and this may be effected from above, by descent of the air from the
upper regions; from below, by ascent, with the assistance of rising
currents created by solar radiations; or, finally, in any and every
direction under the influence of the winds and the general movements of
the atmosphere. Furthermore, the cooling need not be very great in order
to provoke rain under certain conditions of temperature and humidity of
frequent occurrence.
Rain clouds very frequently descend a little below the altitude of the
Puy-de-Dôme. It is, therefore, not difficult, in order to determine the
degree of cooling necessary for the formation of rain, to take advantage
of observations that have been made there. The hygrometer sometimes
remains near saturation without there being precipitation of vapor;
and, supposing that the temperature is near 3° or 4° C., which is about
the mean temperature of the year, it will require a cooling of only one
or two degrees centigrade at most for the air to be unable to hold all
its vapor and for the excess of it to be transformed into rain. This is
confirmed by experiment and observation.
I will mention a remarkable example illustrating this point. Not rarely,
when the west wind is blowing violently on the top of the Puy-de-Dôme,
an east wind, blowing opposite to it, prevails at Clermont. Then an eddy
is formed behind the plateau and the chain of puys that runs from north
to south, a little west of Clermont. This eddy gradually becomes a vast
whirlwind with a horizontal axis, several leagues long, a few kilometres
wide, and seven hundred or eight hundred metres high. It commonly gives
rise to an abundant and continuous formation of black clouds, which
appear in an instant along its length, following its intersection
with the upper current. The phenomenon is frequent, and is sometimes
produced under very interesting conditions, as on a certain day when
the temperature at Clermont was five degrees above zero, centigrade,
while the hygrometer indicated that the air contained seven tenths of
the quantity of vapor required for saturation. Under such conditions
the temperature on the Puy-de-Dôme would have only had to be a very
little above the freezing point for the vapor of the horizontal eddy to
be transformed into rain on meeting the upper current coming from the
west. Now, on the top of the mountain the thermometer marked 4°C. below
the freezing point. Hence, every time the lower east wind increased a
little, this having the effect of carrying the vapor and the air of the
lower regions a little higher, the black clouds could be seen developing
with a recrudescence of intensity. A few instants afterward a torrential
rain fell at Clermont.
In some cases--and such frequently occur in summer--the mingling of
strata of air of different temperatures is effected by ascending
currents. The sky is clear; the moist air in contact with the soil is
warmed under the action of the sun, rises, and more or less quickly
reaches a much colder stratum. Light mists are formed; they may
frequently be seen rising and spreading out over the warmer or moister
spots. On the flanks of the Puy-de-Dôme one may often find himself among
ascending currents of this sort which succeed one another intermittingly
when the air is calm, after a rain; they rise with a velocity of four or
five metres at least per second.
These fogs finally become stationary in a region of the same density
with themselves. There they accumulate and form a cloud or a group of
clouds that go on developing. When penetrated by the rays of the sun,
which they almost wholly absorb, these clouds are warmed up again in
the interior, and budding protuberances are seen, which are especially
developed on the upper parts of the cloud. These protuberances are
formed and grow so rapidly as to almost suggest the presence of a
steam generator within every cloud. The external parts of the cloud,
however, cool very soon by radiation, evaporation, or dissolution,
but especially by their contact with the cold air, into which they
continue going. Hence, when the vapors emitted by the cloud reach its
periphery, they are cooled at once as if in a condenser; they then
take on a rapid movement of descent, which is easily distinguished,
and suffer condensation in their lower parts. As the surface of the
cloud in contact with the cold air around it is considerable in
proportion to that which receives the influence of the solar rays, the
warm ascending currents slacken speed and are extinguished, because
the cloudy mass, drawn on by the higher currents, removes from the
place where it is formed, or because it stops the rays of the sun and
prevents their reaching the ground. There results a more and more
complete condensation, and the watery vapor is at last transformed into
drops of rain. The condensation into rain is accelerated and augmented
when the mass of cloud rises with great rapidity, especially when it
enters abruptly into very cold atmospheric strata. A sudden mixture
of the cloud with the air around it takes place then, and sudden and
abundant rains result like those which are produced at the instant of
thunderstorms.
The formation and mixture of masses of air of different temperatures
are effected by ascending currents in zones of restricted extent, but
sometimes very numerous. Local showers and thunderstorms are produced
in this way. The phenomenon becomes much more important and at the same
time extends over vast regions, when it is brought about by the aid of
the wind and the larger movements of the atmosphere, and general rains
result.
