Size and Constitution of the Sun—The Solar Spots—Intensity of the
Sun’s Light and Heat—The Sun’s Atmosphere—His influence on the
Planets—Atmospheres of the Planets—The Moon has none—Lunar
heat—The Differential Telescope—Temperature of Space—Internal
Heat of the Earth—Zone of constant Temperature—Increase of Heat
With the Depth—Central Heat—Volcanic Action—Quantity of Heat
received from the Sun—Isogeothermal Lines—Line of perpetual
Congelation—Climate—Isothermal Lines—Same quantity of Heat
annually received and radiated by the Earth.
THE sun is a globe 880,000 miles in diameter: what his body may be it is
impossible to conjecture, but it seems to be a dark mass surrounded by
an extensive atmosphere at a certain height in which there is a stratum
of luminous clouds which constitutes the photosphere of the sun. Above
it rises the true solar atmosphere, visible as an aureola or corona
during annular and total eclipses, and probably the cause of the
peculiar phenomena in the photographic image of the sun already
mentioned. Through occasional openings in the photosphere or mottled
ocean of flame, the dark nucleus appears like black spots, often of
enormous size. These spots are almost always comprised within a zone of
the sun’s surface, whose breadth measured on a solar meridian does not
extend beyond 30-1/2° on each side of his equator, though they have been
seen at a distance of 39-1/2°. The dark central part of the spots is
surrounded by a succession of obscure cloudy envelopes increasing in
brightness up to a penumbra, sometimes there are three or more shades,
but it requires a good telescope to distinguish the intermediate ones.
The spots gradually increase in size and number from year to year to a
maximum, and then as gradually decrease to a minimum, accomplishing
regular vicissitudes in periods of about eleven years, and are
singularly connected with the cycles of terrestrial magnetism. From
their extensive and rapid changes, there is every reason to believe that
the exterior and incandescent part of the sun is gaseous.
Doubts have arisen as to the uniformity of the quantity of heat emitted
by the sun. Sir William Herschel was the first to suspect that it was
affected by the quantity and magnitude of the spots on his surface;
Professor Secchi has observed that the spots are less hot than the
luminous part; and now Professor Wolf has perceived that the amount of
heat emitted by the sun varies periodically with the spots every 11·11
years, or nearly nine times in a century, beginning at the commencement
of the present one. He has discovered a sub-period in that of the spots,
which no doubt has an effect on the quantity of solar heat. So the
unaccountable vicissitudes in the temperature of different years may
ultimately be found to depend upon the constitution of the sun himself.
The intensity of the sun’s light diminishes from the centre to the
circumference of the solar disc. His direct light has been estimated to
be equal to that of 5563 wax candles of moderate size placed at the
distance of one foot from an object; that of the moon is probably only
equal to the light of one candle at the distance of 12 feet:
consequently the light of the sun is more than three hundred thousand
times greater than that of the moon. According to Professor Secchi’s
experiments at Rome, the heat of the solar image is almost twice as
great at the centre as at the edge. The maximum heat, however, is not in
the centre, but in the solar equator, and the spots are less hot than
the rest of the surface.
The oceans of light and heat probably arising from electric or chemical
processes of immense energy that continually take place at the sun’s
surface (N. 217) are transmitted in undulations by the ethereal medium
in all directions; but notwithstanding the sun’s magnitude and the
inconceivable intensity of light and heat that must exist at his
surface, as the intensity of both diminishes as the square of the
distance increases, his kindly influence can hardly be felt at the
boundaries of our system. In Uranus the sun must be seen like a small
brilliant star not above the hundred and fiftieth part as bright as he
appears to us, but that is 2000 times brighter than our moon, so that he
is really a sun to Uranus, and may impart some degree of warmth. But if
we consider that water would not remain fluid in any part of Mars, even
at his equator, and that, in the temperate zones of the same planet,
even alcohol and quicksilver would freeze, we may form some idea of the
cold that must reign in Uranus and Neptune. The climate of Venus more
nearly resembles that of the earth, though, excepting at her poles, much
too hot for animal and vegetable life such as they exist here, for she
receives seven times as much light and heat as the earth does; but in
Mercury the mean heat from the intensity of the sun’s rays must be above
that of boiling quicksilver, and water would boil even at his poles.
Thus the planets, though kindred with the earth in motion and form, are,
according to our experience, totally unfit for the habitation of such a
being as man, unless indeed their temperature should be modified by
circumstances of which we are not aware, and which may increase or
diminish the sensible heat so as to render them habitable. In our utter
ignorance it may be observed, that the earth, if visible at all from
Neptune, can only be a minute telescopic object; that from the nearest
fixed star the sun must dwindle to a mere point of light; that the whole
solar system would there be hid by a spider’s thread; and that the
starry firmament itself is only the first series of starry systems, the
numbers of which are bounded alone by the imperfection of our
space-penetrating instruments. In this overwhelming majesty of creation,
it seems rash to affirm that the earth alone is inhabited by intelligent
beings, and thus to limit the Omnipotent, who has made nothing in vain.
Several of the planets have extensive and dense atmospheres: according
to Schroëter the atmosphere of Ceres is more than 668 miles high, and
that of Pallas has an elevation of 465 miles, but not a trace of an
atmosphere can be perceived round Vesta. The attraction of the earth has
probably deprived the moon of hers, for the refractive power of the air
at the surface of the earth is at least a thousand times as great as at
the surface of the moon: the lunar atmosphere must therefore be of a
greater degree of rarity than can be produced by our best air-pumps.
This is confirmed by Arago’s observations during a solar eclipse, when
no trace of a lunar atmosphere could be seen. Since then, however, some
indications of air have been perceived in the lunar valleys. In taking
photographic images of the moon and Jupiter at Rome, Professor Secchi
found that the light of the full moon is to that of the quarter moon as
3 to 1. Jupiter gives a photographic image as bright and vigorous as the
brightest part of the moon; but although the light of Jupiter is less
than that of the moon, he is nearly five times farther from the sun; and
as light diminishes as the square of the distance increases, the light
of Jupiter is proportionally greater than that of the moon, consequently
Jupiter’s atmosphere reflects more light than the dark volcanic soil of
the moon; thus Professor Secchi observes photography may in time reveal
the quality of the materials of which the celestial bodies are formed.
