_glories_, or fog-images, and the coronæ or small concentric coloured
circles which surround the sun or moon when partly obscured by thin
white clouds, are owing to the refraction of the light in the aqueous
particles of the cloud or fog. The colours in the concentric bands of
the coronæ, however, differ from the foregoing; that nearest the sun is
of deep blue, white, and red; the circle exterior to that consists of
purple, blue, green, pale yellow, and red; but the series is very rarely
complete.
Halos, which surround the sun in large circles, or a complicated
combination of circles, are, on the contrary, supposed to be produced by
the light falling on minute crystals of ice suspended in the atmosphere;
they are particularly brilliant and frequent in high latitudes. It is
scarcely possible to give an idea of these beautiful and singular
objects. Sometimes a large coloured circle surrounds the sun or passes
through his centre, which is occasionally touched or cut by segments of
others. One seen at St. Petersburg on the 29th of June, 1790, consisted
of four coloured circles of different sizes intersecting each other,
which were either cut or touched by segments of eight others, and at the
points of intersection mock suns or parhelia appeared. The sky is very
hazy on these occasions. Mock suns, without circles and halos, are by no
means uncommon round both sun and moon, but seldom of that complicated
kind. They are situate between the observer and the sun, whereas the
rainbow is always in that part of the sky opposite the sun, because it
is produced by refraction and reflection of the sun’s rays in the drops
of rain; and when the light is intense and the rain abundant, there are
two concentric bows, the prismatic colour of the innermost of which are
the most vivid, the violet being within and the red outside: sometimes
the inner edge exhibits a repetition of colours in fine fringes, in
which red and green predominate. The colours are reversed in the
exterior bow, the violet being outside and the red on the inner edge.
Besides these two principal and most common bows, supernumerary rainbows
occasionally appear within the interior bow, generally green and violet,
though there are sometimes more or less perfect repetitions of all the
colours. In squally weather a rainbow is sometimes seen on a blue sky
when rain is falling, but it is generally on clouds; it is constantly
seen when the sun shines on the fine drops of fountains and cascades. As
the light of the moon is feeble, lunar rainbows are rare, and, for the
most part, colourless. In the early morning, when the sun throws his
slanting beams across the fields, a miniature bow, with all its vivid
colours, may be seen in each dewdrop as it hangs on the points of the
bending grass.
Light is said to be polarized when, after having been once refracted or
reflected, it is rendered incapable of being again refracted or
reflected at certain angles. For example, if a crystal of brown
tourmaline be cut longitudinally into thin slices, and polished, the
light of a candle may be seen through a slice as if it were glass. But
if one of these slices be held perpendicularly between the eye and the
candle, and a second slice be turned round between the eye and the other
plate of tourmaline, the image of the candle will vanish and come into
view at every quarter-revolution of the plate, varying through all
degrees of brightness down to total or almost total evanescence, and
then increasing again by the same degrees as it had decreased. Thus, the
light, in passing through the first plate of tourmaline, is said to be
polarized because it has been rendered incapable of passing through the
second piece of tourmaline in certain positions.
A ray of light acquires the same property if it be reflected from a pane
of plate-glass at an angle of 57 degrees; it is by that rendered
incapable of being reflected by another pane of plate-glass in certain
definite positions, for the image of the light vanishes and reappears
alternately at every quarter-revolution of the second pane.
