them motion, and of which the tropical waters are deprived. If, dipping
one hand into a basin filled with tropical water at 80° and the other
into one filled with polar water at 32°, we refer to our _sensations_,
we call the water in the one _hot_ and that in the other _cold_; but
so far as the water itself is concerned heat and cold simply mean
difference in the amounts of heat possessed. Both the polar and the
tropical water possess a certain amount of energy in the form of heat,
only the polar water does not possess so much of it as the tropical.
How, then, according to Dr. Carpenter, does polar cold impart motion
to the water? The warm water flowing in upon the polar column becomes
chilled by cold, but it is not cooled below that of the water
underneath; for, according to Dr. Carpenter, the ocean in polar regions
is as cold and as dense underneath as at the surface. The cooled
surface-water does not sink through the water underneath, like the
surface-water of a pond chilled during a frosty night. “The descending
motion in column C will not consist,” he says, “in a successional
descent of surface-films from above downwards, but it will be a
downward movement of the _entire mass_, as if water in a tall jar
were being drawn off through an orifice at the bottom” (§ 29). There
is a downward motion of the entire column, producing an outflow of
water at the bottom towards the equatorial column W, which outflow is
compensated by an inflow from the top of the equatorial column to the
top of the polar column C. But what causes column C to descend? The
cause of the descent is its excess of weight over that of column W.
Column C descends and column W ascends, for the same reason that in
a balance the heavy scale descends and the light scale rises. Column
C descends not simply because it is cold, but because it is _colder_
than column W. Column C descends not simply because in consequence of
being cold it is dense and therefore heavy, but because in consequence
of being cold it is _denser_ and therefore _heavier_ than column W.
It might be as cold as frozen mercury and as heavy as lead; but it
would not on that account descend unless it were heavier than column
W. The descent of column C and ascent of column W, and consequently
the general oceanic circulation, results, therefore, according to Dr.
Carpenter’s explanation, from the _difference_ in the weights of the
two columns; and the difference in the weights of the two columns
results from their _difference_ of density; and the difference of
density of the two columns in turn results from their _difference_ of
temperature. But it has already been proved that the difference of
temperature between the polar and equatorial columns depends wholly on
the difference in the amount of heat received by each from the sun. The
equatorial column W possesses more heat than the polar column C, solely
because it receives more heat from the sun than column C. Consequently
Dr. Carpenter’s statement that the circulation is produced by polar
cold rather than by equatorial heat, is just as much in contradiction
to his own theory as it is to the principles of mechanics. Again, his
admission that the general oceanic circulation “cannot be produced by
the application of heat to the surface,” is virtually a giving up the
whole point in debate; for according to his gravitation theory, and
every form of that theory, the circulation results from _difference_ of
temperature between equatorial and polar seas; but this difference, as
we have seen, is entirely owing to the difference in the amount of heat
received from the sun at these two places. The heat received, however,
is “surface-heat;” for it is at the surface that the ocean receives all
its heat from the sun; and consequently if surface-heat cannot produce
the effect required, nothing else can.
_M. Dubuat’s Experiments._—Referring to the experiments of M. Dubuat
adduced by me to show that water would not run down a slope of 1
in 1,820,000,[82] he says, “Now the experiments of M. Dubuat had
reference, not to the slow restoration of level produced by the motion
of water on itself, but to the sensible movement of water flowing over
solid surfaces and retarded by its friction against them” (§ 22).
Dr. Carpenter’s meaning, I presume, is that if the incline consist
of any solid substance, water will not flow down it; but if it be
made of _water_ itself, _water_ will flow down it. But in M. Dubuat’s
experiments it was only the molecules in actual _contact_ with the
solid incline that could possibly be retarded by friction against it.
The molecules not in contact with the solid incline evidently rested
upon an _incline of water_, and were at perfect liberty to roll down
that incline if they chose; but they did not do so; and consequently M.
Dubuat’s experiment proved that water will not flow over itself on an
incline of 1 in 1,000,000.