The effect of the earth’s atmosphere on lunar heat is remarkable.
Professor Forbes proved that the direct light of the full moon is
incapable of raising a thermometer the one three thousandth part of a
Centigrade degree, at least in England; but at an elevation of 8870 feet
on the Peak of Teneriffe, Mr. Piazzi Smyth found a very sensible heat
from the moon, although she was then 19° south of the equator; so it is
no doubt absorbed by our atmosphere at lower levels.
Some exceedingly interesting experiments might be made by means of a
telescope having a prism attached to its objective extremity, and
furnished with a micrometer, because by it the difference of the
illumination of objects might be determined with extreme accuracy—as for
example, the comparative intensity between the bright and dark parts of
the moon, the comparative intensity of the solar light reflected by the
moon, and the lumière cendré, or the light of the earth reflected on the
moon, whence a comparison might be made between the light of the sun and
that of the earth. Hence also it might be known whether the terrestrial
hemispheres successively visible from the moon are more or less
luminous, according as they contain more land or water, and at the same
time it might be possible to appreciate the more or less cloudy or clear
state of our atmosphere, so that in time we might ultimately find in the
lumière cendré of the moon data upon the mean diaphaneity of different
terrestrial hemispheres which are of different temperatures.
It is found by experience that heat is developed in opaque and
translucent substances by their absorption of solar light, but that the
sun’s rays do not sensibly alter the temperature of perfectly
transparent bodies through which they pass. As the temperature of the
pellucid planetary space can be but little affected by the passage of
the sun’s light and heat, neither can it be sensibly raised by the heat
now radiated from the earth.
Doubtless the radiation of all the bodies in the universe maintains the
ethereal medium at a higher temperature than it would otherwise have,
and must eventually increase it, but by a quantity so evanescent that it
is hardly possible to conceive a time when a change will become
perceptible.
The temperature of space being so low as -239° Fahrenheit, it becomes a
matter of no small interest to ascertain whether the earth may not be
ultimately reduced by radiation to the temperature of the surrounding
medium; what the sources of heat are; and whether they be sufficient to
compensate the loss, and to maintain the earth in a state fit for the
support of animal and vegetable life in time to come. All observations
that have been made under the surface of the ground concur in proving
that there is a stratum at the depth of from 40 to 100 feet throughout
the whole earth where the temperature is invariable at all times and
seasons, and which differs but little from the mean annual temperature
of the country above. According to M. Boussingault, that stratum at the
equator is at the depth of little more than a foot in places sheltered
from the direct rays of the sun; but in our climates it is at a much
greater depth. In the course of more than half a century the temperature
of the earth at the depth of 90 feet, in the cellars of the Observatory
at Paris, has never been above or below 53° of Fahrenheit’s thermometer,
which is only 2° above the mean annual temperature at Paris. This zone,
unaffected by the sun’s rays from above, or by the internal heat from
below, serves as an origin whence the effects of the external heat are
estimated on one side, and the internal temperature of the globe on the
other.
As early as the year 1740 M. Gensanne discovered in the lead-mines of
Giromagny, in the Vosges mountains, three leagues from Béfort, that the
heat of the ground increases with the depth below the zone of constant
temperature. A vast number of observations have been made since that
time, in the mines of Europe and America, by MM. Saussure, Daubuisson,
Humboldt, Cordier, Fox, Reich, and others, which agree, without an
exception, in proving that the temperature of the earth becomes higher
in descending towards its centre. The greatest depth that has been
attained is in the silver-mine of Guanaxato, in Mexico, where M. de
Humboldt found a temperature of 98° at the depth of 285 fathoms, the
mean annual temperature of the country being only 61°. Next to that is
the Dalcoath copper-mine, in Cornwall, where Mr. Fox’s thermometer stood
at 68° in a hole in the rock at the depth of 230 fathoms, and at 82° in
water at the depth of 240 fathoms, the mean annual temperature at the
surface being about 50°. But it is needless to multiply examples, all of
which concur in showing that there is a very great difference between
the temperature in the interior of the earth and at its surface. Mr.
