himself; for it followed from his observations, that such obstructions
as the propped board and the pasteboard roof _kept in the heat_. It
followed also, from the observed effects of clear skies, that clouds
_keep in the heat_. Now, what sort of heat is that which is prevented
from escaping by the interference of screens, whether material or
vaporous? There are three processes by which heat is transmitted from
one body to another,—these are, conduction, convection, and radiation.
The first is the process by which objects in contact communicate
their heat to each other, or by which the heat in one part of a body
is gradually transmitted to another part. The second is the process
by which heat is carried from one place to another by the absolute
transmission of heated matter. The third is that process by which heat
is spread out in all directions, in the same manner as light. A little
consideration will show that the last process is that with which we
are alone concerned; and this important result flows from Dr. Wells’
experiments, that _the rate of the deposition of dew depends on the
rate at which bodies part with their heat by radiation_. If the process
of radiation is checked, dew is less copiously deposited, and _vice
versâ_.
When we consider the case of heat accompanied by light, we understand
readily enough that a screen may interfere with the emission of
radiant heat. We use a fire-screen, for instance, with the object of
producing just such an interference. But we are apt to forget that
what is true of luminous heat is true also of that heat which every
substance possesses. In fact, we do not meet with many instances in
which the effect of screens in preventing the loss of obscure heat is
very noteworthy. There are some, as the warmth of a green-house at
night, and so on; but they pass unnoticed, or are misunderstood. It
was in this way that the explanation of dew-phenomena had been so long
delayed. The very law on which it is founded had been _practically_
applied, while its meaning had not been recognized. “I had often
in the pride of half-knowledge,” says Wells, “smiled at the means
frequently employed by gardeners to protect tender plants from cold,
as it appeared to me impossible that a thin mat, or any such flimsy
substance, could prevent them from attaining the temperature of the
atmosphere, by which alone I thought them liable to be injured. But
when I had seen that bodies on the surface of the earth become, during
a still and serene night, colder than the atmosphere, by radiating
their heat to the heavens, I perceived immediately a just reason for
the practice which I had before deemed useless.”
And now all the facts which had before seemed obscure were accounted
for. It had been noticed that metallic plates were often dry when
grass or wood was copiously moistened. Now, we know that metals part
unwillingly with their heat by radiation, and therefore the temperature
of a metal plate exposed in the open air is considerably higher than
that of a neighbouring piece of wood. For a similar reason, dew is
more freely deposited on grass than on gravel. Glass, again, is a
good radiator, so that dew is freely deposited on glass objects,—a
circumstance which is very annoying to the telescopist. The remedy
employed is founded on Wells’ observations—a cylinder of tin or card,
called a dew-cap, is made to project beyond the glass, and thus to act
as a screen, and prevent radiation.
We can now also interpret the effects of a clear sky. Clouds act the
part of screens, and check the emission of radiant heat from the earth.
This fact has been noticed before, but misinterpreted, by Gilbert White
of Selborne. “I have often observed,” he says, “that cold seems to
descend from above; for when a thermometer hangs abroad on a frosty
night, the intervention of a cloud shall immediately raise the mercury
ten degrees, and a clear sky shall again compel it to descend to its
former gauge.” Another singular mistake had been made with reference
to the power which clouds possess of checking the emission of radiant
heat. It had been observed that on moonlit nights the eyes are apt to
suffer in a peculiar way, which has occasionally brought on temporary
blindness. This had been ascribed to the moon’s influence, and the term
moon-blindness had therefore been given to the affection. In reality,
the moon has no more to do with this form of blindness than the stars
have to do with the formation of dew. The absence of clouds from the
air is the true cause of the mischief. There is no sufficient check
to the radiation of heat from the eyeballs, and the consequent chill
results in temporary loss of sight, and sometimes even in permanent
injury.
Since clouds possess this important power, it is clear that while they
are present in the air there can never be a copious formation of dew,
which requires, as we have seen, a considerable fall in the temperature
of the air around the place of deposition. When the air is clear,
however, radiation proceeds rapidly, and therefore dew is freely formed.
But it might seem that since objects in the upper regions of the air
part with their radiant heat more freely than objects on the ground,
the former should be more copiously moistened with dew than the latter.
That the fact is exactly the reverse is thus explained. The cold
which is produced by the radiation of heat from objects high in the
air is communicated to the surrounding air, which, growing heavier,
descends towards the ground, its place being supplied by warmer air.
Thus the object is prevented from reducing the air in its immediate
neighbourhood to so low a temperature as would be attained if this
process of circulation were checked. Hence, a concave vessel placed
below an object high in air, would serve to increase the deposition
of dew by preventing the transfer of the refrigerated air. We are not
aware that the experiment has ever been tried, but undoubtedly it would
have the effect we have described. An object on the ground grows cold
more rapidly, because the neighbouring air cannot descend after being
chilled, but continues in contact with the object; also cold air is
continually descending from the neighbourhood of objects higher in
air which are parting with their radiant heat, and the cold air thus
descending takes the place of warmer air, whose neighbourhood might
otherwise tend to check the loss of heat in objects on the ground.