_A Begging of the Question at Issue._—“It is to be remembered,” says
Dr. Carpenter, “that, however small the original amount of movement
may be, a _momentum_ tending to its continuance _must_ be generated
from the instant of its commencement; so that if the initiating force
be in constant action, there will be a _progressive acceleration_ of
its rate, until the increase of resistance equalises the tendency to
further acceleration. Now, if it be admitted that the propagation of
the disturbance of equilibrium from one column to another is simply
_retarded_, _not_ prevented, by the viscosity of the liquid, I cannot
see how the conclusion can be resisted, that the constantly maintained
difference of gravity between the polar and equatorial columns really
acts as a _vis viva_ in maintaining a circulation between them” (§ 35).
If it be true, as Dr. Carpenter asserts, that in the case of the
general oceanic circulation advocated by him “viscosity” simply
_retards_ motion, but does not _prevent_ it, I certainly agree with him
“that the constantly maintained difference of gravity between the polar
and equatorial columns really acts as a _vis viva_ in maintaining a
circulation between them.” But to assert that it merely retards, but
does not prevent, motion, is simply _begging the question at issue_.
It is an established principle that if the _force_ resisting motion be
greater than the force tending to produce it, then no motion can take
place and no work can be performed. The experiments of M. Dubuat prove
that the _force_ of the molecular resistance of water to motion is
_greater_ than the _force_ derived from a slope of 1 in 1,000,000; and
therefore it is simply begging the question at issue to assert that it
is _less_. The experiments of MM. Barlow, Rainey, and others, to which
he alludes, are scarcely worthy of consideration in relation to the
present question, because we know nothing whatever regarding the actual
amount of force producing motion of the water in these experiments,
further than that it must have been enormously greater than that
derived from a slope of 1 in 1,000,000.
_Supposed Argument from the Tides._—Dr. Carpenter advances Mr.
Ferrel’s argument in regard to the tides. The power of the moon to
disturb the earth’s water, he asserts, is, according to Herschel,
only 1/11,400,000th part of gravity, and that of the sun not over
1/25,736,400th part of gravity; yet the moon’s attractive force, even
when counteracted by the sun, will produce a rise of the ocean. But as
the disturbance of gravity produced by difference of temperature is far
greater than the above, it ought to produce circulation.
It is here supposed that the force exerted by gravity on the ocean,
resulting from difference of temperature, tending to produce the
general oceanic circulation, is much greater than the force exerted
on the ocean by the moon in the production of the tides. But if we
examine the subject we shall find that the opposite is the case. The
attraction of the moon tending to lift the waters of the ocean acts
directly on every molecule from the surface to the bottom; but the
force of gravity tending to produce the circulation in question acts
directly on only a portion of the ocean. Gravity can exercise no direct
force in impelling the underflow from the polar to the equatorial
regions, nor in raising the water to the surface when it reaches the
equatorial regions. Gravity can exercise no direct influence in pulling
the water horizontally along the earth’s surface, nor in raising it
up to the surface. The pull of gravity is always _downwards_, never
_horizontally_ nor upwards. Gravity will tend to pull the surface-water
from the equator to the poles because here we have _descent_. Gravity
will tend to sink the polar column because here also we have _descent_.
But these are the only parts of the circuit where gravity has any
tendency to produce motion. Motion in the other parts of the circuit,
viz., along the bottom of the ocean from the poles to the equator and
in raising the equatorial column, is produced by the _pressure_ of the
polar column; and consequently it is only _indirectly_ that gravity may
be said to produce motion in those parts. It is true that on certain
portions of the ocean the force of gravity tending to produce motion is
greater than the force of the moon’s attraction, tending to produce the
tides; but this portion of the ocean is of inconsiderable extent. The
total force of gravity acting on the entire ocean tending to produce
circulation is in reality prodigiously less than the total force of the
moon tending to produce the tides.