Fox’s observations on the temperature of springs which rise at profound
depths in mines afford the strongest testimony. He found considerable
streams flowing into some of the Cornish mines at the temperature of 80°
or 90°, which is about 30° or 40° above that of the surface, and also
ascertained that nearly 2,000,000 gallons of water are daily pumped from
the bottom of the Poldice mine, which is 176 fathoms deep at 90° or
100°. As this is higher than the warmth of the human body, Mr. Fox
justly observes that it amounts to a proof that the increased
temperature cannot proceed from the persons of the workmen employed in
the mines. Neither can the warmth of mines be attributed to the
condensation of the currents of air which ventilate them. Mr. Fox, whose
opinion is of high authority in these matters, states that, even in the
deepest mines, the condensation of the air would not raise the
temperature more than 5° or 6°; and that, if the heat could be
attributed to this cause, the seasons would sensibly affect the
temperature of mines, which it appears they do not where the depth is
great. Besides, the Cornish mines are generally ventilated by numerous
shafts opening into the galleries from the surface or from a higher
level. The air circulates freely in these, descending in some shafts and
ascending in others. In all cases Mr. Fox found that the upward currents
are of a higher temperature than the descending currents; so much so,
that in winter the moisture is often frozen in the latter to a
considerable depth; the circulation of air, therefore, tends to cool the
mine instead of increasing the heat. Mr. Fox has also removed the
objections arising from the comparatively low temperature of the water
in the shafts of abandoned mines, by showing that observations in them,
from a variety of circumstances which he enumerates, are too discordant
to furnish any conclusion as to the actual heat of the earth. The high
temperature of mines might be attributed to the effects of the fires,
candles, and gunpowder used by the miners, did not a similar increase
obtain in deep wells, and in borings to great depths in search of water,
where no such causes of disturbance occur. In a well dug with a view to
discover salt in the canton of Berne, and long deserted, M. de Saussure
had the most complete evidence of increasing heat. The same has been
confirmed by the temperature of many wells, both in France and England,
especially by the Artesian wells, so named from a peculiar method of
raising water first resorted to in Artois, and since become very
general. An Artesian well consists of a shaft a few inches in diameter,
bored into the earth till a spring is found. To prevent the water being
carried off by the adjacent strata, a tube is let down which exactly
fills the bore from top to bottom, in which the water rises pure to the
surface. It is clear the water could not rise unless it had previously
descended from high ground through the interior of the earth to the
bottom of the well. It partakes of the temperature of the strata through
which it passes, and in every instance has been warmer in proportion to
the depth of the well; but it is evident that the law of increase cannot
be obtained in this manner. Perhaps the most satisfactory experiments on
record are those made by MM. Auguste de la Rive and F. Marcet during the
year 1833, in a boring for water about a league from Geneva, at a place
318 feet above the level of the lake. The depth of the bore was 727
feet, and the diameter only between four and five inches. No spring was
ever found; but the shaft filled with mud, from the moisture of the
ground mixing with the earth displaced in boring, which was peculiarly
favourable for the experiments, as the temperature at each depth may be
considered to be that of the particular stratum. In this case, where
none of the ordinary causes of disturbance could exist, and where every
precaution was employed by scientific and experienced observers, the
temperature was found to increase regularly and uniformly with the depth
at the rate of about 1° of Fahrenheit for every 52 feet. Professor Reich
of Freyberg has found that the mean of a great number of observations
both in mines and wells is 1° of Fahrenheit for every 55 feet of depth;
and from M. Arago’s observations in the Grenelle Artesian well at Paris,
the increase is 1° of Fahrenheit for every 45 feet. Though there can be
no doubt as to the increase of temperature in penetrating the crust of
the earth, there is still much uncertainty as to the law of increase,
which varies with the nature of the soil and other local circumstances;
but, on an average, it has been estimated at the rate of 1° for every 50
or 60 feet, which corresponds with the observations of MM. Marcet and De
la Rive. In consequence of the rapid increase of internal heat, thermal
springs, or such as are independent of volcanic action, rising from a
great depth, must necessarily be very rare and of a high temperature;
and it is actually found that none are so low as 68° of Fahrenheit; that
of Chaudes Aigues, in Auvergne, is about 136°. In many places warm water
from Artesian wells will probably come into use for domestic purposes,
and it is even now employed in manufactories near Stutgardt, in Alsace,
&c.
It is hardly to be expected that at present any information with regard
to the actual internal temperature of the earth should be obtained from
that of the ocean, on account of the mobility of fluids, by which the
colder masses sink downwards, while those that are warmer rise to the
surface. Nevertheless, it may be stated that the temperature of the sea
decreases with the depth between the tropics; while, on the contrary,
all our northern navigators found that the temperature increases with
the depth in the polar seas. The change takes place about the 70th
parallel of latitude. Some ages hence, however, it may be known whether
the earth has arrived at a permanent state as to heat, by comparing
secular observations of the temperature of the ocean if made at a great
distance from the land.
Should the earth’s temperature increase at the rate of 1° for every 50
feet, it is clear that at the depth of 200 miles the hardest substances
must be in a state of fusion, and our globe must in that case either be
encompassed by a stratum of melted lava at that depth, or it must be a
ball of liquid fire 7600 miles in diameter, enclosed in a thin coating
of solid matter; for 200 miles are nothing when compared with the size
of the earth. No doubt the form of the earth, as determined by the
pendulum and arcs of the meridian, as well as by the motions of the
moon, indicates original fluidity and subsequent consolidation and
reduction of temperature by radiation; but whether the law of increasing
temperature is uniform at still greater depths than those already
attained by man, it is impossible to say. At all events, internal
fluidity is not inconsistent with the present state of the earth’s
surface, since earthy matter is as bad a conductor of heat as lava,
which often retains its heat at a very little depth for years after its
surface is cool. Whatever the radiation of the earth might have been in
former times, certain it is that it goes on very slowly in our days; for
M. Fourier has computed that the central heat is decreasing from
radiation by only about the 1/30000th part of a degree in a century. If
so, there can be no doubt that it will ultimately be dissipated; but as
far as regards animal and vegetable life, it is of very little
consequence whether the centre of our planet be liquid fire or ice,
since its condition in either case could have no sensible effect on the
climate at its surface. The internal fire does not even impart heat
enough to melt the snow at the poles, though nearer to the centre than
any other part of the globe.
The immense extent of active volcanic fire is one of the causes of heat
which must not be overlooked.
The range of the Andes from Chile to the north of Mexico, probably from
Cape Horn to Behring Straits, is one vast district of igneous action,
including the Caribbean and the West Indian Islands on one hand; and
stretching quite across the Pacific Ocean, through the Polynesian
Archipelago, the New Hebrides, the Georgian and Friendly Islands, on the
other. Another chain begins with the Aleutian Islands, extends to
Kamtschatka, and from thence passes through the Kurile, Japanese, and
Philippine Islands, to the Moluccas, whence it spreads with terrific
violence through the Indian Archipelago, even to the Bay of Bengal.
Volcanic action may again be followed from the entrance of the Persian
Gulf to Madagascar, Bourbon, the Canaries, and Azores. Thence a
continuous igneous region extends through about 1000 geographical miles
to the Caspian Sea, including the Mediterranean, and extending north and
south between the 35th and 40th parallels of latitude; and in central
Asia a volcanic region occupies 2500 square geographical miles. The
volcanic fires are developed in Iceland in tremendous force; and the
antarctic land discovered by Sir James Ross is an igneous formation of
the boldest structure, where a volcano in high activity rises 12,000
feet above the perpetual ice of these polar deserts, and within 19-1/2°
of the south pole. Throughout this vast portion of the world the
subterraneous fire is often intensely active, producing such violent
earthquakes and eruptions that their effects, accumulated during
millions of years, may account for many of the great geological changes
of igneous origin that have already taken place in the earth, and may
occasion others not less remarkable, should time—that essential element
in the vicissitudes of the globe—be granted, and their energy last.