Here, also, we recognize the cause of the second peculiarity detected
by Aristotle—namely, that dew is only formed copiously in serene
weather. When there is wind, it is impossible that the refrigerated air
around an object which is parting with its radiant heat, can remain
long in contact with the object. Fresh air is continually supplying the
place of the refrigerated air, and thus the object is prevented from
growing so cold as it otherwise would.
In conclusion, we should wish to point out the important preservative
influence exercised during the formation of dew. If the heat which is
radiated from the earth, or from objects upon it, during a clear night,
were not repaired in any way, the most serious injury would result to
vegetation. For instance, if the sun raised no vapour during the day,
so that when night came on the air was perfectly dry, and thus the
radiant heat passed away into celestial space without compensation, not
a single form of vegetation could retain its life during the bitter
cold which would result. But consider what happens. The sun’s heat,
which has been partly used up during the day in supplying the air
with aqueous vapour, is gradually given out as this vapour returns to
the form of water. Thus the process of refrigeration is effectually
checked, and vegetation is saved from destruction. There is something
very beautiful in this. During the day, the sun seems to pour forth his
heat with reckless profusion, yet all the while it is being silently
stored up; during the night, again, the earth seems to be radiating
her heat too rapidly into space, yet all the while a process is going
on by which the loss of heat is adequately compensated. Every particle
of dew which we brush from the blades of grass, as we take our morning
rambles, is an evidence of the preservative action of nature.
_THE LEVELLING POWER OF RAIN._
It has been recognized, ever since geology has become truly a science,
that the two chief powers at work in remodelling the earth’s surface,
are fire and water. Of these powers one is in the main destructive, and
the other preservative. Were it not for the earth’s vulcanian energies,
there can be no question that this world would long since have been
rendered unfit for life,—at least of higher types than we recognize
among sea creatures. For at all times igneous causes are at work,
levelling the land, however slowly; and this not only by the action
of sea-waves at the border-line between land and water, but by the
action of rain and flood over inland regions. Measuring the destructive
action of water by what goes on in the lifetime of a man, or even
during many successive generations, we might consider its effects very
slight, even as on the other hand we might underrate the effects of the
earth’s internal fires, were we to limit our attention to the effects
of upheaval and of depression (not less preservative in the long run)
during a few hundreds or thousands of years. As Lyell has remarked in
his “Principles of Geology,” “our position as observers is essentially
unfavourable when we endeavour to estimate the nature and magnitude of
the changes now in progress. As dwellers on the land, we inhabit about
a fourth part of the surface; and that portion is almost exclusively
a theatre of decay, and not of reproduction. We know, indeed, that
new deposits are annually formed in seas and lakes, and that every
year some new igneous rocks are produced in the bowels of the earth,
but we cannot watch the progress of their formation; and as they are
only present to our minds by the aid of reflection, it requires an
effort both of the reason and the imagination to appreciate duly their
importance.” But that they are actually of extreme importance, that in
fact all the most characteristic features of our earth at present are
due to the steady action of these two causes, no geologist now doubts.
I propose now to consider one form in which the earth’s aqueous
energies effect the disintegration and destruction of the land. The sea
destroys the land slowly but surely, by beating upon its shores and by
washing away the fragments shaken down from cliffs and rocks, or the
more finely divided matter abstracted from softer strata. In this work
the sea is sometimes assisted by the other form of aqueous energy—the
action of rain. But in the main, the sea is the destructive agent by
which shore-lines are changed. The other way in which water works the
destruction of the land affects the interior of land regions, or only
affects the shore-line by removing earthy matter from the interior of
continents to the mouths of great rivers, whence perhaps the action of
the sea may carry it away to form shoals and sandbanks. I refer to the
direct and indirect effects of the downfall of rain. All these effects,
without a single exception, tend to level the surface of the earth. The
mountain torrent whose colour betrays the admixture of earthy fragments
is carrying those fragments from a higher to a lower level. The river
owes its colour in like manner to earth which it is carrying down to
the sea level. The flood deposits in valleys matter which has been
withdrawn from hill slopes. Rainfall, acts, however, in other ways, and
sometimes still more effectively. The soaked slopes of great hills give
way, and great landslips occur. In winter the water which has drenched
the land freezes, in freezing expands, and then the earth crumbles and
is ready to be carried away by fresh rains; or when dry, by the action
even of the wind alone. Landslips, too, are brought about frequently in
the way, which are even more remarkable than those which are caused by
the unaided action of heavy rainfalls.
