Atlantic Monthly Vol. 6, No. 33, July, 1860 by Various
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Various >> Atlantic Monthly Vol. 6, No. 33, July, 1860
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19 THE
ATLANTIC MONTHLY.
A MAGAZINE OF LITERATURE, ART, AND POLITICS.
* * * * *
VOL. VI.--JULY, 1860.--NO. XXXIII.
* * * * *
METEOROLOGY.
A GLANCE AT THE SCIENCE.
The purpose of this article is to present, in a brief and
simple manner, the leading principles on which the science of
Meteorology is founded,--rather, however, in the spirit of an
inquirer than of a teacher. For, notwithstanding the rapid
progress it has made within the last thirty years, it is far
from having the authority of an exact science; many of its
phenomena are as yet inexplicable, and many differences of
opinion among the learned remain unreconciled on points at first
sight apparently easy to be settled.
Meteorology has advanced very far beyond its original limits.
Spherical vapor and atmospheric space give but a faint idea of its
range. We find it a leading science in Physics, and having intimate
relations with heat, light, electricity, magnetism, winds, water,
vegetation, geological changes, optical effects, pneumatics,
geography,--and with climate, controlling the pursuits and
affecting the character of the human race. It is so intimately
blended, indeed, with the other matters here named, as scarcely to
have any positive boundary of its own; and its vista seems ever
lengthening, as we proceed.
Without dwelling upon the numerous consequences which flow from
meteorological influences, let us see what is properly included under
the subject of Meteorology. And first, of the Atmosphere.
This is a gaseous, vapor-bearing, elastic fluid, surrounding the
earth. Its volume is estimated at 1/29th, and its weight at about
43/1000ths, that of the globe. It is composed of 21 parts in weight
of Oxygen and 77 of Nitrogen, with a little Carbonic Acid,
Aqueous Vapor, and a trace of Carburetted Hydrogen. There are
numerous well-known calculations of the proportions of the various
constituents of the atmosphere, which we owe to Priestley, Dalton,
Black, Cavendish, Liebig, and others; but that given by Professor
Ansted is sufficiently simple and intelligible. In 10 volumes or
parts of it, he gives to
Oxygen, the great supporter of life 2.100
Nitrogen, (not condensible under 50
atmospheres, and not respirable or
combustible,) 7.750
Aqueous Vapor .l42
Carbonic Acid .004
Carburetted Hydrogen .004
______
10.000
and he adds a trace of Ammoniacal Vapor. It is _usual_ to state the
proportions of air as being 1 Oxygen to 4 Nitrogen.
It is a curious fact, that, while there are six varieties of compounds
of nitrogen and oxygen, but one of these is fitted to sustain life,
and that is our atmosphere.
It is well enough to note, that, when we use the word volume or
measure, in speaking of the atmosphere or any gaseous body, we adopt
the theory of Gay-Lussac, who discovered that gases unite with each
other in definite proportions whenever they enter into combination.
This theory led to important results; for by knowing the elements of a
compound gas, we easily determine its specific gravity.
It has been attempted to apply the principle to organic bodies;
but it has not yet been carried to a full and satisfactory
conclusion. It may be noticed, too, that Dalton affirmed that
simple substances unite with each other in definite weights
to form compound substances, thus supporting the idea of
Lussac. These discoveries were made about the same time,
Dalton having the credit of originating them. Various modifications
of the principle have been from time to time presented to public
attention.
Whether the constituents of the atmosphere are chemically or
mechanically combined,--one of the things about which the learned are
not fully agreed,--it is found to be chemically the same in its
constituents, all over the world, whether collected on mountains or on
plains, on the sea or on the land, whether obtained by aeronauts miles
above the earth or by miners in their deepest excavations. On the
theory of its mechanical combination, however, as by volume, and that
each constituent acts freely for itself and according to its own laws,
important speculations (conclusions, indeed) have arisen, both as
regards temperature and climatic differences. It should be observed,
that volume, as we have used the word, is the apparent space occupied,
and differs from mass, which is the _effective_ space occupied, or the
real bulk of matter, while density is the relation of mass to volume,
or the quotient resulting from the division of the one by the other.
Those empty spaces which render the volume larger than the mass are
technically called its pores.
Has the composition of the atmosphere changed in the lapse of years?