Sir Charles Lyell, who has shown the power of existing causes with great
ingenuity, estimates that on an average twenty eruptions take place
annually in different parts of the world; and many must occur or have
happened, even on the most extensive and awful scale, among people
equally incapable of estimating their effects and of recording them. We
should never have known the extent of the fearful eruption which took
place in the island of Sumbawa, in 1815, but for the accident of Sir
Stamford Raffles having been governor of Java at the time. It began on
the 5th of April, and did not entirely cease till July. The ground was
shaken through an area of 1000 miles in circumference; the tremors were
felt in Java, the Moluccas, a great part of Celebes, Sumatra, and
Borneo. The detonations were heard in Sumatra, at the distance of 970
geographical miles in a straight line; and at Ternate, 720 miles in the
opposite direction. The most dreadful whirlwinds carried men and cattle
into the air; and with the exception of 26 persons, the whole population
of the island perished to the amount of 12,000. Ashes were carried 300
miles to Java in such quantities that the darkness during the day was
more profound than ever had been witnessed in the most obscure night.
The face of the country was changed by the streams of lava, and by the
upheaving and sinking of the soil. The town of Tomboro was submerged,
and water stood to the depth of 18 feet in places which had been dry
land. Ships grounded where they had previously anchored, and others
could hardly penetrate the mass of cinders which floated on the surface
of the sea for several miles to the depth of two feet. A catastrophe
similar to this, though of less magnitude, took place in the island of
Bali in 1808, which was not heard of in Europe till years afterwards.
The eruption of Coseguina in the Bay of Fonseca, which began on the 19th
of January, 1835, and lasted many days, was even more dreadful and
extensive in its effects than that of Sumbawa. The ashes during this
eruption were carried by the upper current of the atmosphere as far
north as Chiassa, which is upwards of 400 leagues to the windward of
that volcano. Many volcanoes supposed to be extinct have all at once
burst out with inconceivable violence. Witness Vesuvius, on historical
record; and the volcano in the island of St. Vincent in our own days,
whose crater was lined with large trees, and which had not been active
in the memory of man. Vast tracts are of volcanic origin where volcanoes
have ceased to exist for ages. Whence it may be inferred that in some
places the subterraneous fires are in the highest state of activity, in
some they are inert, and in others they appear to be extinct. Yet there
are few countries that are not subject to earthquakes of greater or less
intensity; the tremors are propagated like a sonorous undulation to such
distances that it is impossible to say in what point they originate. In
some recent instances their power must have been tremendous. In South
America, so lately as 1822, an area of 100,000 square miles, which is
equal in extent to the half of France, was raised several feet above its
present level—a most able account of which is given in the ‘Transactions
of the Geological Society,’ by an esteemed friend of the author’s, the
late Mrs. Graham, who was present during the whole time of that
formidable earthquake, which recurred at short intervals for more than
two months, and who possessed talents to appreciate, and had
opportunities of observing, its effects under the most favourable
circumstances at Valparaiso, and for miles along the coast where it was
most intense. A considerable elevation of the land has again taken place
along the coast of Chile, in consequence of the violent earthquake which
happened on the 20th of February, 1835. In 1819 a ridge of land
stretching for 50 miles across the delta of the Indus, 16 feet broad,
was raised 10 feet above the plain. The reader is referred to Sir
Charles Lyell’s excellent ‘Principles of Geology,’ already mentioned,
for most interesting details of the phenomena and extensive effects of
volcanoes and earthquakes, too numerous to find a place here. It may
however be mentioned that innumerable earthquakes are from time to time
shaking the solid crust of the globe, and carrying destruction to
distant regions, progressively though slowly accomplishing the great
work of change. A most disastrous instance took place on the 15th of
December, 1857, in the Neapolitan provinces of La Basilicata and
Principato Citeriore, where the destruction was extensive and terrible;
the number of victims, according to the official accounts, being
returned at upwards of ten thousand. These terrible engines of ruin,
fitful and uncertain as they may seem, must, like all durable phenomena,
have a law which may in time be discovered by long-continued and
accurate observations.
The shell of volcanic fire that girds the globe at a small depth below
our feet has been attributed to different causes. By some it is supposed
to originate in an ocean of incandescent matter, still existing in the
central abyss of the earth. Some conceive it to be superficial, and due
to chemical action, in strata at no very great depth when compared with
the size of the globe. The more so as matter on a most extensive scale
is passing from old into new combinations, which, if rapidly effected,
are capable of producing the most intense heat. According to others,
electricity, which is so universally diffused in all its forms
throughout the earth, if not the immediate cause of the volcanic
phenomena, at least determines the chemical affinities that produce
them. It is clear that a subject so involved in mystery must give rise
to much speculation, in which every hypothesis is attended with
difficulties that observation alone can remove.
But the views of Mr. Babbage and Sir John Herschel on the general cause
of volcanic action, and the changes in the equilibrium of the internal
heat of the globe, accord more with the laws of mechanics and radiant
heat than any that have been proposed. The theory of these distinguished
philosophers, formed independently of each other, is equally consistent
with observed phenomena, whether the earth be a solid crust encompassing
a nucleus of liquid lava, or that there is merely a vast reservoir or
stratum of melted matter at a moderate depth below the superficial
crust. The author is indebted to the kindness of Sir Charles Lyell for
the perusal of a most interesting letter from Sir John Herschel, in
which he states his views on the subject.