Babinet, in his Studies on the Sciences of Observation, explains the
formation of rain by supposing that when the wind meets an obstacle,
it ascends; the moving air cools in rarefying, and deposits its excess
of vapor over saturation. This fact, when it occurs, should indeed
contribute to the condensation of the vapor contained in the air; but it
does not afford an adequate explanation of all rains; for, first, how
can it rain on the vast oceans which present no obstacles to cause the
air to ascend? It is necessary to suppose that internal movements of the
atmosphere intervene in the production of rain.
Monk, Mason, de Saussure, and many others fix the prime condition for
the formation of rain in the superposition of two beds of cloud. This
assertion, although it is still repeated in a number of treatises
on physics, is inexact. A single stratum of cloud--yes, a solitary
cloud--has been seen, on the Puy-de-Dôme, to produce rain and lightning,
with thunder.
Frequently, under the influence of the centers of perturbation which
often exist south of the Alps, a vast sea of clouds, the upper face of
which does not exceed an altitude varying from seven hundred to twelve
hundred metres, covers all central France, and probably other countries.
Only the high table-lands and mountains rise above this stratum of
clouds over which the sun shines in a perfectly clear sky. Yet rain is
found in such strata of clouds, however homogeneous they may be, and
it rains in the regions they cover. I have long been able to affirm
this fact, important because it destroys old errors elaborated in the
isolation of the study, and to support it with authentic proof.
We may witness the formation of rain when we rise into the usual region
of the clouds, either in balloon ascents or by climbing mountains.
The phenomenon may be observed under five aspects: First, we may
find ourselves in a fog of greater or less thickness, the hygrometer
indicating that the air is nearly saturated with vapor, without one
being able to detect the fall of the smallest liquid particle, and
without exterior objects being moistened. Second, while we can not
observe the fall of a single liquid drop, however small, everything
enveloped in the cloud will be rapidly moistened. We are in the
atmospheric stratum where the rain is beginning to form. Inhabitants
of mountainous regions say at such times that there is a wet fog. At
the top of the Puy-de-Dôme, when this condition lasts for a day, we can
collect three, four, or five millimetres of water. Third, we may remark,
in the fog, the fall of exceedingly fine droplets, which we can hardly
distinguish--it is drizzling. Fourth, the rain is falling, while we are
still in the fog; and, fifth, the rain is falling and we are below the
fog--that is, below the clouds.
These five aspects may be present in the same cloud, when we will find
them in the order given in successive strata, one beneath another; so
that, entering such a cloud from the upper part, we may traverse, in
regular order, "dry" fog, wet fog, fog with drizzle, fog with rain, and,
as we leave the cloud at the bottom, rain without fog. Mr. Glaisher,
the English scientific aëronaut, thus records his experience in an
ascension he made July 1, 1863: "We let ourselves drop at eight hundred
metres, and went into a fog which was dry for the first thirty metres,
but shortly afterward became moist. As we descended, the fog seemed to
become more charged with water, and seemed very dark beneath us; at five
hundred or six hundred metres we heard the sound of the rain striking
the trees, so violent was the fall."
Rain drops, in fact, grow as they fall, whether by continuance of
condensation, or by union with other drops. They should, therefore, be
larger when they issue from the cloud in proportion as the region where
drizzle is formed is higher above the base of the cloud. There is,
however, a limit to the size they can attain, for the velocity of their
fall increases with their mass, and they are divided by the resistance
of the air.
The five aspects under which we have regarded the formation of rain are
evidently five phases distinguished by our senses in the progressive
transformation which the vapor of water undergoes in passing to the
liquid state. It also sometimes happens that the condensation of the
vapor in a cloud can only reach the first or second stage of the
transformation without extending to the other stages. At other times it
stops at the third phase, that of drizzling, which may then, as rain
does, cross atmospheric regions below the cloud, and reach the ground,
provided the base of the cloud is not too high and the air passed
through is not too dry. In short, we may conclude that the formation of
rain is due simply to variations in the temperature and moisture of the
air. There is, however, another element, the intervention of which is
indispensable, if not to reduce the vapor to water, at least to cause
that water to fall in rain, or under the form of drops. This element is
the atmospheric dust.
We designate generally as atmospheric dust all the corpuscles which the
atmospheric envelope of the earth holds in suspension; but distinctions
should be made. Some dust occurs in the air fortuitously and for the
moment, such as troubles us in dry weather when the wind is blowing.
This is coarse, and so evident that we say "It is dusty," and soon falls
by its weight to the ground. There is other dust which remains in the
air almost permanently. It becomes visible to the eye when illuminated
against a dark background, as when a sunbeam comes into a dark room.
Other dust may be studied under a microscope of low power; and still
other, and the largest proportion of that in the atmosphere, is so
fine that it can not be distinguished, even with the most powerful
instruments.