On this point both French and German philosophers have largely
speculated. It is computed that it contains about two millions of
cubic geographical miles of oxygen, and that 12,500 cubic geographical
miles of carbonic acid have been breathed out into the air or
otherwise given out in the course of five thousand years. The
inference, then, should be, that the latter exists in the air in the
proportion of 1 to 160, whereas we find but 4 parts in 10,000. Dumas
and Bossingault decided that no change had taken place, verifying
their conclusion by experiments founded on observations for more than
thirty-five years. No _chemical_ combination of oxygen and nitrogen
has ever been detected in the atmosphere, and it is presumed none will
be.
* * * * *
The atmosphere possesses, as may be readily imagined, many important
characteristics. One of these is Weight.
This is demonstrated by simple, yet decisive experiments. The
discovery of the _fact_ is attributed to the illustrious Galileo, but
to modern science we owe all the certainty, variety, and elegance of
the demonstration. A vessel containing a quantity of air is weighed;
the air is exhausted from it and it is weighed again. An accurate
scale will then detect the difference of weight. A cubic foot of air
weighs 1.2 oz. Hence a column of air of one inch in diameter and a
mile in height weighs 44 oz.
The atmosphere is supposed to have an elevation of from 45 to 50
miles, but its weight diminishes in proportion to its height. The
whole pressure at the surface of the earth is estimated to be 15 lbs.
to the square inch; a person of ordinary size is consequently pressed
upon by a weight of from 13 to 14 tons. Happily for us, the pressure
from without is counteracted by the pressure from within.
The weight of the air is of great importance in the economy of Nature,
since it prevents the excessive evaporation of the waters upon the
earth's surface, and limits its extent by unalterable laws. Water
boils at a certain temperature when at the earth's surface, where the
weight of the atmosphere is greatest, but at different temperatures at
different elevations from the surface. At the level of the sea it
boils at 212 deg.. On the high plains of Quito, 8,724 feet above the sea,
it boils at 194 deg., and an egg cannot be cooked there in an open vessel.
At Potosi the boiling-point is still lower, being 188 deg., and the
barometrical column stands at 18 deg.. Indeed, the experiment is often
exhibited at our chemical lectures, of a flask containing a small
quantity of water, which, exhausted of air, is made to boil by the
ordinary heat of the hand.
Fahrenheit proposed to ascertain the height of mountains by this
principle, and a simple apparatus was contrived for the purpose, which
is now in successful use. The late Professor Forbes of Edinburgh,
whose untimely death the friends of science have had so much reason to
deplore, ascertained that the temperature of boiling water varied
arithmetically with the height, and at the rate of one degree of the
thermometric scale for every 549.05 feet. Multiplying the difference
of the boiling-point by this number of feet, we have the elevation.
The weight of the atmosphere, as indicated by the barometer, is also a
means for ascertaining the height of mountains or of plains; but
correction must be made for the effects of expansion or contraction,
and for capillarity, or the attraction between the mercury and the
glass tube, at least whenever great exactness is required. Tables for
the convenience of calculation are given in several scientific works,
and particularly in a paper of Professor Forbes, Ed. Trans. Vol. 15.
Briefly, however, we may state, that between 0 deg. and 32 deg., 34
thousandths of an inch must be allowed for depression or contraction,
and between 32 deg. and 52 deg. 33 thousandths. The weight of the atmosphere
is not only affected by rarefaction, but by currents of air, which
give it a sudden density or rarity. Those who have ascended mountains
have experienced both these changes.
A common experiment to prove the weight of air is that of the
Magdeburg Hemispheres, a simple contrivance of Otto Guericke, a
merchant of that city. It is a part of every complete philosophical
apparatus. It consists of brass caps, which, when joined together, fit
tightly and become a globe. The air within being exhausted, it will be
found difficult to separate them. If the superficies be 100 square
inches and the height of the mercury be 30 inches, the atmosphere will
press on these hemispheres with a weight of 1,475 lbs, requiring the
efforts of seven or eight powerful men to tear them asunder. One of
these instruments, of the diameter of a German ell, required the
strength of 24 horses to separate it. The experiment was publicly made
in 1650 at the Imperial Diet at Rendsborg, in the presence of the
Emperor Ferdinand III. and a large number of princes and nobles, much
to their astonishment.