Supposing that the globe is merely a solid crust, resting upon fluid or
semi-fluid matter, whether extending to the centre or not, the transfer
of pressure from one part of its surface to another by the degradation
of existing continents, and the formation of new ones, would be
sufficient to subvert the equilibrium of heat in the interior, and
occasion volcanic eruptions. For, since the internal heat of the earth
is transmitted outwards by radiation, an accession of new matter on any
part of the surface, like an addition of clothing, by keeping it in,
would raise the temperature of the strata below, and in the course of
ages would even reduce those at a great depth to a state of fusion. Some
of the substances might be converted into gases; and should the
accumulation of new matter take place at the bottom of the sea, as is
generally the case, this lava would be mixed with water in a state of
ignition in consequence of the enormous pressure of the ocean, and of
the newly superimposed matter which would prevent it from expanding into
steam. Now Sir Charles Lyell has shown, with his usual talent, that the
quantity of matter carried down by rivers from the surface of the
continents is comparatively trifling, and that the great transfer to the
bottom of the ocean is produced at the coast-line by the action of the
sea; hence, says Sir John Herschel, “the greatest accumulation of local
pressure is in the central area of the deep sea, while the greatest
local relief takes place along the abraded coast-lines. Here then should
occur the chief volcanic vents.” As the crust of the earth is much
weaker on the coasts than elsewhere, it is more easily ruptured, and, as
Mr. Babbage observes, immense rents might be produced there by its
contraction in cooling down after being deprived of a portion of its
original thickness. The pressure on the bottom of the ocean would force
a column of lava mixed with ignited water and gas to rise through an
opening thus formed, and, says Sir John Herschel, “when the column
attains such a height that the ignited water can become steam, the joint
specific gravity of the column is suddenly diminished, and up comes a
jet of mixed steam and lava, till so much has escaped that the matter
deposited at the bottom of the ocean takes a fresh bearing, when the
evacuation ceases and the crack becomes sealed up.”
This theory perfectly accords with the phenomena of nature, since there
are very few active volcanoes at a distance from the sea, and the
exceptions that do occur are generally near lakes, or they are connected
with volcanoes on the maritime coasts. Many break out even in the bottom
of the ocean, probably owing to some of the supports of the superficial
crust giving way, so that the steam and lava are forced up through the
fissures.
Finally, Mr. Babbage observes that, “in consequence of changes
continually going on, by the destruction of forests, the filling up of
seas, the wearing down of elevated lands, the heat radiated from the
earth’s surface varies considerably at different periods. In consequence
of this variation, and also in consequence of the covering up of the
bottom of the sea by the detritus of the land, the surfaces of equal
temperature within the earth are continually changing their form, and
exposing thick beds near the exterior to alterations of temperature. The
expansion and contraction of these strata may form rents and veins,
produce earthquakes, determine volcanic eruptions, elevate continents,
and, possibly, raise mountain chains.”
The numerous vents for the internal heat formed by volcanoes, hot
springs, and the emission of steam, so frequent in volcanic regions, no
doubt maintain the tranquillity of the interior fluid mass, which seems
to be perfectly inert unless when put in motion by unequal pressure.
But, to whatever cause the increasing heat of the earth and the
subterranean fires may ultimately be referred, it is certain that,
except in some local instances, they have no sensible effect on the
temperature of its surface. It may therefore be concluded that the heat
of the earth, above the zone of uniform temperature, is entirely owing
to the sun.
The power of the solar rays depends much upon the manner in which they
fall, as we readily perceive from the different climates on our globe.
Although the sun is about three millions of miles nearer to the earth in
winter than in summer, his rays strike the atmosphere in the northern
hemisphere so obliquely that it absorbs a much greater quantity of heat
than when they are more direct (N. 217). Indeed it is so great that,
when the sun has an altitude of 30°, one half of his heat is absorbed by
the atmosphere, and it increases very rapidly as he sinks towards the
horizon. However, that heat is not lost: it is most beneficial to the
earth, being really the heat which supplies the greatest part of that
which is radiated into space during the absence of the sun. Professor
Dove has shown, by taking at all seasons the mean of the temperatures of
points on the earth’s surface diametrically opposite to each other, that
the average temperature of the whole earth’s surface in June, when we
are farthest from the sun, considerably exceeds that in December, when
we are nearest to him, owing to the excess of water in the southern
hemisphere, and that of land in the northern, which gives a general
insular climate to the former, and a continental climate to the latter.
The observations of the north polar navigators, and those of Sir John
Herschel at the Cape of Good Hope, show that the direct heating
influence of the solar rays is greatest at the equator, and that it
diminishes gradually as the latitude increases. At the equator the
maximum is 48-3/4°, while in Europe it has never exceeded 29-1/2°.
M. Pouillet has estimated with singular ingenuity, from a series of
observations made by himself, that the whole quantity of heat which the
earth receives annually from the sun is such as would be sufficient to
melt a stratum of ice covering the whole globe 46 feet deep. Part of
this heat is radiated back into space; but by far the greater part
descends into the earth during the summer, towards the zone of uniform
temperature, whence it returns to the surface in the course of the
winter, and tempers the cold of the ground and the atmosphere in its
passage to the ethereal regions, where it is lost, or rather where it
combines with the radiation from the other bodies of the universe in
maintaining the temperature of space. The sun’s power being greatest
between the tropics, the heat sinks deeper there than elsewhere, and the
depth gradually diminishes towards the poles; but the heat is also
transmitted laterally from the warmer to the colder strata north and
south of the equator, and aids in tempering the severity of the polar
regions.
The mean heat of the earth, above the stratum of constant temperature,
is determined from that of springs; and, if the spring be on elevated
ground, the temperature is reduced by computation to what it would be at
the level of the sea, assuming that the heat of the soil varies
according to the same law as the heat of the atmosphere, which is about
1° of Fahrenheit’s thermometer for every 333·7 feet. From a comparison
of the temperature of numerous springs with that of the air, Sir David
Brewster concludes that there is a particular line passing nearly
through Berlin, at which the temperature of springs and that of the
atmosphere coincide; that in approaching the arctic circle the
temperature of springs is always higher than that of the air, while,
proceeding towards the equator, it is lower.
