_The Physical Geography of the Sea_. By M. F. MAURY. New York:
Harper & Brothers. 1857.
_Climatology of the United States and of the Temperate Latitudes
of the North American Continent_. By LORIN BLODGET. Philadelphia: J.
B. Lippincott & Co. 1857.
_Proceedings of the British Association for the Advancement of
Science_. 1857.
An eloquent philosopher, depicting the deplorable results that would
follow, if some future materialist were "to succeed in displaying to
us a mechanical system of the human mind, as comprehensive,
intelligible, and satisfactory as the Newtonian mechanism of the
heavens," exclaims, "Fallen from their elevation, Art and Science
and Virtue would no longer be to man the objects of a genuine and
reflective adoration." We are led, in reflecting upon the far more
probable success of the meteorologist, to similar forebodings upon
the dulness and sameness to which social intercourse will be reduced
when the weather philosophers shall succeed in subjecting the changes
of the atmosphere to rules and predictions,--when the rain shall
fall where it is expected, the wind blow no longer "where it listeth,"
and wayward man no longer find his counterpart in nature. But we
console ourselves by contemplating the difficulties of the problem,
and the improbability, that, in our generation at least, we shall be
deprived of these subjects of general news and universal interest.
During the last half-century, the progress of experimental
philosophy in the direction of the weather, though its results are
for the most part of a negative character, has yet been sufficient
to excite the apprehensions of the philanthropist. We have unlearned
many fables and false theories, and have made great advancement in
that knowledge of our ignorance, which is the only true foundation
of positive science.
The moon has been deposed from the executive chair, though she still
has her supporters and advocates; and an innumerable host of minor
causes are found to constitute, upon strictly republican principles,
the ruling power of the winds and the rain. That regularity, however
complicated, which reason still demands, and expects even from the
weather, is not found to be so simple as our rules and signs of the
weather indicate; for the operation of these innumerable causes is
so complicated, that the repetition of similar phenomena or similar
combinations of causes, to any great extent, is the most improbable
of events. Perhaps the meteorologist will ultimately find that
Nature has succeeded, in what seems, indeed, to be her aim, in
completely retracing her steps, and reducing the operation of that
simple and regular system of causes, which she brought out of chaos,
back to a confusion of detail, from which all law and regularity are
obliterated.
Meteorological observations have, however, determined many regular
and constant causes and a few regular phenomena. The method pursued
in these investigations is, for the most part, the elimination, by
general averages, of limited and temporary changes in the elements
of the weather, and the determination of those changes which depend
upon the constant influences of locality, of season, and of constant
or slowly varying causes. These constant influences constitute the
climate; and the study of climates is thus the first step towards
the solution of the problem of the weather. Climates, in their
changes and distribution, are very important elements in the
determination of the movements of the weather, and are to the
meteorologist what the elements of the planetary orbits are to the
astronomer; but, unlike planetary perturbations, the weather makes
the most reckless excursions from its averages, and obscures them by
a most inconsequent and incalculable fickleness.
Whether mechanical science will hereafter succeed in calculating
these perturbations of climate, as we may style the weather, or will
find the problem beyond its capacity, it will yet, doubtless, account
for much that is now obscure, as observation brings the facts more
distinctly to view. We propose to give a brief general survey of the
mechanics of the atmosphere in its present state, and to indicate
the nature and limits of our knowledge on this subject.
Among the first noticed and most remarkable features of regularity
in atmospheric changes are constant, periodic, and prevailing winds.
The most remarkable instances of these are the trade-winds of the
torrid zone, the monsoons of the Indian Ocean, and the prevailing
southwest wind of our northern temperate latitudes. Of these, the
trade-winds are the most important to science, as furnishing the key
to that general explanation of the winds which was first advanced by
the distinguished Halley.
In Halley's celebrated theory, the trade-winds are explained as the
effects of the unequal distribution of the sun's heat in different
latitudes. The air of the equator, heated more than the northern or
southern air, expands more, and overflows, moving in the upper
regions of the atmosphere toward the poles; while the lower, colder
air on both sides moves toward the equator to preserve equilibrium.
Thus an extensive circulation is carried on. The air that moves from
the equator in the upper atmosphere, gradually sinking to the surface
of the earth, finally ceases to move toward the poles, and returns
as an undercurrent to the equator, where it again rises and moves
toward the poles.