This extremely fine and light dust is disseminated to heights that may
exceed fifteen or twenty or more miles. Cyclones, volcanic eruptions,
and immense prairie fires are the principal causes of its production
and expansion in the atmosphere. Mr. Aitken, a Scotch meteorologist,
has made some remarkable experiments to demonstrate the existence of
this dust. For that purpose he employed a very ingenious method, which
permitted him to count all the particles, even those which could not be
seen with a microscope. The principle of his method is as follows: If
we fill a receiver with air that has been deprived of all its dust by
passing it through a liquid, and saturate it with vapor, and then by
cooling cause the vapor to condense, the resultant water is deposited
directly. If the receiver is filled with air not cleared of its dust,
the cooling of the mixture of air and vapor provokes first the formation
of a fog that marks the presence of dust, because each particle of dust
becomes a nucleus, a center of condensation, for the vapor. Finally,
if the cooling is carried far enough, the water formed falls in very
fine droplets, each one of which incloses a dust particle. Mr. Aitken
has succeeded in counting these droplets, by introducing only a very
small volume of dusty air into the receiver and finally filling it with
absolutely pure air. He has thus found that the external air contains
on the average 32,000 particles of dust per cubic centimetre after a
rain of considerable duration, and 130,000 particles in fine weather.
There are 1,860,000 particles in the same volume of air in the middle
of a room, and 5,420,000 particles near the ceiling. The figures look
fanciful, but they are exact, for they have been corroborated by
numerous consistent experiments and agree with the determinations that
have been made by other methods.
As to the formation of rain, it should be observed that absolutely pure
air can not give either fog or drops of water when it is supersaturated
with vapor. If there were no dust in the atmosphere we should have no
clouds or rain. The sky would always be clear, and the sun would shine
uninterruptedly as long as it was above the horizon. There would be no
dawn or twilight, and day and night would succeed one another instantly,
without transition. Atmospheric water would be deposited only when in
contact with things, as in Aitken's experiments, very much as dew is
deposited.
The causes of the formation of rain are evidently the same everywhere.
The secondary conditions change only according to climates; but they
vary so much that rains are distributed very unequally over the earth.
According to Desanis, the quantity of vapor contained in a column of
air as high as the atmosphere would give, in France, a layer of water
about four centimetres thick. Few rain storms would furnish so much;
but there are storms sometimes that give much more. On August 17, 1888,
seven centimetres of water fell at Clermont in five hours; and September
12, 1875, the pluviometer measured ten centimetres for the whole day.
Still more copious rains fall in some tropical countries; at Purneah, in
India, eighty-nine centimetres have fallen in twenty-four hours.
Mr. John Murray has calculated, from the charts of Elias Loomis, that
the quantity of rain falling every year over the whole earth would form
a bed of water averaging nine hundred and seventy millimetres in depth.
When we consider the annual quantities of rain in particular regions or
localities, we find the numbers exceedingly variable, and some of them
surprising. Clermont receives 630 millimetres, and the mean of the fall
in Europe is about the same. About one metre falls on the western coast
of Iceland, two metres in Norway, 2.80 metres in Scotland, 4.60 metres
at Vera Cruz, 5.20 metres at Buitenzorg, in the Dutch East Indies, 7.10
metres at Maranhão, Brazil, and 12.50 metres at Cherrapunji, in British
India. On the other hand, it rarely rains in some regions of the globe
north and south of the equator; as in the center of the Sahara and of
Arabia, the plateau of eastern Persia and Beluchistan, the desert of
Kalahari, and the desert of Atacama. The plains or pampas of the eastern
slopes of the Andes, in about 23° south latitude, are likewise subject
to extreme droughts, in one of which, lasting three years, three million
head of cattle perished.--_Translated for the Popular Science Monthly
from Ciel et Terre._
FOOTNOTE:
[12] An address before the Society of Horticulture and Viticulture at
Clermont-Ferrand.
DREAM AND REALITY.
BY M. CAMILLE MELINAND.
There is a very striking resemblance between dreams and waking
perceptions. We see in dreams objects, persons, and events identical
with those of the waking state. The belief in their reality is as
complete as in that of what we see when awake; the emotions are as deep
and vivid. Pleasures have a delicious savor, and pains are even more
intense than those of the reality--as, for instance, those of nightmare,
and the distresses to which we give ourselves up in full. In all cases
these dream troubles seem as real as those of life, and are taken by us
quite as seriously; and the existence of everything we see and feel is
as evident as in life.