The most energetic action of aqueous destructive forces is seen when
water which has accumulated in the higher regions of some mountain
district breaks its way through barriers which have long restrained it,
and rushes through such channels as it can find or make for itself into
valleys and plains at lower levels. Such catastrophes are fortunately
not often witnessed in this country, nor when seen do they attain the
same magnitude as in more mountainous countries. It would seem, indeed,
as though they could attain very great proportions only in regions
where a large extent of mountain surface lies above the snow-line.
The reason why in such regions floods are much more destructive than
elsewhere will readily be perceived if we consider the phenomena of one
of these terrible catastrophes.
Take, for instance, the floods which inundated the plains of Martigny
in 1818. Early in that year it was found that the entire valley of the
Bagnes, one of the largest side-valleys of the great valley of the
Rhône, above Geneva, had been converted into a lake through the damming
up of a narrow outlet by avalanches of snow and ice from a loftier
glacier overhanging the bed of the river Dranse. The temporary lake
thus formed was no less than half a league in length, and more than
200 yards wide, its greatest depth exceeding 200 feet. The inhabitants
perceived the terrible effects which must follow when the barrier
burst, which it could not fail to do in the spring. They, therefore,
cut a gallery 700 feet long through the ice, while as yet the water
was at a moderate height. When the waters began to flow through this
channel, their action widened and deepened it considerably. At length
nearly half the contents of the lake were poured off. Unfortunately,
as the heat of the weather increased, the middle of the barrier slowly
melted away, until it became too weak to withstand the pressure of
the vast mass of water. Suddenly it gave way; and so completely that
all the water in the lake rushed out in half an hour. The effects of
this tremendous outrush of the imprisoned water were fearful. “In
the course of their descent,” says one account of the catastrophe,
“the waters encountered several narrow gorges, and at each of these
they rose to a great height, and then burst with new violence into the
next basin, sweeping along forests, houses, bridges, and cultivated
land.” It is said by those who witnessed the passage of the flood at
various parts of its course, that it resembled rather a moving mass
of rock and mud than a stream of water. “Enormous masses of granite
were torn out of the sides of the valleys, and whirled for hundreds
of yards along the course of the flood.” M. Escher the engineer tells
us that a fragment thus whirled along was afterwards found to have a
circumference of no less than sixty yards. “At first the water rushed
on at a rate of more than a mile in three minutes, and the whole
distance (forty-five miles) which separates the Valley of Bagnes from
the Lake of Geneva was traversed in little more than six hours. The
bodies of persons who had been drowned in Martigny were found floating
on the further side of the Lake of Geneva, near Vevey. Thousands of
trees were torn up by the roots, and the ruins of buildings which had
been overthrown by the flood were carried down beyond Martigny. In
fact, the flood at this point was so high, that some of the houses in
Martigny were filled with mud up to the second story.”
It is to be noted respecting this remarkable flood, that its effects
were greatly reduced in consequence of the efforts made by the
inhabitants of the lower valleys to make an outlet for the imprisoned
waters. It was calculated by M. Escher that the flood carried down
300,000 cubic feet of water every second, an outflow five times as
great as that of the Rhine below Basle. But for the drawing off of the
temporary lake, the flood, as Lyell remarks, would have approached in
volume some of the largest rivers in Europe. “For several months after
the _débâcle_ of 1818,” says Lyell, “the Dranse, having no settled
channel, shifted its position continually from one side to the other of
the valley, carrying away newly erected bridges, undermining houses,
and continuing to be charged with as large a quantity of earthy
matter as the fluid could hold in suspension. I visited this valley
four months after the flood, and was witness to the sweeping away of
a bridge and the undermining of part of a house. The greater part of
the ice-barrier was then standing, presenting vertical cliffs 150 feet
high, like ravines in the lava-currents of Etna, or Auvergne, where
they are intersected by rivers.” It is worthy of special notice that
inundations of similar or even greater destructiveness have occurred in
the same region at former periods.
It is not, however, necessary for the destructive action of floods
in mountain districts that ice and snow should assist, as in the
Martigny flood. In October, 1868, the cantons of Tessin, Grisons, Uri,
Valois, and St. Gall, suffered terribly from the direct effects of
heavy rainfall. The St. Gothard, Splugen, and St. Bernhardin routes
were rendered impassable. In the former pass twenty-seven lives were
lost, besides many horses and waggons of merchandise. On the three
routes more than eighty persons in all perished. In the small village
of Loderio alone, no less than fifty deaths occurred. The damage in
Tessin was estimated at £40,000. In Uri and Valois large bridges were
destroyed and carried away. Everything attested the levelling power
of rain; a power which, when the rain is falling steadily on regions
whence it as steadily flows away, we are apt to overlook.