If a thin plate of mica be interposed when the image of the candle has
vanished, the darkness will instantly disappear, and a succession of the
most gorgeous colours will come into view, varying with every
inclination of the mica, from the richest reds to the most vivid greens,
blues, and purples. The most splendid colours arranged in symmetrical
forms are exhibited by thin plates of an infinite variety of substances
besides mica. They display some of the most beautiful objects in nature,
and show differences otherwise inappreciable in the arrangement of the
molecules of crystalline bodies.[144]
M. Arago discovered that the light of the sun is polarized by the
reflection of the atmosphere, but not equally so on every part of the
sky; the polarization is least in the vicinity of the sun, and greatest
at 90° from him, for there his light is reflected at an angle of 45°,
which is the polarizing angle for air.[145] There are three points in
the sky where the light is not polarized: one of these neutral points,
discovered by M. Arago, is 18° 30ʹ above the point diametrically
opposite to the sun when he is in the horizon; the second neutral point,
discovered by M. Babinet, is 18° 30ʹ above the sun when he is rising or
setting; and the third, discovered by Sir David Brewster, is 15° or 16°
below the sun. These points vary with the height of the sun, and the two
latter rise and coincide in his centre when he is in the zenith.[146]
Now, the portion of polarized light sent to the eye from any part of a
clear sky is in a plane passing through that point, the eye of the
observer, and the centre of the sun. If that point be the north pole of
the heavens, it is clear that, as the sun moves in his diurnal course,
the plane will move with him as an hour circle, and may be used as a
dial to determine the hour of the day. Professor Wheatstone, by whom
that beautiful application of the polarization of the atmosphere has
been made, has constructed a clock, of very simple form, which shows the
time of day with great accuracy, and which has many advantages over a
sun-dial.
ELECTRICITY.
Electricity pervades the earth, the air, and all substances, without
giving any visible sign of its existence when in a latent state, but,
when elicited, it exhibits forces capable of producing the most sudden,
violent, and irresistible effects. It is roused from its dormant state
by every disturbance in the chemical, mechanical, or calorific condition
of matter, and then experience shows that bodies in one electric state
repel, and in another they attract each other. Probably their mutual
attraction and repulsion arise from the redundancy and defect of
electricity; in the first case they are said to be positively, in the
latter negatively electric.[147] When they have different kinds of
electricity they attract each other, and, when not opposed, the
electricity coalesces with great rapidity, producing the flash,
explosion, and shock, and that with the more violence the greater the
tension or pressure of the electricity on the surrounding air which
resists its escape. Equilibrium is then restored, and the electricity
remains latent till called forth by a new exciting cause. The electrical
state of substances is easily disturbed, for, without contact, positive
electricity tends to produce negative electricity in a body near it, and
_vice versâ_: the latter is then said to be electric by induction.
The electricity of the atmosphere arises from evaporation, and the
chemical changes that are in perpetual progress on the globe; no
electricity, however, is developed by the evaporation of pure water, but
it arises abundantly from water containing matter susceptible of
chemical action during the evaporation; consequently, the ocean is one
of the greatest sources of atmospheric electricity; combustion is
another, and a large portion arises from vegetation. The air, when pure,
is almost always positively electric; but as the chemical changes on the
earth sometimes produce positive and sometimes negative electricity, it
is subject to great local variations; a passing cloud or a puff of wind
produces a change, and a distant storm renders it negative for the time,
but the earth is always in a negative state. The quantity of electricity
varies with the hours of the day and the seasons; it is more powerful in
the day than in the night, in winter than in summer, and it diminishes
from the equator to the poles. It thunders daily in many places, in
others never, as on the east coast of Peru and in the Arctic regions,
except where there are violent volcanic explosions, which always
generate electricity, as in Iceland. Wherever there are no trees or high
objects to conduct it to the ground, the quantity of positive
electricity increases with the height above the surface of the earth.
Violent thunder-storms take place on the tops of the Andes and Himalaya
mountains, at heights of 26,650 feet above the plains.
Electricity becomes very strong when dew is deposited, and in some cases
it is strongly developed in fogs. Mr. Cross found it so powerful on one
occasion, that it was dangerous to approach the apparatus for measuring
its intensity. A continued succession of explosions lasted nearly five
hours, and the stream of fire between the receiving-ball and the
atmospheric conductor was too vivid to look at. M. Peltier has found
that the common fogs arising from the mere condensation of the moisture
in the air are neutral, but that others, which are produced by
exhalations from the earth, are sometimes positive, sometimes negative;
the subject, however, requires further investigation.