As compared with water, the air (the barometer indicating 30 deg., and the
thermometer 55 deg.) is 833 times lighter.
It is this weight of the atmosphere which counterbalances that of a
column of mercury 29 inches in height, and a column of water 32 to 34
feet in height.
The old quaint notion of Nature's abhorring a vacuum was found to be
practically only an assertion that the air had weight. The ordinary
pump, commonly called the suction-pump, is constructed on this
principle. The weight of the atmosphere at the level of the sea is
found to be the same all over the world.
* * * * *
We find the atmosphere with another characteristic,--Elasticity.
However it may be compressed, air returns, on liberation, to its
original volume, and while thus perfectly elastic it is also the most
compressible of bodies. This elasticity arises from the repulsive
force of its particles, and is always equal to the compressive force
which it balances. A glass vessel full of air, placed under a receiver
and then exhausted by the air-pump, will burst into atoms. Water, on
the other hand, is almost the reverse. Twenty cubic inches, introduced
into a cannon whose sides are three inches thick, cannot be compressed
into nineteen inches without bursting it. This non-elastic property of
water, with another, that of communicating, when under the action of
any force, an equal pressure in all directions, led to the invention
of the hydraulic press.
The elasticity of the air enables fishes to rise and sink in water,
through the action of the air-bladder.
The sudden compression of air liberates its latent heat, and produces
fire. On this principle the pneumatic tinder-box is constructed.
Brockhaus says that air has as yet been compressed only into
one-eighth of its original bulk.
For every degree of heat between the freezing-point and the boiling-point,
32 deg. and 212 deg., the expansion of air is about 1/490th part, so that
any invention which seeks to use rarefied air as a motive power
must employ a very intense degree of heat, enough to fuse many kinds
of metals.
To the celebrated Mr. Boyle and to Henry Cavendish, both of Great
Britain, we are indebted for most of what we know of this particular
property of the air.
* * * * *
Density, or closeness, is another quality of the atmosphere. It has
been found to be 770 times less than that of water, and 770 cubic
inches of air weigh as much as a cubic inch of water. It is in direct
ratio with its elasticity, and there are tables by which it may be
determined at different altitudes. At the surface of the earth, this
density is indicated as 1; at 2-1/2 miles, as 1/2; at 5 miles, as 1/4;
and so on, the difference being in a geometrical progression.
As we proceed in the consideration of our general subject, we shall
find, under the appropriate heads, that density is not without
material influence on reflection and refraction, on transparency and
the transmission of light, the presence or absence of moisture, and
the amount of heat at the earth's surface,--and we might add, on
health, and the increase or diminution of the vital energies.
* * * * *
Temperature is another branch of our subject, and one involving a
series of subordinate topics on which volumes have been written, and
to which are still devoted the labors of the most learned men of our
day. In this place, merely an out-line can be attempted.
Temperature is the degree of heat or cold in the particles of all
bodies, which is perceptible by sensation, and is measurable by their
expansion or contraction. It is the key to the theory of the winds, of
rain, of aerial and oceanic currents, of vegetation and climate with
all their multifarious and important differences. While the inclined
position of the earth on its axis and its movement in its elliptical
orbit influence the general amount of heat, it is rather to the
consequences of these in detail that we are called when we speak of
temperature. If the sun shone on a uniformly level surface, everywhere
of the same conducting and radiating power, there would be but little
difficulty in tracing the monotonous effects of temperature.
The reformer Luther, as eccentric as he was learned and sincere, is
reported to have said, that, if he had been consulted at the Creation,
he would have placed the sun directly over the centre of the world and
kept it there, to give unchanging and uniform light and heat! It is
certainly much better that he was not consulted. In that case, every
parallel of latitude would have been isothermal, or of equal mean
annual temperature. The seasons would have been invariable in
character. Some portions of the earth would have been scorched to
crispness, others locked up in never-changing ice.
Vegetation, instead of being universal, would have been confined to a
narrow zone; and the whole human race would have been driven together
into one limited habitable space, to interfere with, incommode, and
destroy each other. The arrangement is best as it is.
We find very important modifications of temperature, occasioned not
only by astronomical influences, but by local causes and geographical
characteristics. For while, as a general rule, the nearer we approach
the equator, the warmer we shall be, yet temperature is greatly
affected by mountains, seas, currents of air or water, by radiation,
by forests, and by vegetation. It is found, in fact, that the lines of
temperature, (the happy conception of Humboldt,) when they are traced
upon the map, are anything but true zones or circles.
