We were surprised very much some time ago at considering how much of
the surface of the globe is covered by the waters of the lakes and
oceans, and took the opportunity then of adverting to the importance
of water in the general economy of nature. When, however, we pass to
the consideration of the magnitude of the earth itself, the relative
proportion of water appears to be much less considerable.
Although there are many places in the great Atlantic and Pacific Oceans
where the depth of water is very great, yet it has been deduced from
principles that are not liable to much error, that the general or average
depth does not exceed three miles. It may appear very strange that we
can assert any thing positive about the depth of water in those seas,
that are to the lines used for sounding quite unfathomable; but it is
effected very simply. Every person has seen a wave advancing along the
level surface of a canal, and by observing with a watch, it could easily
be found to move more quickly at some times than at others. The deeper
any part of the canal is, the more rapidly does the wave move along; and
partly by experiment, and partly by reasoning, the connection between the
depth of the water and velocity of the wave has been discovered. Now,
the tide which comes to Dublin every twelve hours is produced by the
influence of the sun and moon on the vast body of water in the Southern
Pacific Ocean; and the great wave there formed turns round Cape Horn,
and passes up the Atlantic Ocean, to arrive at the coasts of Europe and
North America. The time occupied by this great wave in passing from one
end to the other of the Atlantic can thus be known, and, precisely as in
a canal, the depth of water thus calculated.
The circumference of the earth at its widest part is about 25,000, and
its diameter 8000 miles. Hence the sheet of water which constitutes
the ocean forms but 3-4000ths of its thickness, or nearly the same
proportion as if we took an eighteen inch globe, and having spilled
water on its surface, allowed all the excess of water to drain off. The
remaining wetness would represent pretty nearly the condition of the
waters of the ocean on the surface of the earth. By this means we can
form, though obscurely, to our minds, an idea of the great magnitude of
the earth itself. This magnitude renders also very inconsiderable those
inequalities on the surface of the earth which constitute our highest
ridges of mountains. A true model of Mont Blanc, the highest of European
mountains, if constructed on the eighteen inch globe before referred to,
would be unfelt by a finger drawn along its surface, and it would be
only some of the highest peaks of the Andes and Himalayah that could be
distinctly felt. Where man also employs his most gigantic energies and
greatest efforts of skill to penetrate below the surface, forming mines
by which the supplies of coal, of iron, of copper, and other minerals,
have been obtained from the earliest times, the cavities that he makes
can only be compared with the trace given by the point of a pin that had
lightly touched the globe, and which would require a favourable incidence
of light to see.
The earth is therefore almost perfectly a smooth and solid ball. It is,
however, almost certain that it was not always solid. It is, on the
contrary, almost certain that at a period far exceeding in remoteness
any time of which mere human indications can be found, the globe of the
earth was one mass of liquid matter, heated to a degree exceeding our
most intense fires, and wherein were melted all together the various
elements which have since arranged themselves into their present forms.
From having been thus liquid, the earth, which, revolving on its axis,
produces by the side it turns to the sun the alternating day and night,
has bulged out where the rotation of the surface is most rapid, at the
equator, and has become flattened at the extremities of its axis, at the
poles, just as a thin hoop which we spin round becomes compressed. The
amount of this flattening is however very small. The equatorial diameter
of the earth being accurately 7925, and the polar diameter being 7898,
the compression is 27 miles.
To account for the existence of this compression, the earth must have
been originally liquid, for otherwise the rotation on its axis could not
have generated this regular form. If it had been solid when it began to
revolve, it should either have retained its original form, or it should
have broken in pieces; but certainly unless it had been liquid, it could
not have arrived at the exact degree of flattening which its velocity of
rotation should have produced in a liquid mass.
The intensely heated and liquid earth, revolving in the cold and empty
spaces of the planetary system, gradually must have lost its excess
of heat. Cooling most rapidly at the surface, it there solidified,
and generated the first rocks. The loss of heat still going on, the
solidification proceeded to a greater and greater depth, and should
ultimately have reduced the earth to the same temperature as the empty
space among the stars. The temperature of space has been calculated
to be almost the same as that in the winter at Melville Island, in
northernmost America, that is, 56 deg. below zero, or as far below the
freezing point of water as the temperature of the hottest water that the
hand can bear is above it. The earth is, however, not allowed to cool to
that degree. It receives from the sun by radiation a quantity of heat
which counteracts its tendency to cool, and hence the mean temperature
of the surface of the earth has remained the same from the earliest
historical epochs. In fact, at the surface we can find no trace of that
original and internal great heat, the temperature of the surface of the
earth being regulated altogether by the effect of the sun’s rays; but
if we dig down to a moderate depth, about 45 feet, the influence of the
sun becomes insensible. Within that space also we can detect a very
curious and important arrangement of the heat. It is not that the whole
surface becomes warmed in summer and cold in winter, but the heat which
is received from the sun in one summer travels by conduction beneath the
surface, and is succeeded by the heat of the next summer, an intervening
and cooler layer corresponding to the winter time, so that at a depth of
20 feet we may detect the heat which had fallen upon the surface four or
five years before, this space of 45 feet being formed of numerous layers
like the coatings of an onion, one for each year, until becoming less and
less distinct, according as the depth increases, they join together in
forming the layer of invariable temperature in which all the effect of
the sun’s heat is lost.