Now the air of the equator, moving with the earth's rotary motion,
has a greater velocity than the earth itself at high northern or
southern latitudes, and consequently appears to gain an eastward
motion in its progress toward the poles. Without friction, this
relative eastward motion would increase as the air moves toward the
poles, and diminish at the same rate as the air returns, till at the
equator the velocity of the earth and of the air would again be equal;
but friction reduces the motion of the returning air to that of the
earth, at or near the calms of the tropics; so that the air, passing
the tropics, gains a relative westward motion in its further
progress through the torrid zone. The southwestward motion thus
produced between the tropic of Cancer and the equator is the
well-known trade-wind.
Now, according to this theory, the prevailing winds of our temperate
latitudes ought to have a southeastward motion as far as the calms
of Cancer or "the horse latitudes." Moreover, instead of these calms,
there should still be a southward motion. But observation has shown,
that though the prevailing lower winds of our latitude move eastward,
still their motion is toward the north rather than the south; so
that they appear to contradict the theory by which the trade-winds
are explained.
To account for these anomalies, Lieut. Maury has invented a very
ingenious hypothesis, which is published in his "Physical Geography
of the Sea." He supposes that the air, which passes from the equator
toward the poles in the upper regions of the atmosphere, is brought
down to the surface of the earth beyond the calms of the tropics,
and that it thence proceeds with an increasing eastward motion,
appearing in our northern hemisphere as the prevailing northeastward
winds. Approaching the poles with a spiral motion, the air there
rises, according to this hypothesis, in a vortex, and returns toward
the equator in the upper atmosphere, gradually acquiring a westward
motion; till, returning to the tropics, it is again brought down to
the earth, and thence proceeds, with a still increasing westward
motion, as the trade-winds. At the equator the air rises again, and,
according to Lieut. Maury, crosses to the other side, and proceeds
through a similar course in the other hemisphere.
The rising of the air at the equator is supposed to cause the
equatorial rains; and the drought of the tropics is also explained
by that descent of the air, in these latitudes, which this
hypothesis supposes.
Now although this hypothesis explains the phenomena, it has still
met with great opposition. The motions which Lieut. Maury supposes
can hardly be accounted for without resorting, as is usual in such
cases, to electricity or magnetism,--to some occult cause, or some
occult operation of a known cause. Moreover, it has been difficult
for the mechanical philosopher to understand how the winds manage to
cross each other, as Lieut. Maury supposes them to do, at the
equator and the tropics, without getting into "entangling alliances."
If this hypothesis were advanced, not as a physical explanation of
the phenomena, but, like the epicycles and eccentrics of Ptolemy,
"to save the appearances," its ingenuity would be greatly to its
author's credit; but, like the epicycles and eccentrics, though it
represents the phenomena well enough, it contradicts laws of motion,
now well known, which ought to be familiar to every physical
philosopher. But these speculations of Lieut. Maury will now be
superseded by a new theory of atmospheric movements, an account of
which was presented by its author, Mr. J. Thompson, at the recent
meeting of the British Association for the Advancement of Science. [1]
[Footnote 1: A fuller discussion of this theory the author
reserved for the Royal Society. The _London Athenaeum_ gives a brief
abstract of his paper, in its report of the proceedings of the
Association.]
Mr. Thompson's theory takes account of forces, hitherto unnoticed,
which are generated by the eastward circulation of the atmosphere in
high latitudes. He shows that these forces cause the prevailing
northeastward under-current of our latitudes, while above this, yet
below the highest northeastward current, the air ought still to move
southward according to Halley's theory.
This under-current is not the immediate effect of differences of
temperature, but a secondary effect induced by the friction of the
earth's surface and the continual deflection of the air's eastward
motion from a great circle, (in which the air tends to move,) into
the small circle of the latitude, in which the air actually does move.
The force of this deflection, measured by the centrifugal force of
the air as it circulates around the pole, retards the movement from
the equator, and finally wholly suspends it; so that the upper air
circulates around in the higher latitudes as water may be made to
circulate in a pail; and the air is drawn away from the polar
regions as this circulatory motion is communicated to it, and tends
to accumulate in the middle latitudes, as the circulating water is
heaped up around the sides of the pail. Hence, in the middle
latitudes there is a greater weight of air than at the poles, and
this tends to press the lower air to higher latitudes. Centrifugal
force, however, balances this pressure, so long as the lower air
moves with the velocity of the upper strata; but as the friction of
the earth retards its motion and diminishes its centrifugal force,
it gradually yields to the pressure of the air above it, and moves
toward the poles. Near the polar circles it is again retarded by its
increasing centrifugal force, and it returns through the middle
regions of the atmosphere.