Though in long-continued mild rains there are no traces of electricity,
yet, when rain or snow falls from the higher regions of the atmosphere,
it is more or less developed, sometimes positive, sometimes negative,
depending a good deal on the direction of the wind. The atmosphere being
positively electric, negative rain is supposed to arise from the
evaporation of the drops in passing through dry air; the vapour carries
off the positive electricity and leaves the drop in a negative state—a
circumstance which seems to be confirmed by the electricity of cascades,
near which there always is more or less negative electricity; the
positive flows into the earth, while the other remains united to the
drops of the cascade.
The inductive action of the earth upon the clouds, and of the different
strata of clouds on each other, produces great variations in their
electrical state. If rain falls from the lowermost of two strata of
positively electrical clouds, the inductive action of the earth renders
the under surface positive and the upper negative, and the rain is
positive. By-and-bye the under surface of the cloud and the earth become
neutral; and after a time the lower cloud becomes charged with negative
electricity by the induction of the upper strata, and the rain is then
negatively electric. Clouds are very differently charged; grey clouds
have negative—red, white, and orange clouds positive electricity; and
when clouds differently charged meet, an explosion takes place. When the
sky is clear and the air calm and warm, a succession of small white
fleecy clouds rising rapidly above the horizon, and flying swiftly in
the very high regions of the atmosphere, is a certain presage of a
thunder-storm.
Electricity of each kind is probably elicited by the friction of
currents of air, or masses of clouds moving rapidly in different
directions, as in thunder-storms, when small white clouds are seen
flying rapidly over the black mass; yet the quick and irregular motion
of clouds in storms is probably owing to the strong electrical
attraction and repulsion among themselves, though both may be concerned
in these hostile encounters. When two clouds differently charged by the
sudden condensation of vapour, and driven by contending winds, approach
within a certain distance, the thickness of the coating of electricity
increases on the two adjacent sides, and, when the accumulation becomes
so great as to overcome the coercive pressure of the atmosphere between
them, a discharge takes place which occasions a flash of lightning. The
actual quantity of electricity in any part of a cloud is very small. The
intensity of the flash depends upon the extent of surface occupied by
the electricity, which acquires its intensity by its instantaneous
condensation.
The air, being a non-conductor, does not convey the electricity from the
clouds to the earth, but it acquires from them an opposite electricity,
and when the tension is very great the force of the electricity becomes
irresistible, and an interchange takes place between the clouds and the
earth, but the motion of the lightning is so rapid that it is difficult
to ascertain when it goes from the clouds to the earth, or from the
earth to the clouds, though there is no doubt it does both: explosions
have burst from the ground, and people have been killed by them.
When the air is highly rarified by heat, its coercive power is
diminished, so that the electricity escapes from the clouds in the form
of diffuse lambent sheets of lightning without thunder or rain,
frequently seen in warm summer evenings, sometimes even near the zenith,
and quite different from that sheet-lightning at the horizon which is in
general only the reflection of the forked lightning of a distant storm.
When the quantity of electricity developed by the sudden condensation of
vapour is very great, the lightning is always forked; its zigzag form is
occasioned by the unequal conducting power of the air, by which it is
sometimes divided into several branches. The author once saw a flash
divide into four parallel streams—a very uncommon occurrence.
Occasionally, in very great storms, the lightning sends off lateral
branches. It often appears as a globe of fire moving so slowly that it
is visible for several seconds, while the flashes of forked lightning do
not last the millionth part of a second. Professor Wheatstone, who has
measured the velocity of lightning by experiments of great ingenuity,
found that it far surpasses the velocity of light, and would encircle
the globe in the twinkling of an eye. This inconceivable velocity is
beautifully exemplified in the electric telegraph, by which the most
violent and terrific agent in nature is rendered obedient to man, and
conveys his thoughts as rapidly as they are formed. The colour of
lightning is generally a dazzling white or blue, though in highly
rarified air it is rose-colour or violet.