If we dig down still farther, the earth, though having lost the heating
power of the sun, becomes sensibly warmer. The greater the depth to
which we descend, the higher is the temperature found to be. Thus, where
deep sinkings have been made for mines or wells, the air or water at the
bottom is found to be much higher in temperature than at the invariable
layer which gives the mean temperature of the place. This observation was
first made in the case of the deep mines in Cornwall, and, although for
some time ascribed to the presence of the workmen and the burning lamps,
has since been verified by observations in all parts of Europe, and such
agreement found, that the law connecting the temperature with the depth
has been at least approximately determined.
It is found, counting from the invariable layer, that the temperature
increases about one degree of Fahrenheit’s scale for every fifty feet in
depth. Thus, a well having been sunk at Rudersdorff to a depth of 630
feet, the water at the bottom was found to be 67 degrees, while the mean
temperature was 50 degrees. In a coal mine at Newcastle, which reaches
to a depth of 1584 feet, the mean temperature of the surface being 48
degrees, the thermometer was found to stand at 73 degrees in the lowest
part of the mine, and hence the elevation of temperature was 25 degrees.
Observations elsewhere vary between these limits; but the general result
is, that the rise is one degree for about every fifty feet, as above
stated.
When we consider the great magnitude of the earth, and observe the
rapidity with which the increase of temperature occurs, it will strike
every person that we in reality inhabit a mere pellicle or skin, which
has formed by cooling upon the surface, whilst all the internal mass of
our globe may still be in the same state of igneous fusion and tumultuous
action of elements, from which its present mineral constitution on the
surface has resulted. For although it has cooled so far that at the
surface all traces of its central fires have disappeared, yet at a mile
and a half below the surface the temperature is such as should boil
water: at a depth of five miles, lead would melt. Thirty miles below the
surface, cast iron, and all those rocks which are generally the product
of volcanoes in action, as trap and basalt, would fuse; and hence we may
consider those terrific phenomena which have so frequently desolated some
of the most beautiful districts of the earth, as being minute apertures
or cracks in the thin coating of our planet, and giving vent from time to
time to some small portions of the internal fires which work beneath.
Additional evidence of the existence of this central heat may be derived
from the peculiarity of springs. Those springs which carry off and are
supplied with water from the surface, change their temperature with the
season, being in winter cold, but in summer warm. Others, deriving their
waters from a deeper layer of soil, as from the stratum of constant heat,
are always the same, and, possessing the mean temperature of the place,
feel warm in winter and cold in summer. Such springs exist in every
country, and are very useful in ascertaining the mean temperature, for
in place of watching a thermometer for a year, and taking averages, it
is only necessary to select with proper caution such a deeply supplied
spring, and by observing the temperature of its waters, the mean
temperature of the place is found.
A certain quantity of the water which is absorbed by the ground after
rain must penetrate to a great depth, must descend, in fact, until at 1½
miles it is boiled and driven up again to find some outlet as a spring.
In rising up, however, it is for the most part cooled; but having charged
itself with various saline and metallic bodies, under the most favourable
circumstances of high temperature and pressure, it issues as a hot
mineral spring or spa. On getting into the air, it generally abandons a
great part of what it had dissolved, and forms in many cases enormous
depositions of solid rock.
A company in Paris have formed the idea of using the water thus heated by
the powers below, for the purposes of public baths. The neighbourhood of
Paris is peculiarly fitted for what are termed Artesian wells, in which
the water often rises considerably above the surface of the ground. Under
the auspices of this company, a well has been sunk already to the depth
of 1600 feet, and water obtained at 77 degrees; but to obtain natural
hot water at a temperature of 100 degrees, which would be required for
bathing purposes, an additional depth of probably as much more will be
required. It is said the projectors are not now sanguine of its pecuniary
success.
* * * * *
THE SECRET OF SUCCESS IN LIFE.--In no department of life do men rise
to eminence who have not undergone a long and diligent preparation;
for whatever be the difference in the mental power of individuals, it
is the cultivation of the mind alone that leads to distinction. John
Hunter was as remarkable for his industry as for his talents, of which
his museum alone forms a most extraordinary proof; and if we look around
and contemplate the history of those men whose talents and acquirements
we must esteem, we find that their superiority of knowledge has been
the result of great labour and diligence. It is an ill-founded notion
to say that merit in the long-run is neglected. It is sometimes joined
to circumstances that may have a little influence in counteracting it,
as an unfortunate manner and temper; but generally it meets with its
due reward. The world are not fools--every person of merit has the best
chance of success; and who would be ambitious of public approbation, if
it had not the power of discriminating?--_Physic and Physicians._
* * * * *
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