It is not, however, necessary to go beyond our own country for
evidence of the destructive action of water. We have had during the
past few years very striking evidence in this respect, which need
scarcely be referred to more particularly here, because it will be
in the recollection of all our readers. Looking over the annals of
the last half-century only, we find several cases in which the power
of running water in carrying away heavy masses of matter has been
strikingly shown. Consider, for instance, the effects of the flood in
Aberdeenshire and the neighbouring counties, early in August, 1829. In
the course of two days a great flood extended itself over “that part of
the north-east of Scotland which would be cut off by two lines drawn
from the head of Loch Rannoch, one towards Inverness and the other to
Stonehaven.” The total length of various rivers in this region which
were flooded amounted to between 500 and 600 miles. Their courses were
marked everywhere by destroyed bridges, roads, buildings, and crops.
Sir T. D. Lauder records “the destruction of thirty-eight bridges, and
the entire obliteration of a great number of farms and hamlets. On the
Nairn, a fragment of sandstone fourteen feet long by three feet wide
and one foot thick, was carried about 200 yards down the river. Some
new ravines were formed on the sides of mountains where no streams had
previously flowed, and ancient river channels, which had never been
filled from time immemorial, gave passage to a copious flood.” But
perhaps the most remarkable effect of these inundations was the entire
destruction of the bridge over the Dee at Ballater. It consisted of
five arches, spanning a waterway of 260 feet. The bridge was built of
granite, the pier, resting on rolled pieces of granite and gneiss.
We read that the different parts of this bridge were swept away in
succession by the flood, the whole mass of masonry disappearing in the
bed of the river. Mr. Farquharson states that on his own premises the
river Don forced a mass of 400 or 500 tons of stones, many of them of
200 or 300 pounds’ weight, up an inclined plane, rising six feet in
eight or ten yards, and left them in a rectangular heap about three
feet deep on a flat ground, the heap ending abruptly at its lower
extremity.” At first sight this looks like an action the reverse of
that levelling action which we have here attributed to water. But in
reality it indicates the intense energy of this action; which drawing
heavy masses down along with swiftly flowing water, communicates to
them so great a momentum, that on encountering in their course a rising
slope, they are carried up its face and there left by the retreating
flood. The rising of these masses no more indicates an inherent
uplifting power in running water, than the ascent of a gently rising
slope by a mass which has rolled headlong down the steep side of a
hill indicates an upward action exerted by the force of gravity.
Even small rivers, when greatly swollen by rain, exhibit great energy
in removing heavy masses. Thus Lyell mentions that in August, 1827, the
College, a small river which flows down a slight declivity from the
eastern watershed of the Cheviot Hills, carried down several thousand
tons’ weight of gravel and sand to the plain of the Till. This little
river also carried away a bridge then in process of building, “some of
the arch stones of which, weighing from half to three-quarters of a ton
each, were propelled two miles down the rivulet.” “On the same occasion
the current tore away from the abutment of a mill-dam a large block
of greenstone porphyry, weighing nearly two tons, and transported it
to a distance of nearly a quarter of a mile. Instances are related as
occurring repeatedly, in which from 1000 to 3000 tons of gravel are in
like manner removed by this streamlet to still greater distances in one
day.”
It may appear, however, to the reader that we have in such instances
as these the illustration of destructive agencies which are of their
very nature limited within very narrow areas. The torrent, or even
the river, may wear out its bed or widen it, but nevertheless can
hardly be regarded as modifying the aspect of the region through which
it flows. Even in this respect, however, the destructive action of
water is not nearly so limited as it might appear to be. Taking a few
centuries or a few thousand years, no doubt, we can attribute to the
action of rivers, whether in ordinary flow or in flood, little power
of modifying the region which they drain. But taking that wider survey
(in time) of fluviatile work which modern science requires, dealing
with this form of aqueous energy as we deal with the earth’s vulcanian
energies, we perceive that the effects of river action in the course of
long periods of time are not limited to the course which at any given
time a river may pursue. In carrying down material along its course
to the sea, a river is not merely wearing down its own bed, but is so
changing it that in the course of time it will become unfit to drain
the region through which it flows. Its bottom must of necessity become
less inclined. Now although it will then be lower than at present,
and therefore be then even more than now the place to which the water
falling upon the region traversed by the river will naturally tend,
it will no longer carry off that water with sufficient velocity.
Three consequences will follow from this state of things. In the
first place there will be great destruction in the surrounding region
through floods because of inadequate outflow; in the second place, the
overflowing waters will in the course of time find new channels, or in
other words new rivers will be formed in this region; thirdly, owing to
the constant presence of large quantities of water in the depressed bed
of the old river, the banks on either side will suffer, great landslips
occurring and choking up its now useless channel. Several rivers are
undergoing these changes at the present time, and others, which are
manifestly unfit for the work of draining the region through which they
flow (a circumstance attested by the occurrence of floods in every wet
season), must before long be modified in a similar way.