The sudden compression of the air during the passage of lightning must
convert a great quantity of latent into sensible heat, for heat in a
latent or insensible state exists in all bodies independent of their
temperature. Heat is absorbed and becomes insensible to the thermometer
when solids become liquids, and when liquids are changed to vapour; and
it again becomes sensible when vapour is condensed, and when liquids
become solid. When water freezes, all the heat that kept it liquid is
given out; and when ice melts, it absorbs heat from everything near it.
The air is full of heat in a latent state, whatever its temperature may
be, but it can be squeezed out by sudden compression so as to kindle
tinder. Every aërial wave, every sound, every word spoken must set free
an infinitesimal quantity of heat; so everything that tends to rarify
the air must cause it to absorb a proportional quantity.
The rolling noise of thunder is probably owing to the difference between
the velocity of lightning and that of sound. Thunder may be regarded as
originating in every point of a flash of lightning at the same instant;
and as sound takes a considerable time to travel, it will arrive first
from the nearest point; and if the flash run in a direct line from a
person, the noise will come later and later from the remote points of
its path, in a continued roar. Should the direction of the flash be
inclined, the succession of sounds will be more rapid and intense; and
if the lightning describe a circular course above a person, the sound
will arrive at the same instant from every point with a stunning
crash.[148]
In passing to the earth, lightning follows the best conductors—metals by
preference, then damp substances—which is the reason why men and animals
are so often struck. If it meets with a bad conductor, it shivers it to
pieces and scatters the fragments to a considerable distance. A powerful
flash scatters gunpowder, while a feeble one ignites it; the hardest
trees are split and torn to shreds; when a tree is struck, the heat of
the flash converts the sap into steam, the expansive force of which
shivers the tree. The surface of rocks is vitrified by it; and when it
falls on a sandy soil, its course underground is marked by vitrified
tubes many feet long.
Thunder-storms occur daily within the region of the Variables, which is
also the region of storms: in countries under the influence of the
monsoons they are tremendous at the changes of these periodical winds;
where the trade-winds prevail they are hardly known, though electrical
discharges are frequent at their limits. In Greece and Italy there are
about 40 thunder-storms annually, which occur in spring and autumn,
while north of the Alps they chiefly take place in summer. There are
about 24 in the year on the coasts of the Atlantic and in Germany, but
they are much more frequent among mountains than on plains. In the
interior of the old continent they rarely occur in winter, and
three-fourths of the number happen in summer. They are of such rare
occurrence in high latitudes, that in a residence of 6 years in
Greenland Sir Charles Geiseke only heard it thunder once.
Some storms arise from the contention of opposite currents in the air;
others are occasioned by currents of warm air ascending from the earth,
which are suddenly condensed as they enter the upper regions of the
atmosphere, and, as this sometimes happens at the hottest hour of the
day, these storms are periodical for many successive days, recurring
always at the same hour. Sometimes they extend over a great expanse of
country, and the lightning darts from all points of the compass. A
person may be killed at the distance of 20 miles from the explosion by
the _back stroke_. If the two extremities of a highly-charged cloud dip
towards the earth, they will repel the electricity of the earth, if it
be of the same kind with their own, and will attract the other kind; and
if a discharge should take place at one end of the cloud, the
equilibrium will instantly be restored by a flash from that part of the
earth which is under the other, sufficiently strong to destroy life, and
it is the most dangerous, though never so strong as the direct stroke.
When thunder-clouds are very low, there is frequently no lightning; the
electricity produced by induction is so powerful that it escapes from
pointed objects in the shape of flame without heat, known as St. Elmo’s
fire. These flames are not unfrequently seen at the topmasts of ships
and the extremities of their yard-arms. Bodies between the clouds and
earth may be electrized by induction, and their electricity will be seen
in the form of flame, as showers of phosphorescent snow.
Phosphorescence is ascribed to electricity; various substances emit
light when decaying, as fish and wood. Although many marine animals are
phosphorescent, yet the luminous appearance which the sea often assumes
is not always to be attributed to them, but probably to the decaying
animal matter it contains.