Still we oppose the dream to the reality. The waking world is our true,
our only world; the world of the dream seems to us purely interior and
chimerical. The incoherence and absurdity of our dreams surprise and
amuse us, and we are amazed to find that we have been able to believe,
while asleep, in such foolish things. In short, dreaming is synonymous
to us with illusion, phantasmagoria, and falsehood. The clearest of
the prevailing theories about dreams rest upon the postulate that
waking perceptions are the true ones, and the visions of the dream are
false. They have answers to the three questions we are used to ask
concerning dreams--Where do they come from? why are they incoherent?
and why do we take their visions for realities? They explain dreams
as former sensations reviving within us under different combinations,
and as therefore simply confused reflexes of the reality. Dreams may,
however, sometimes be produced by a present impression suffered by one
of our senses, half awakened--a contact, the way we are lying, and the
condition of the organic functions being thus the causes or occasions
of dreams. The incoherence of dreams seems no more mysterious in these
theories, and is explained as the result of two causes--the slumber of
the "reflecting" faculties, judgment, reason, the will, the exercise of
choice and control; and, secondly, the unrestricted reign of imagination
and the association of ideas. Our faith in the reality of the things
dreamed is accounted for by the mechanical play of the images, the law
being set up that every image that is not opposed by stronger images
appears to us a real object. The problem, therefore, resolves itself:
the senses being asleep, the images that arise within us are not
contradicted by normal sensations, and that is why we take them for
realities. Further, our reflective faculties, being likewise dormant,
can not contradict the images, in the absence of sensations, reasonings,
or recollections. Hence a credence, as absolute as unreasonable. We
purpose to show that there is something artificial and prejudiced in the
classical theory of opposition between dreams and waking, which assigns
illusion, confusion, and incoherence to the former, and solid and
permanent reality to the latter, and that the difference between them is
not so clean cut.
Most persons in talking of this subject say that they are sure of the
reality of things when awake because their different senses concur in
attesting it. They see a tree, and satisfy themselves that it is a tree
by going up and touching it. They smell a rose, and go find the rose,
look at it and handle it; while in dreams we are not able to apply
these supplementary tests. The distinction is imaginary, for our senses
likewise seem to support one another in dreams. We dream not only that
we see an object, but also that we feel and hear it. When I dream of
meeting a friend, I believe that I see him and shake hands with him and
hear him speak. There is, therefore, a complete identity of the two
conditions as to this point, and the thing that appears to me in a dream
is a "bundle of sensations," visual, tactile, auditive, muscular, and
often olfactory, just as it appears to me when awake.
We are told of another difference. When awake, we find others agreeing
with us in recognizing the reality of things. I see a tree, and so
do those with me; I show it to them, and they look at it; I feel of
it, and they touch it; I hear the rustling of the leaves, and so do
they. Our perceptions in practical life are thus tested by comparison
with those of others, whereas in our dreams we have our solitary and
fanciful visions all within ourselves, with none to participate in our
perceptions of them.
This supposed contrast is no more real than the former one. What is
true is that when we are once awake we change our point of view, and our
vision of the night then seems to have been wholly interior, solitary,
and subjective. But, notwithstanding the common illusion, _while we are
dreaming_ affairs pass, to us, exactly as when we are awake. It is true
that in the waking state we find ourselves mingled with other men, who
perceive the same objects that we do. Do we not sometimes dream that we
are one of an audience looking at a play? that we are talking with a
friend, and exchange views with him? and that we understand one another
perfectly? There is, therefore, in this aspect, not a difference but
identity between the dream and the waking. The interior condition, the
sensation, the credence, are identical. The dreaming man believes, sees,
and feels himself in intercourse with his fellows, just as the man awake
believes, sees, and feels it. When we wake, we discover our mistake, but
what of that? It does not prevent us from believing completely in it
while we are asleep. And this is the point; for, after all, am I sure
that I shall not awake some day from what I now call my waking life? And
who knows whether I shall not then judge that I have been dreaming a
solitary dream? It may be added that the agreement of witnesses is not a
decisive sign by which to distinguish the reality from the dream. There
are collective hallucinations.
We come now to a more important difference, which includes the principle
and has a characteristic apparently essentially distinguishing the
dream--its looseness, disorder, inconstancy, and incoherence. In the
dream visions succeed one another without connection; no law determines
their order; an unrestricted fancy reigns among them, and the normal is
broken up in them at every point. We are transported instantaneously
from one country to another. We pass without transition from childhood
to age, and causes have the strangest effects. The most essential laws
of thought are constantly violated. There are facts without any causes,
metamorphoses, magical disappearances. Even the absurd is realized,
and the "principle of contradiction" does not seem to be any more
respected than the others. We are at the same time in two places; we
pronounce words, we hold conversations of which we can not when we
wake recover the thread, so strange is their logic, so fugitive the
sense, and so fanciful the combination. A practiced psychologist, M.
Delbœuf, succeeded in taking down in the morning the last phrase of a
book which he had been reading in a dream, and which had seemed then
remarkably lucid. Here it is: "The man raised by the woman and separated
by aberrations pushes facts disengaged by the analysis of the tertiary
nature into the way of progress."