The line of the greatest mean warmth is not coincident with the
equator, but falls to the north of it. This line at 160 deg. W. Long, from
Greenwich is 4 deg. below the geographical equator; at 80 deg. it is about 6 deg.
north, sweeping along the coast of New Granada; at 20 deg. it comes down
and touches the equator; at 40 deg. E. Long., it crosses the Red Sea about
16 deg. north of the equator, and at 120 deg. it falls at Borneo, several
degrees below it;--and the points of the greatest heat, in this line,
are in Abyssinia, nearer the tropic of Cancer than to the equator. On
the other hand, the greatest mean cold points, according to the
opinions of Humboldt, Sir David Brewster, and others, do not coincide,
as would seem natural, with the geographical poles, but they are both
to be found in the northern hemisphere, in Latitude 80 deg., 95 deg.E. Long.
and 100 deg. W. Long. from Greenwich. The western is ascertained to be
4-1/2 deg. colder than the eastern or Siberian. If this be the fact,--but
it is not positively admitted,--an open sea at the pole may be
considered as probable, on the ground of its having a higher mean
temperature than is found at 80 deg.. Kaemptz places one of these cold
points at the north of Barrow's Straits,--the other near Cape Taimur,
in Siberia. Burghaus, in his Atlas, transfers the American cold pole
to 78 deg. N. Lat. It is perhaps too early to determine rigorously the
true temperature of these points.
A noticeable fact also is this,--that places in the same latitude
rarely receive the same amount of heat. Quebec, in British America,
and Drontheim, in Norway, enjoy about the same quantity, while the
former is in 47 deg. and the latter in 68 deg. N. Lat. The mean winter
temperature of Pekin, 39 deg. 45' N. Lat., is 5 deg. below the freezing-point;
while at Naples, which is north of Pekin, it seldom, if ever, goes
below it, and Paris, 500 miles farther north, has a mean winter
temperature of 6 deg. above the freezing-point. The city of New York,
about 11 deg. south of London, has a winter temperature of much greater
severity. The mean temperature of the State of New York, as determined
by a long series of observations, is 44 deg. 31'.
The mean temperature of countries is found to be very stable, and but
very small variations have been detected in modern times. But that
there have been important climatic changes, since the Christian era,
cannot be doubted, unless we doubt history. Not many centuries ago, it
was a common thing for all the British rivers to freeze up during the
winter, and to remain so for several months. If space permitted, an
interesting statement could he made of the changes which have taken
place in vegetation in Greenland, and throughout certain northern
parts of Europe,--also in Palestine, Greece, and other southern
countries,--while we know that the earth's inclination upon its axis
has been unchanged.
Mrs. Somerville remarks, that, though the temperature of any one place
may be subject to very great variations, yet it never differs from the
mean state more than a few degrees.
Without this atmospheric covering of ours, it is considered that the
temperature of the earth at its surface would be the same as that of
the celestial spaces, supposed to be at least 76 deg. below zero, or
_possibly_, says Humboldt, 1400 deg. below! Human life, without our
atmosphere, could not exist for a single moment.
It is computed, that, if the annual heat received by the earth on its
surface could be equally distributed over it, it would melt, in the
course of a year, a stratum of ice 46 feet thick, though it covered
the whole globe, and as a consequence the amount of unradiated heat
would render it uninhabitable.
The relative position of the sun affects temperature, rather than its
distance. In winter the earth is three millions of miles nearer the
sun than in summer, but the oblique rays of the former season reach us
in less quantity than the more direct The distribution of land and
water, the nature of the soil, the indentation of bays, the elevation
of land above the sea-level, insularity, etc., all, as we have already
suggested, have a modifying influence on temperature.
The atmosphere possesses also a reflecting and refracting power,
arising from its varying density, and, perhaps, in the latter case,
somewhat from its lenticular outline.
But for this property we should have no twilight. The sun, instead of
sending up his beams while 18 deg. below the visible horizon, would come
upon us out of an intense darkness, pass over our sky a brazen
inglorious orb, and set in an instant amid unwelcome night.