The aurora is decidedly an electrical phenomenon. It generally appears
soon after sunset in the form of a luminous arch stretching more or less
from east to west, the most elevated point being always in the magnetic
meridian of the place of the observer: across the arch the coruscations
are rapid, vivid, and of various colours, darting like lightning to the
zenith, and at the same time flitting laterally with incessant velocity.
The brightness of the rays varies in an instant: they sometimes surpass
the splendour of stars of the first magnitude, and often exhibit colours
of admirable transparency, blood-red at the base, emerald-green in the
middle, and clear yellow towards their extremity. Sometimes one, and
sometimes a quick succession of luminous currents run from one end of
the arch or bow to the other, so that the rays rapidly increase in
brightness; but it is impossible to say whether the coruscations
themselves are actually affected by a horizontal motion of translation,
or whether the more vivid light is conveyed from ray to ray. The rays
occasionally dart far past the zenith, vanish, suddenly reappear, and,
being joined by others from the arch, form a magnificent corona or
immense dome of light. The segment of the sky below the arch is quite
black, as if formed by dense clouds; yet M. Struve is said to have seen
stars in it, consequently the blackness must be from contrast. The lower
edge of the arch is evenly defined; its upper margin is fringed by the
coruscations, their convergence towards the north, and that of the arch
itself, being probably an effect of perspective.
Either the aurora must be high above the earth, or its coruscations must
be very extensive, since the same display is visible at places wide
asunder. It has frequently been seen in North America and all over the
north of Europe at the same time, sometimes even as far south as Italy,
yet Sir Edward Parry certainly saw a ray dart from it to the ground near
him. M. Struve, Admiral Wrangel, and others who have had many
opportunities of seeing the aurora in high latitudes, assign a very
moderate elevation to it. The arch probably passes through the magnetic
pole; hence, in the north of Greenland it lies south of the observer,
and Sir Edward Parry saw it to the south in Melville Island, which is in
70° N. lat.; consequently it must appear in the zenith in some places.
Dr. Faraday conjectures that the electric equilibrium of the earth is
restored by the aurora conveying the electricity from the poles to the
equator, for it appears in the high southern latitudes, as well as in
the northern; and the Rev. G. Fisher has lately suggested, that, as the
principal display of the aurora takes place at or near the margin of the
polar ice, the electricity may be conveyed by the conducting power of
the frozen particles which abound in the air in these latitudes, and
which, being rendered fitfully luminous by the passage of the
electricity, produce the arch and the ever-varying flashes of the
aurora.
The aurora has a powerful influence on the magnetic needle, even in
places where the display is not seen. Its vibrations seem to be slower
or quicker according as the auroral light is quiescent or in motion, and
the disturbances of the magnetic needle and the auroral displays were
simultaneous at Toronto, in Canada, on 13 days out of 24, the remaining
days having been clouded; and contemporaneous observations show that on
these 13 days there were also magnetic disturbances at Prague and at Van
Diemen’s Land, so that the “occurrence of aurora at Toronto on these
occasions may be viewed as a local manifestation connected with magnetic
effects, which, whatever may have been their origin, probably prevailed
on the same day over the whole surface of the globe.”[149]
MAGNETISM.
Magnetism is one of those unseen imponderable existences which, like
electricity and heat, are known only by their effects. It is certainly
identical with electricity, for, although it never comes naturally into
evidence, magnets can be made to exhibit all the phenomena of electrical
machines.
Terrestrial magnetism, which pervades the whole earth, is extremely
complicated; it varies both with regard to space and time, and,
probably, depends upon the heat of the sun, upon his motion in the
ecliptic, which produces changes of temperature, on galvanic currents
circulating through the surface of the globe, and possibly on the
earth’s rotatory motion.
The distribution of terrestrial magnetism is determined by the
declination-needle, or mariner’s compass, and the dipping-needle; they
consist of magnetized needles or bars of steel, so suspended that the
declination-needle revolves in a horizontal direction, and the
dipping-needle moves in a plane perpendicular to the horizon. The north
end of the declination-needle or magnet points to the north, and the
south end to the south, and it only remains at rest when in that
position. The direction of the needle is the magnetic meridian of the
place of observation.