Thus there are two systems of atmospheric circulation in each
hemisphere. The principal one extends from the equator to high
middle latitudes and partly overlies the other, which extends from
the tropical calms to the polar circles. These two circulations move
in opposite directions; like two wheels, when one communicates its
motion to the other by the contact of their circumferences.
In the middle latitudes the lower current of the principal
circulation lies upon the upper current of the secondary circulation,
and both move together toward the equator. This principal lower
current first touches the earth's surface beyond the tropical calms,
and having lost its relative eastward motion and now tending westward,
it appears as the trade-wind, very regular and constant; while the
upper secondary current returns, without reaching the tropics, as an
undercurrent, and in our latitude appears as the prevailing
northeastward wind,--a very feeble motion, usually lost in the
weather winds and other disturbances, and only appearing distinctly
in the general average.
Mr. Thompson illustrates the effect of the friction of the earth's
surface on the eastward circulation of the air by a very simple
experiment with a pail of water. If we put into the pail grains of
any material a little heavier than water, and then give the water a
rotatory motion by stirring it, the grains ought, by the centrifugal
force imparted to them, to collect around the sides of the pail; but,
sinking to the bottom, they do in fact tend to collect at the centre,
carried inward by those currents which the friction of the sides and
bottom indirectly produces.
Thus Mr. Thompson's beautiful and philosophical theory completes
that of Halley, and explains all those apparent anomalies which have
hitherto seemed irreconcilable with the only rational account of the
trade-winds. The rainless calms of the tropics are explained by this
theory without that crossing and interference of winds which Lieut.
Maury supposes; for the secondary circulation returns as an
under-current toward the poles without reaching the tropics, and the
dry lower current of the principal circulation passes over the
tropical latitudes, in its gradual descent, before it reaches the
earth as the trade-winds.
These trade-winds, absorbing moisture from the sea, precipitate it
as they rise again, and produce the constant equatorial rains; and
these rains, doubtless, tend much more powerfully than the mere
unequal distribution of heat to direct the wind toward the equator;
for the fall of rain rapidly diminishes the pressure of the air and
disturbs its equilibrium, so that violent winds are frequently
observed to blow toward rainy districts. Thus, primarily, the unequal
distribution of heat, and, more immediately, the equatorial rains
cause the principal circulation of our atmosphere; and this
indirectly produces the secondary circulation of Mr. Thompson's
theory. Both these regular movements are, however, greatly disturbed,
and especially the latter, by winds which are occasioned by local
and irregular rains.
In these movements and their causes we have the general outline of
our subject, within which we must now sketch the weather. The causes
of atmospheric movement, which we have thus far considered, are the
unequal distribution of the sun's heat, the absorption and
precipitation of moisture, the direct and the inductive action of
the earth's rotation and friction. If to these we should add the
tidal action of the sun's and moon's attractions, we should perhaps
complete the list of _vera causae_ which are certainly known to
exert a more or less general influence upon the atmosphere. But this
short list is long enough, as we shall soon see.
If the earth were wholly covered with water of a uniform depth, its
climates would be distributed with greater regularity, and the
perturbations of climate would be comparatively small and regular;
though even under such circumstances there would still exist a
tendency to discontinuity and complexity of movements from that
influence of rain, the peculiar character of which we shall soon
consider.
The irregular distribution of land and water, and the peculiar
action of each in imparting the heat of the sun to the incumbent air,--
the irregular distribution of plains and mountains, and their various
effects in different positions and at different altitudes,--the
distribution of heat effected by ocean currents,--all these tend to
produce permanent derangements of climate and great irregularities
in the weather. To these we must add what the astronomer calls
disturbing actions of the second order,--effects of the disturbances
themselves upon the action of the disturbing agencies,--effects of
the irregular winds upon the distribution of heat and rain, and upon
the action of lands and seas, mountains and plains. Though such
disturbances are comparatively insignificant in the motions of the
planets, yet in the weather they are often more important than the
primary causes.
The aggregate and permanent effect of all these disturbing causes,
primary and secondary, is seen in that irregular distribution of
climates, which the tortuous isothermal lines and the mottled
raincharts illustrate. The isothermal lines may be regarded as the
topographical delineations of that bed of temperatures down which
the upper atmosphere flows from the equator toward the poles, till
its downward tendency is balanced by the centrifugal force of its
eastward motion. This irregular bed shifts from month to month, from
day to day, and even from hour to hour; and the lines that are drawn
on the maps are only averages for the year or the season.