Since the warmth of the superficial strata of the earth decreases from
the equator to the poles, there are many places in both hemispheres
where the ground has the same mean temperature. If lines were drawn
through all those points in the upper strata of the globe which have the
same mean annual temperature, they would be nearly parallel to the
equator between the tropics, and would become more and more irregular
and sinuous towards the poles. These are called isogeothermal lines. A
variety of local circumstances disturb their parallelism, even between
the tropics.
The temperature of the ground at the equator is lower on the coasts and
islands than in the interior of continents; the warmest part is in the
interior of Africa; but it is obviously affected by the nature of the
soil, especially if it be volcanic.
Much has been done to ascertain the manner in which heat is distributed
over the surface of our planet, and the variations of climate, which, in
a general view, mean every change of the atmosphere, such as of
temperature, humidity, variations of barometric pressure, purity of air,
the serenity of the heavens, the effects of winds, and electric tension.
Temperature depends upon the property which all bodies possess, more or
less, of perpetually absorbing and emitting or radiating heat. When the
interchange is equal, the temperature of a body remains the same; but,
when the radiation exceeds the absorption, it becomes colder, and _vice
versâ_. In order to determine the distribution of heat over the surface
of the earth, it is necessary to find a standard by which the
temperature in different latitudes may be compared. For that purpose it
is requisite to ascertain, by experiment, the mean temperature of the
day, of the month, and of the year, at as many places as possible
throughout the earth. The annual average temperature may be found by
adding the mean temperatures of all the months in the year, and dividing
the sum by twelve. The average of ten or fifteen years will give it
approximately; for, although the temperature in any place maybe subject
to very great variations, yet it never deviates more than a few degrees
from its mean state, which consequently offers a good standard of
comparison. As a standard, however, much greater accuracy is required.
If climate depended solely upon the heat of the sun, all places having
the same latitude would have the same mean annual temperature. The
motion of the sun in the ecliptic, indeed, occasions perpetual
variations in the length of the day, and in the direction of the rays
with regard to the earth; yet, as the cause is periodic, the mean annual
temperature from the sun’s motion alone must be constant in each
parallel of latitude; for it is evident that the accumulation of heat in
the long days of summer, which is but little diminished by radiation
during the short nights, is balanced by the small quantity of heat
received during the short days in winter, and its radiation in the long,
frosty, and clear nights. In fact, if the globe were everywhere on a
level with the surface of the sea, and of uniform substance, so as to
absorb and radiate heat equally, the mean heat of the sun would be
regularly distributed over its surface in zones of equal annual
temperature parallel to the equator, from which it would decrease to
each pole as the square of the cosine of the latitude; and its quantity
would only depend upon the altitude of the sun and atmospheric currents.
The distribution of heat, however, in the same parallel, is very
irregular in all latitudes except between the tropics, where the
isothermal lines, or the lines passing through places of equal mean
annual temperature, are more nearly parallel to the equator. The causes
of disturbance are very numerous; but such as have the greatest
influence, according to M. de Humboldt, to whom we are indebted for the
greater part of what is known on the subject, are the elevation of the
continents, the distribution of land and water over the surface of the
globe exposing different absorbing and radiating powers; the variations
in the surface of the land, as forests, sandy deserts, verdant plains,
rocks, &c.; mountain-chains covered with masses of snow, which diminish
the temperature; the reverberation of the sun’s rays in the valleys,
which increases it; and the interchange of currents, both of air and
water, which mitigates the rigour of climates; the warm currents from
the equator softening the severity of the polar frosts, and the cold
currents from the poles tempering the intense heat of the equatorial
regions. To these may be added cultivation, though its influence extends
over but a small portion of the globe, only a fourth part of the land
being inhabited.
Temperature decreases with the height above the level of the sea, as
well as with the latitude. The air in the higher regions of the
atmosphere is much cooler than that below, because the warm air expands
as it rises, by which its capacity for heat is increased, a great
proportion becomes latent or absorbed, and less of it sensible. A
portion of air at the surface of the earth whose temperature is 70° of
Fahrenheit, if carried to the height of two miles and a half, would
expand so much that its temperature would be reduced 50°; and in the
ethereal regions the temperature is 239° below the zero point of
Fahrenheit.
The height at which snow lies perpetually decreases from the equator to
the poles, and is higher in summer than in winter; but it varies from
many circumstances. Snow rarely falls when the cold is intense and the
atmosphere dry. Extensive forests produce moisture by their evaporation;
and high table-lands, on the contrary, dry and warm the air, because the
air at great elevations is too rare to absorb much of the sun’s heat. In
the Cordilleras of the Andes, plains of only twenty-five square leagues
from their extent raise the temperature as much as 3° or 4° above what
is found at the same altitude on the rapid declivity of a mountain,
consequently the line of perpetual snow varies according as one or other
of these causes prevails. Aspect in general has also a great influence;
yet the line of perpetual snow is much higher on the northern than on
the southern side of the Himalaya, partly because the air is nearly
deprived of its moisture by precipitation before it arrives at the
northern side of the mountains. On the whole, it appears that the mean
height between the tropics at which the snow lies perpetually is about
15,207 feet above the level of the sea; whereas snow does not cover the
ground continually at the level of the ocean till near the north pole.
In the southern hemisphere, however, the cold is greater than in the
northern. In Sandwich Land, between the 54th and 58th degrees of
latitude, perpetual snow and ice extend to the sea-level; and in the
island of S. Georgia, in the 53rd degree of south latitude, which
corresponds with the latitude of the central counties of England,
perpetual snow descends even to the level of the ocean. It has been
shown that this excess of cold in the southern hemisphere cannot be
attributed to the winter being longer than ours by 7-3/4 days. It is
probably owing to the open sea surrounding the south pole, which permits
the icebergs to descend to a lower latitude by 10° than they do in the
northern hemisphere, on account of the numerous obstructions opposed to
them by the islands and continents about the north pole. Icebergs from
the Arctic seas seldom float farther to the south than the Azores;
whereas those that come from the south pole descend to as low a latitude
as that of the Cape of Good Hope.