The north end of the dipping-needle bends or dips below the horizon in
the northern hemisphere, and the south end bends or dips beneath it in
the southern hemisphere, and between the two there is a line which
encircles the whole earth, where the dipping-needle remains horizontal.
That line, which is the magnetic equator or line of no dip, crosses the
terrestrial equator in several places, extending alternately on each
side, but never deviating more than 12 degrees from it. The deviation is
greater in that part of the Pacific where there are most islands, and it
is greatest both to the south and north in traversing the continents of
Africa and America; thus, it appears that the configuration of the land
and water has an influence on terrestrial magnetism. North and south of
the magnetic equator the needle dips more and more, till at last it
becomes perpendicular to the horizon in two points, or rather linear
spaces, known as the north and south magnetic poles, which are quite
distinct from the poles of the earth’s rotation. One, whose position was
determined by Captain Ross, is in 70° N. lat. and 97° W. long., while
that in the southern hemisphere, determined by Sir James Ross, in the
interior of Victoria Island, is in 70° S. lat. and 162° E. long. Lines
of equal dip are such as may be drawn on a globe through all those
places where the dipping-needle makes the same angle with the horizon.
The angle of the dip is not always the same: according to Colonel
Sabine, who is the highest authority on this subject, it has been
decreasing in the northern hemisphere, for the last fifty years, at the
rate of three minutes annually: it is also subject to variations of
short periods, and it seems to be affected by shocks of earthquakes,
even when very distant.
The intensity of the magnetic force is as variable and even more
complicated than the other magnetic phenomena: it is measured by the
number of vibrations made by the declination-needle in a given time. It
is very different in different parts of the earth, but there are four
points in which the intensity is greater than anywhere else. Two of
these are in the northern and two in the southern hemisphere; they
neither coincide with the poles of the earth’s rotation nor with the
magnetic poles, nor are they all of equal intensity.
One of these foci of maximum magnetic intensity is situate in North
America, south-west from Hudson’s Bay; another is in northern Siberia in
120° E. long. In the southern hemisphere, one of the points of maximum
magnetic intensity is in the South Atlantic in 20° S. lat. and 324° E.
long., and the other is situate in 60° S. lat. and 131° 20ʹ E.
long.[150] In consequence of the unequal intensity of the force in these
4 foci, the decrease in magnetic power from them towards the equator is
extremely irregular, so that the dynamic equator, which is a line
supposed to be drawn through all the points on the earth where the
intensity is the least, encircles the globe in a waving line, which
neither coincides with the geographical nor magnetic equator; it forms
the division between the magnetic intensities in the two hemispheres.
Lines drawn on a globe through all the points where the magnetic
intensity is the same are so complicated that it is scarcely possible to
convey an idea of them in words. They form a series of ovals round each
of the foci of maximum force, then a figure of 8 in each hemisphere
having a focus and its ovals in each loop, then they open into tortuous
lines which encompass the globe, but which become less so as they
approach the dynamic equator. The complication is increased by the foci
in the two hemispheres being unsymmetrically placed with regard to one
another, as well as by the difference in their intensities.
The declination or horizontal needle only remains at rest when in a
magnetic meridian, that is, when it points to the north and south
magnetic poles. The magnetic meridians coincide with the geographical
meridians in some places, and in these the magnet points to the true
north and south, that is, to the poles of the earth’s rotation. But if
it be carried successively to different longitudes, it will deviate
sometimes to the east, sometimes to the west of the true north.
Imaginary lines on the globe, passing through all places where the
magnet points to the poles of the earth’s rotation, are lines of no
variation; and lines passing through all places where the magnet
deviates by an equal quantity from the geographical meridians are lines
of equal variation; they are also very irregular, and form two closed
systems or loops,—that is, they surround two points, one in northern
Siberia and another in the Pacific, nearly in the meridian of the
Pitcairn Islands and the Marquesas.[151]
The whole magnetic system is perpetually undergoing secular and
periodical changes, which are so irregular and complicated that half a
century is sufficient to alter the form and position of all the lines
that have been mentioned. The foci of magnetic intensity, and the whole
system represented by the magnetic lines, are moving along the two
hemispheres in opposite directions; those in the northern hemisphere are
going from west to east, and those in the southern from east to west;
and as the foci of maximum intensity move with different velocities, the
forms, as well as the places, of the curves are slowly, yet continually,
changing. The weaker magnetic focus in the northern hemisphere moved
through 50 degrees of longitude in 250 years.