In the midst of these irregular, but continuous agencies, the rain
introduces a peculiar discontinuity, and turns irregularity into
discord. We have shown that the rain is an immediate cause of wind;
but how is the rain itself produced? For so marked an effect we
naturally seek a special cause; but no adequate single cause has
ever been discovered. The combination of many conditions, probably,
is necessary, such as a peculiar distribution of heat and moisture
and atmospheric movements; though the immediate cause of the fall of
rain is doubtless the rising, and consequent expansion and cooling,
of the saturated air.
The winds that blow hither and thither, vainly striving to restore
equilibrium to the atmosphere, burden themselves with the moisture
they absorb from the seas; and this moisture absorbs their heat,
retards their motion, and slowly modifies the forces which impel them.
Now when the saturated air, extending far above the surface of the
earth, and carried in its movements still higher, is relieved of an
incumbent weight of air, it becomes rarefied, and its temperature
and capacity for moisture are simultaneously diminished; its moisture,
suddenly precipitated, appears as a cloud, the particles of which
collect into rain-drops and fall to the earth. Thus the air suddenly
loses much of its weight, and instead of restoring equilibrium to
the troubled atmosphere, it introduces a new source of disturbance.
Though the weight of the air is diminished by the fall of rain, yet
the bulk is increased by the expansive force of the latent heat
which the condensed vapors set free. Thus the rainy air expands
upwards and flows outwards, and no longer able to balance the
pressure of the surrounding air, it is carried still higher by
inblowing winds, which rise in turn and continue the process, often
extending the storm over vast areas. The force of these movements is
measured partly by the force of latent heat set free, and partly by
the mechanical power of the rain-fall, a very small fraction of
which constitutes the water-power of all our rivers. Such a fruitful
source of disturbance, generated by so slight an accident as the
upward movement of the saturated air, expanded by its own agency to
so great an extent, so sudden and discontinuous in its action, so
obscure in its origin, and so distinct in its effects,--such a
phenomenon defies the powers of mathematical prediction, and rouses
all the winds to sedition.
A storm not only disturbs the lower winds, but its influences reach
even to the upper movements. The sudden expansion and rising of the
rainy air delay these movements, which afterwards react as violent
winds.
The forces stored away by the gradual rise of vapor and its
absorption of heat, and then suddenly exhibited in a mechanical form
by the effects of rain, afford an illustration of that principle of
conservation and economy of power, of which there are so many
examples in modern science. No power is ever destroyed. Whether
exhibited as heat or mechanical force, in the products and forces of
chemical or of vital action, in movement or in altered conditions of
motion,--whether changed by the growth of plants into fuel or into
food, and converted again to heat by combustion or by vital processes,
and brought out as mechanical power in the steam-engine or in the
horse,--it is still the same power, and is measured in each of its
forms by an invariable standard. It first appears as the heat of the
sun, and a portion escapes at once back into space, while the rest
passes first through a series of transformations. A part is changed
into moving winds or into suspended vapor, and a part into fuel or
food. From conditions of motion it is changed into motion; from
motion it is changed by friction or resistance into heat, electric
force, molecular vibrations, or into new conditions of motion, and
passing through its course of changes, it remains embroiled in its
permanent effects or escapes into space as heat.
Though mechanical science will probably never be able to predict the
beginning or duration of storms, it will yet, doubtless, be able to
account for all their general features, and for such distinct local
peculiarities as observation may determine. Great advancement has
already been made in the determination of prevailing winds and in
the study of storms. Two theories have been brought forward upon the
general movements of storms; both have been proved, to the entire
satisfaction of their advocates, by the storms themselves; and
probably both are, with some limitations, true. The first of these
theories we have already described. According to it, the winds move
inward toward the centre of the storm; according to the other theory,
they blow in a circumference around the centre.
Observations upon storms of small extent, such as thunder-storms or
tornadoes, show very clearly that the winds blow toward the stormy
district. But when observations are made upon the winds within the
district of such extensive storms as sometimes visit the United
States, the directions of the wind are found to be so various, that
the advocates of either theory, making due allowance for local
disturbances, can triumphantly refute their adversaries. In such
storms there are doubtless many centres or maxima of rain, and
whether the wind move around or toward these centres, it would
inevitably get confused.
The opinion, that the winds move around the central point or line of
the storm, was strenuously maintained by the late Mr. Redfield,
whose activity in his favorite pursuit has connected his name
inseparably with meteorology. Others have maintained the same opinion,
and the rotatory motion of the tropical hurricanes is offered as a
principal proof. It is obvious from the causes of motion already
considered, that, if the air is carried far, by its tendency toward a
rainy district, it will acquire a secondary relative motion from its
change of latitude; and this, in our hemisphere, if the air move
toward the south, will be westward,--if toward the north, eastward.