The influence of mountain-chains does not wholly depend upon the line of
perpetual congelation. They attract and condense the vapours floating in
the air, and send them down in torrents of rain. They radiate heat into
the atmosphere at a lower elevation, and increase the temperature of the
valleys by the reflection of the sun’s rays, and by the shelter they
afford against prevailing winds. But, on the contrary, one of the most
general and powerful causes of cold arising from the vicinity of
mountains is the freezing currents of wind which rush from their lofty
peaks along the rapid declivities, chilling the surrounding valleys:
such is the cutting north wind called the bise in Switzerland.
Next to elevation, the difference in the radiating and absorbing powers
of the sea and land has the greatest influence in disturbing the regular
distribution of heat. The extent of the dry land is not above the fourth
part of that of the ocean; so that the general temperature of the
atmosphere, regarded as the result of the partial temperatures of the
whole surface of the globe, is most powerfully modified by the sea.
Besides, the ocean acts more uniformly on the atmosphere than the
diversified surface of the solid mass does, both by the equality of its
curvature and its homogeneity. In opaque substances the accumulation of
heat is confined to the stratum nearest the surface. The seas become
less heated at their surface than the land, because the solar rays,
before being extinguished, penetrate the transparent liquid to a greater
depth and in greater numbers than in the opaque masses. On the other
hand, water has a considerable radiating power, which, together with
evaporation, would reduce the surface of the ocean to a very low
temperature, if the cold particles did not sink to the bottom on account
of their superior density. The seas preserve a considerable portion of
the heat they receive in summer, and from their saltness do not freeze
so soon as fresh water. So that, in consequence of all these
circumstances, the ocean is not subject to such variations of heat as
the land, and, by imparting its temperature to the winds and by its
currents, it diminishes the rigour of climate on the coasts and in the
islands, which are never subject to such extremes of heat and cold as
are experienced in the interior of continents, though they are liable to
fogs and rain from the evaporation of the adjacent seas. On each side of
the equator to the 48th degree of latitude, the surface of the ocean is
in general warmer than the air above it. The mean of the difference of
the temperature at noon and midnight is about 1°·37, the greatest
deviation never exceeding from 0°·36 to 2°·16, which is much cooler than
the air over the land.
On land the temperature depends upon the nature of the soil and its
products, its habitual moisture or dryness. From the eastern extremity
of the Sahara desert quite across Africa, the soil is almost entirely
barren sand; and the Sahara desert itself extends over an area of
194,000 square leagues, equal to twice the area of the Mediterranean
Sea, and raises the temperature of the air by radiation from 90° to
100°, which must have a most extensive influence. On the contrary,
vegetation cools the air by evaporation and the apparent radiation of
cold from the leaves of plants, because they absorb more caloric than
they give out. The graminiferous plains of South America cover an extent
ten times greater than France, occupying no less than about 50,000
square leagues, which is more than the whole chain of the Andes, and all
the scattered mountain-groups of Brazil. These, together with the plains
of North America and the steppes of Europe and Asia, must have an
extensive cooling effect on the atmosphere if it be considered that in
calm and serene nights they cause the thermometer to descend 12° or 14°,
and that in the meadows and heaths in England the absorption of heat by
the grass is sufficient to cause the temperature to sink to the point of
congelation during the night for ten months in the year. Forests cool
the air also by shading the ground from the rays of the sun, and by
evaporation from the boughs. Hales found that the leaves of a single
plant of helianthus three feet high exposed nearly forty feet of
surface; and, if it be considered that the woody regions of the river
Amazons, and the higher part of the Orinoco, occupy an area of 260,000
square leagues, some idea may be formed of the torrents of vapour which
rise from the leaves of the forests all over the globe. However, the
frigorific effects of their evaporation are counteracted in some measure
by the perfect calm which reigns in the tropical wildernesses. The
innumerable rivers, lakes, pools, and marshes interspersed through the
continents absorb caloric, and cool the air by evaporation; but, on
account of the chilled and dense particles sinking to the bottom, deep
water diminishes the cold of winter, so long as ice is not formed.
In consequence of the difference in the radiating and absorbing powers
of the sea and land, their configuration greatly modifies the
distribution of heat over the surface of the globe. Under the equator
only one-sixth part of the circumference is land; and the superficial
extent of land in the northern and southern hemispheres is in the
proportion of three to one. The effect of this unequal division is
greater in the temperate than in the torrid zones, for the area of land
in the northern temperate zone is to that in the southern as thirteen to
one, whereas the proportion of land between the equator and each tropic
is as five to four. It is a curious fact, noticed by Mr. Gardner, that
only one twenty-seventh part of the land of the globe has land
diametrically opposite to it. This disproportionate arrangement of the
solid part of the globe has a powerful influence on the temperature of
the southern hemisphere. But, besides these greater modifications, the
peninsulas, promontories, and capes, running out into the ocean,
together with bays and internal seas, all affect temperature. To these
may be added the position of continental masses with regard to the
cardinal points. All these diversities of land and water influence
temperature by the agency of the winds. On this account the temperature
is lower on the eastern coasts both of the New and Old World than on the
western; for, considering Europe as an island, the general temperature
is mild in proportion as the aspect is open to the Atlantic Ocean, the
superficial temperature of which, as far north as the 45th and 50th
degrees of latitude, does not fall below 48° or 51° of Fahrenheit, even
in the middle of winter. On the contrary, the cold of Russia arises from
its exposure to the northern and eastern winds. But the European part of
that empire has a less rigorous climate than the Asiatic, because it
does not extend to so high a latitude.
The interposition of the atmosphere modifies all the effects of the
sun’s heat. The earth communicates its temperature so slowly, that M.
Arago has occasionally found as much as from 14° to 18° of difference
between the heat of the soil and that of the air two or three inches
above it.