The declination is subject to periodic variations depending upon the
position of the moon, and to annual variations arising from the motion
of the sun in the ecliptic, as well as to horary variations
corresponding to changes of temperature from the diurnal rotation of the
earth.
Throughout the middle latitudes of the northern hemisphere the north end
of the magnet has a mean motion from east to west from eight in the
morning till half-past one, it then moves to the east till evening,
after which it makes another excursion to the west, and returns again to
its original position at eight in the morning. The extent of its
variation is greater in the day than in the night, in summer than in
winter. It decreases from the middle latitudes in Europe, where it is 13
or 14 minutes, to the equator, where it is only 3 or 4; but at the
equator the variations are performed with extreme regularity. The horary
motions of the south end of the magnet in the southern hemisphere are
accomplished in an exactly opposite direction. Between these two
magnetic hemispheres there is a line passing through an infinity of
places, and very nearly coinciding with the line of minimum magnetic
intensity, where the horary phenomena of both hemispheres are combined,
each predominating alternately at opposite seasons. At St. Helena, which
is one of the places in question, and nearly on the line of minimum
intensity, the horary motion of the north end of the magnet corresponds
in direction during one half of the year with the movement in the
northern hemisphere, and in the other half of the year the direction at
the same hours corresponds with that in the southern hemisphere, the
passage from the one to the other being at the equinoxes, when the
diurnal variations at the usual hours partake more or less of the
characteristics of both on different days.[152]
It thus appears that there are six points on the earth peculiarly
remarkable for magnetic phenomena, all of which are distinct from one
another, and from the poles of the earth’s rotation—namely, two magnetic
poles where the dipping-needle makes an angle of 90 degrees with the
horizon. The magnetic equator corresponds with these in every point of
which the angle of the dip is zero: it encircles the earth, and
intersects the terrestrial equator, but does not coincide with it. The
other four points are the foci of maximum magnetic intensity, and to
them the dynamical equator or line of minimum magnetic intensity
corresponds, also surrounding the earth in an irregular line, but which
coincides with neither the terrestrial nor magnetic equator. Besides
these, and either partly or nearly coinciding with the line of minimum
intensity, is that line which is supposed to pass through all places
where the horary variations of the magnet partake of the phenomena of
each hemisphere alternately.
The earth’s magnetism is subject to vast unaccountable commotions or
storms of immense extent, which occur at irregular intervals, and are of
short duration. In 1818, a magnetic storm, shown by a violent agitation
of the needle, took place at the same time over 47 degrees of longitude,
extending through all the countries from Paris to Kasan; and on the 25th
of September, 1841, one of these storms was simultaneously observed at
Toronto in North America, at the Cape of Good Hope, Prague in Europe, at
Macao in China, and there is reason to believe that it extended to Van
Diemen’s Land. Similar storms have happened simultaneously in Sicily and
at Upsala in Sweden; others of less extent and shorter periods more
frequently occur, and are, like the greater storms, not to be attributed
to any known cause.
M. Necker de Saussure has traced a marked coincidence between the
prevailing direction of the stratified masses of the mountain chains and
that of the curves of equal magnetic intensity. The coincidence is
perfect in the Ural chain, for there the lines of force tend north and
south; and they do not deviate much from the stratification in the great
plains of European Russia. There is every reason to believe that a
coincidence takes place in the Scandinavian mountains, for a line of
equal magnetic intensity passes parallel to the Norwegian coast. In
Scotland, a line almost coincides with the Grampians; and as it becomes
less northerly before reaching Portugal and Spain, it is there also in
singular coincidence with the sierras on the table-land; the Pyrenees,
however, form an exception to the law. A magnetic line follows the break
of the chain of the Alps with great precision. The intersection of two
upheavals makes these mountains alter their direction from S.W. and N.E.
to E. nearly, and near to that change the magnetic line takes a similar
bend, and coincides with the Caucasus, Taurus, Hindoo-Coosh, Himalaya,
and Chinese mountains, after which it again tends to the north, and
follows the Yablonia chain to Behring’s Straits.