Reflection is the rebound of the rays of light or heat from an
opposing surface at the same angle as that at which they fall upon it.
These are called angles of incidence and reflection, and are equal.
Refraction is the bending of a ray passing obliquely from a rarer into
a denser medium. This may be observed when a rod is placed slantingly
in a vessel of clear water; the part immersed will appear bent or
broken. This is ordinary refraction. Terrestrial refraction is the
same thing, occurring whenever there is a difference of density in the
aerial strata.
The atmosphere absorbs some portion of the light which it receives. It
is not all reflected or refracted or even penetrative.
Objects seen under various degrees of light, either convected or
retarded by different media, appear near or distant, distinct or
confused. Thus, we are often surprised at the apparent nearness and
brightness of an opposite shore or neighboring island, in some
conditions of the air, while at other times they seem distant and lie
in shadowy obscurity.
The looming up of a vessel on the water is another common instance of
the principle of refraction.
It has been noticed by almost every one, that, during the warm and
moist nights of summer, the moon, as she rises above the horizon,
appears much larger than when at the zenith. So the setting sun is
seen of apparently increased size. Sir John Herschel asserts that the
appearance is an illusion, and so do some others. Professor Carey
says, that, if we look through a paper tube at the moon when on the
horizon, the paper being folded so as to make the aperture of its
exact size, and then look again at it when it reaches the zenith, we
shall find there is no difference.
On the other hand, an experiment is offered by a German Professor, of
the name of Milo, of this kind: If we look through a tube so
constructed as to have one side filled with spirits of wine and the
other with common air, the half of the object seen through the former
will be found to appear much larger to the eye than the other half
seen through the latter.
It is laid down, that, where extraordinary refraction takes place
laterally or vertically, the visual angle of the spectator is
singularly enlarged, and objects are magnified, as if seen through a
telescope. Dr. Scoresby, a celebrated meteorologist and navigator,
mentions some curious instances of the effects of refraction seen by
him in the Arctic Ocean.
Many remarkable phenomena attend this state of the atmosphere, known
as the Fata Morgana of Sicily, the Mirage of the Desert, the Spectre
of the Brocken, and the more common exhibitions of halos, coronae, and
mock suns. The Mountain House at Catskill has repeatedly been seen
brightly pictured on the clouds below. Rainbows are also due to this
condition of the atmosphere.
We might occupy the remainder of the space allowed us by enlarging on
various topics which belong to this part of our subject. The twilight
gray, the hues of the evening and morning sky, the peculiarity of the
red rays of light, the scintillation of stars, their flashing changes
of colors, are all meteorological in their character, as well as
strikingly beautiful and interesting.
* * * * *
Polarity of light is another of the wonders of which Meteorology takes
cognizance. The celebrated Malus, in 1808, while looking at the light
of the setting sun shining upon the windows of the Luxembourg, was led
to the discovery that a beam of light which was reflected at a certain
angle from transparent and opaque bodies, or by transmission through
several plates of uncrystallized bodies, or of bodies crystallized and
possessing the property of double refraction, changed its character,
so as to have sides, to revolve around poles peculiar to itself, and
to be incapable of a second reflection. The angle of polarity was
found to be 54 deg..
The beam of polarized light was also found to have the peculiar
property of penetrating into the molecules of bodies, illuminating
them and, enabling the eye to determine as to their structure. The
production of beautiful spectres, prismatic colors of gorgeous hues,
and the most remarkable system of rings, has followed the discovery,
and important results are expected from the continuation of the
researches. It has already enabled the astronomer to determine what
heavenly bodies do or do not shine with their own light. The subject
is still under investigation.
* * * * *
Color from light comes also under the notice of the meteorologist. The
received opinion is, that there is no inherent color in any object we
look at, but that it is in the light itself which falls upon and is
reflected from the object. Each object, having a particular reflecting
surface of its own, throws back light at its own angle, absorbing some
rays and dispersing others, while it preserves its own. In this sense
it may be said that the rose has no color,--its hues are only
borrowed. If the idea should be carried out, it would certainly
destroy much of the poetry of color. Thus, in praising the modest
blush which crimsons the cheek of beauty, we should destroy all its
charm, if we attributed it to a sudden change in the reflecting
surface of the epidermis,--a mere mechanical rushing of blood to the
skin, and a corresponding change in its angle of reflection!
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