The circumstances which have been enumerated, and many more, concur in
disturbing the regular distribution of heat over the globe, and occasion
numberless local irregularities. Nevertheless the mean annual
temperature becomes gradually lower from the equator to the poles. But
the diminution of mean heat is most rapid between the 40th and 45th
degrees of latitude both in Europe and America, which accords perfectly
with theory; whence it appears that the variation in the square of the
cosine of the latitude (N. 127), which expresses the law of the change
of temperature, is a maximum towards the 45th degree of latitude. The
mean annual temperature under the equator in America is about 81-1/2° of
Fahrenheit: in Africa it is said to be nearly 83°. The difference
probably arises from the winds of Siberia and Canada, whose chilly
influence is sensibly felt in Asia and America, even within 18° of the
equator.
The isothermal lines are nearly parallel to the equator, till about the
22nd degree of latitude on each side of it, where they begin to lose
their parallelism, and continue to do so more and more as the latitude
augments. With regard to the northern hemisphere, the isothermal line of
59° of Fahrenheit passes between Rome and Florence in latitude 43°; and
near Raleigh in North Carolina, latitude 36°: that of 50° of equal
annual temperature runs through the Netherlands, latitude 51°; and near
Boston in the United States, latitude 42-1/2°: that of 41° passes near
Stockholm, latitude 59-1/2°; and St. George’s Bay, Newfoundland,
latitude 48°: and lastly, the line of 32°, the freezing point of water,
passes between Ulea in Lapland, latitude 66°, and Table Bay, on the
coast of Labrador, latitude 54°.
Thus it appears that the isothermal lines, which are nearly parallel to
the equator for about 22°, afterwards deviate more and more. From
observations made during the numerous voyages in the Arctic Seas, it is
found that the isothermal lines of Europe and America entirely separate
in the high latitudes, and surround two poles of maximum cold: one, in
79° N. lat. and 120° E. long., has a mean temperature of 2° Fahrenheit;
and the other, whose temperature was determined by Sir David Brewster to
be 3-1/2° Fahrenheit, from the observations of Sir Edward Parry is near
Melville Island. The pole of the earth’s rotation, whose mean
temperature is probably not below 15° Fahrenheit, is nearly midway
between the two; and the line which joins these points of maximum cold
is almost coincident with that diameter of the polar basin which bisects
it, and passes through its two great outlets into the Pacific and
Atlantic Oceans, a most remarkable feature, and strongly indicative of
the absence of land, and of the prevalence of a materially milder
climate in the polar Ocean, probably not under 15° Fahrenheit.[12] It is
believed that two corresponding poles of maximum cold exist in the
southern hemisphere, though observations are wanting to trace the course
of the southern isothermal lines with the same accuracy as the northern.
The isothermal lines, or such as pass through places where the mean
annual temperature of the air is the same, do not always coincide with
the isogeothermal lines, which are those passing through places where
the mean temperature of the ground is the same. Sir David Brewster, in
discussing this subject, finds that the isogeothermal lines are always
parallel to the isothermal lines; consequently the same general formula
will serve to determine both, since the difference is a constant
quantity obtained by observation, and depending upon the distance of the
place from the neutral isothermal line. These results are confirmed by
the observations of M. Kupffer of Kasan during his excursions to the
north, which show that the European and the American portions of the
isogeothermal line of 32° of Fahrenheit actually separate, and go round
the two poles of maximum cold. This traveller remarked, also, that the
temperature both of the air and of the soil decreases most rapidly
towards the 45th degree of latitude.
It is evident that places may have the same mean annual temperature, and
yet differ materially in climate. In one, the winters may be mild and
the summers cool; whereas another may experience the extremes of heat
and cold. Lines passing through places having the same mean summer or
winter temperature are neither parallel to the isothermal, the
geothermal lines, nor to one another, and they differ still more from
the parallels of latitude. In Europe, the latitude of two places which
have the same annual heat never differs more than 8° or 9°; whereas the
difference in the latitude of those having the same mean winter
temperature is sometimes as much as 18° or 19°. At Kasan, in the
interior of Russia, in latitude 55°·48, nearly the same with that of
Edinburgh, the mean annual temperature is about 37°·6; at Edinburgh it
is 47°·84. At Kasan the mean summer temperature is 64°·84, and that of
winter 2°·12; whereas at Edinburgh the mean summer temperature is
58°·28, and that of winter 38°·66. Whence it appears that the difference
of winter temperature is much greater than that of summer. At Quebec the
summers are as warm as those in Paris, and grapes sometimes ripen in the
open air: whereas the winters are as severe as in Petersburgh; the snow
lies five feet deep for several months, wheel carriages cannot be used,
the ice is too hard for skating, travelling is performed in sledges, and
frequently on the ice of the river St. Lawrence. The cold at Melville
Island on the 15th of January, 1820, according to Sir Edward Parry, was
55° below the zero of Fahrenheit’s thermometer; and when Dr. Kane was on
the northern coast of Greenland it was 70° below that point; yet the
summer heat during the day in these high latitudes is insupportable.
Observations tend to prove that all the climates of the earth are
stable, and that their vicissitudes are only periods or oscillations of
more or less extent, which vanish in the mean annual temperature of a
sufficient number of years. This constancy of the mean annual
temperature of the different places on the surface of the globe shows
that the same quantity of heat which is annually received by the earth
is annually radiated into space; and that would be the case even if the
quantity of heat emitted by the sun should vary with his spots, for, if
more were received, more would be radiated. Nevertheless, a variety of
causes may disturb the climate of a place; cultivation may make it
warmer; but it is at the expense of some other place, which becomes
colder in the same proportion. There may be a succession of cold summers
and mild winters, but in some other country the contrary takes place to
effect the compensation; wind, rain, snow, fog, and the other meteoric
phenomena, are the ministers employed to accomplish the changes. The
distribution of heat may vary with a variety of circumstances; but the
absolute quantity lost and gained by the whole earth in the course of a
year, if not invariably the same, is at least periodical.