In Africa, the lines of equal magnetic force coincide with the Komri,
and with the lofty sea-coast range which unites the mountains of
Abyssinia with those at the Cape of Good Hope. Throughout North America
the lines of equal force coincide with the Alleghanies, and on the coast
of the Pacific they take the direction of the Rocky Mountains. In
Mexico, the stratified rocks are parallel to the mountains of Anahuac,
which is the same with the direction of the magnetic curves, and a
similar coincidence takes place in the Parima ranges, and in the
coast-chain of Venezuela. The Andes, and the lines of equal magnetic
intensity, are completely discordant, for they cross one another; but
lines of equal magnetic force stretch from the southern promontories of
America and Asia to the mountains of Victoria Land.
There is strong presumptive evidence of the influence of the electric
and magnetic currents on the formation and direction of the mountain
masses and mineral veins, but their slow persevering action on the
ultimate atoms of matter has been placed beyond doubt by the formation
of rubies and other gems, as well as various other mineral substances,
by voltaic electricity.
The existence of electric currents on the surface of the earth has been
deduced from terrestrial magnetism, and from the connection between the
diurnal variations of the magnet and the apparent motion of the sun;
also from the electro-magnetic properties of metalliferous veins, and
from atmospheric electricity, which is continually passing between the
air and the earth.
Dr. Faraday’s brilliant discoveries have changed the received opinions
with regard to the magnetic properties of matter. Although all bodies
are magnetic, they show that it assumes a totally different form in
different substances. For example, if a bar of iron be freely suspended
between the poles of an electro-magnet, or very powerful horse-shoe
magnet, it will be attracted by both poles, and will rest in the
direction between them—that is, on the line of force. But if a bar of
bismuth be suspended in the same manner, it will be repelled by both
poles, and will assume a direction at right angles to that which the
iron took, and thus the same force, whether electric or magnetic,
produces opposite effects upon these two metals. Substances affected
after the manner of iron are magnetic—those affected after the manner of
bismuth are said to be _diamagnetic_. All substances come under one or
other of these two classes: the diamagnetic are infinitely more abundant
than the magnetic; almost all bodies on earth belong to that class. Many
of the metals, acids, oils, sugar, starch, animal matter, flame, and all
the gases, whether light or heavy, have the diamagnetic property less or
more, but oxygen less than any other, and that is the reason why
atmospheric air is the most feebly diamagnetic of all substances at its
natural temperature; for when very hot it becomes more diamagnetic, and
if extremely cold it takes a place among the magnetic class. Important
results with regard to the magnetic state of the globe will undoubtedly
be deduced from this new property of matter, and Dr. Faraday’s
observations on that subject show that he is not without such
anticipations.
“When we consider the magnetic condition of the earth as a whole,
without reference to its possible relation to the sun, and reflect upon
the enormous amount of diamagnetic matter which forms its crust; and
when we remember that magnetic curves of a certain amount of force,
universal in their presence, are passing through these matters, and
keeping them constantly in a state of tension, and therefore of action,
we cannot doubt that some great purpose, of utility to the system and to
us its inhabitants, is fulfilled by it. If the sun have anything to do
with the magnetism of the globe, then it is possible that part of this
effect may be due to the action of the light that comes to us from that
body; and in that view the air seems most strikingly placed round our
sphere, investing it with a transparent diamagnetic, which, therefore,
is permeable to his rays, and, at the same time, moving with great
velocity across them. Such conditions seem to suggest the possibility of
magnetism being thence generated.”