Hence the motion of the air from both directions toward a stormy
district is deflected to the right side of the storm; and this gives
rise to that motion from right to left which is observed in the
hurricanes of the northern hemisphere.
To suppose, as many do, that regular winds, arising from constant
and extensive causes, can come into bodily conflict and preserve
their identity and original impetus for days, without immediate and
strongly impelling forces to sustain their motion, implies a
profound ignorance of mechanical science, and is little better than
those ancient superstitions which gave a personal identity to the
winds. The momentum of ordinary winds is a feeble force in
comparison with those forces of pressure and friction which
continually modify it. Hence sudden changes in the direction and
intensity of winds must primarily arise from similar changes in
these forces. But there are no known forces which change so suddenly,
except the pressure and latent heat of suspended vapor; and therefore
the fall of rain is the only adequate known cause of those
storm-winds which, interpolated among the gentler winds, keep the
atmosphere in perpetual commotion.
Storms have, however, certain habits and peculiarities, more or less
regular and distinct, which depend upon locality and season. And
this is what ought to be expected; for, though the storms themselves
are essentially anomalous, yet many of the causes which cooperate to
induce them are constant or periodic, while others are subject to
but slight perturbations. It is obvious that no more moisture can be
precipitated than has been evaporated, and that the winds only gain
suddenly by the fall of rain the forces which they have lost at their
leisure in the absorption of moisture. Thus the rage of the storm is
kept within bounds, and though the exact period at which the winds
are set free cannot be determined, yet their force and frequency
must be subject to certain limitations. The study of the habits and
peculiarities of storms is of the greatest importance to navigation
and agriculture, and these arts have already been benefited by the
labors of the meteorologist.
The lawlessness of the weather, within certain limitations, though
discouraging to the physical philosopher, has yet its bright side
for the student of final causes. The uses of the weather and its
adaptation to organic life are subjects of untiring interest. The
progression of the seasons, varied by differences of latitude, is
also diversified and adapted to a fuller development of organic
variety by irregularities of climate.
The regular alternations of day and night, summer and winter, dry
seasons and wet, are adapted to those alternations of organic
functions which belong to the economy of life. The vital forces of
plants and of the lower orders of animals have not that
self-determining capacity of change which is necessary to the
complete development of life; but they persist in their present mode
of action, and, when they are not modified by outward changes,
reduce life to its simplest phases. Changes of growth are effected
by those apparent hardships to which life is subject; and progression
in new directions is effected by retrogression in previous modes of
growth. The old leaves and branches must fall, the wood must be
frost-bitten or dried, the substance of seeds must wither and then
decay, the action of leaves must every night be reversed, vines and
branches must be shaken by the winds, that the energies and the
materials of new forms of life may be rendered active and available.
Some of the outward changes of nature are regular and periodic, while
others, without law or method, are apparently adapted by their
diversity to draw out the unlimited capacities and varieties of life;
so that as inorganic nature approaches a regulated confusion, the
more it tends to bring forth that perfect order, of which fragments
appear in the incomplete system of actual organic life.
The classification of organic forms presents to the naturalist, not
the structure of a regular though incomplete development, but the
broken and fragmentary form of a ruin. We may suppose, then, with a
recent physiological writer, that the creation of those organic
forms which constitute this fragmentary system was effected in the
midst of an elemental storm, a regulated confusion, uniting all the
external conditions which the highest capacities and the greatest
varieties of organized life require for their fullest development;
and that as the storm subsided into a simpler, but less genial
diversity,--into the weather,--whole orders and genera and species
sank with it from the ranks of possible organic forms. The weather,
fallen from its high estate, no longer able to develope, much less to
create new forms, can only sustain those that are left to its care.
Man finds himself everywhere mirrored in nature. Wayward, inconstant,
always seeking rest, always impelled by new evils, the greatest of
which he himself creates,--protecting and cherishing or blighting and
destroying the fragmentary life of a fallen nature,--incapable
himself of creating new capacities, but nourishing in prosperity and
quickening in adversity those that are left,--he sees the workings of
his own life in the strife of the elements. His powers and activities
are related to his spiritual capacities, as inorganic movements are
related to an organizing life. The resurrection of his higher nature
is like a new creation, secret, sudden, inconsequent. "The wind
bloweth where it listeth, and thou hearest the sound thereof, but
canst not tell whence it cometh, and whither it goeth; so is every
one that is born of the Spirit."
* * * * *