It is no doubt a somewhat difficult problem to determine accurately
the total amount of force exercised by gravity on the ocean; but for
our present purpose this is not necessary. All that we require at
present is a very rough estimate indeed. And this can be attained by
very simple considerations. Suppose we assume the mean depth of the
sea to be, say, three miles. The mean depth may yet be found to be
somewhat less than this, or it may be found to be somewhat greater;
a slight mistake, however, in regard to the mass of the ocean will
not materially affect our conclusions. Taking the depth at 3 miles,
the force or direct pull of gravity on the entire waters of the ocean
tending to the production of the general circulation will not amount to
more than 1/24,000,000,000th that of gravity, or only about 1/2,100th
that of the attraction of the moon in the production of the tides. Let
it be observed that I am referring to the force or pull of gravity,
and not to hydrostatic pressure.
The moon, by raising the waters of the ocean, will produce a slope of 2
feet in a quadrant; and because the raised water sinks and the level is
restored, Mr. Ferrel concludes that a similar slope of 2 feet produced
by difference of temperature will therefore be sufficient to produce
motion and restore level. But it is overlooked that the restoration of
level in the case of the tides is as truly the work of the moon as the
disturbance of that level is. For the water raised by the attraction of
the moon at one time is again, six hours afterwards, pulled down by the
moon when the earth has turned round a quadrant.
No doubt the earth’s gravity alone would in course of time restore
the level; but this does not follow as a logical consequence from Mr.
Ferrel’s premises. If we suppose a slope to be produced in the ocean by
the moon and the moon’s attraction withdrawn so as to allow the water
to sink to its original level, the raised side will be the heaviest and
the depressed side the lightest; consequently the raised side will tend
to sink and the depressed side will tend to rise, in order that the
ocean may regain its static equilibrium. But when a difference of level
is produced by difference of temperature, the raised side is always the
lightest and the depressed side is always the heaviest; consequently
the very effort which the ocean makes to maintain its equilibrium
tends to prevent the level being restored. The moon produces the tides
chiefly by means of a simple yielding of the entire ocean considered as
a mass; whereas in the case of a general oceanic circulation the level
is restored by a _flow_ of water at or near the surface. Consequently
the amount of friction and molecular resistance to be overcome in the
restoration of level in the latter case is much greater than in the
former. The moon, as the researches of Sir William Thomson show, will
produce a tide in a globe composed of a substance where no currents or
general flow of the materials could possibly take place.
_Pressure as a Cause of Circulation._—We shall now briefly refer to
the influence of pressure (the indirect effects of gravity) in the
production of the circulation under consideration. That which causes
the polar column C to descend and the equatorial column W to ascend,
as has repeatedly been remarked, is the difference in the weight of
the two columns. The efficient cause in the production of the movement
is, properly speaking, gravity; _cold_ at the poles and _heat_ at the
equator, or, what is the same thing, the _excess_ of heat received
by the equator over that received by the poles is what maintains the
difference of temperature between the two columns, and consequently is
that also which maintains the difference of weight between them. In
other words, difference of temperature is the cause which maintains
the _state of disturbed equilibrium_. But the efficient cause of
the circulation in question is gravity. Gravity, however, could not
act without this state of disturbed equilibrium; and difference of
temperature may therefore be called, in relation to the circulation,
a necessary _condition_, while gravity may be termed the _cause_.
Gravity sinks column C _directly_, but it raises column W _indirectly_
by means of pressure. The same holds true in regard to the motion of
the bottom-waters from C to W, which is likewise due to pressure. The
pressure of the excess of the weight of column C over that of column W
impels the bottom-water equatorwards and lifts the equatorial column.
But on this point I need not dwell, as I have in the preceding chapter
entered into a full discussion as to how this takes place.
We come now to the most important part of the inquiry, viz., how is
the surface-water impelled from the equator to the poles? Is pressure
from behind the impelling force here as in the case of the bottom-water
of the ocean? It seems to me that, in attempting to account for the
surface-flow from the equator to the poles, Dr. Carpenter’s theory
signally fails. The force to which he appeals appears to be wholly
inadequate to produce the required effect.
The experiments of M. Dubuat, as already noticed, prove that, any slope
which can possibly result from the difference of temperature between
the equator and the poles is wholly insufficient to enable gravity to
move the waters; but it does not necessarily prove that the _pressure_
resulting from the raised water at the equator may not be sufficient to
produce motion. This point will be better understood from the following
figure, where, as before, P C represents the polar column and E W the
equatorial column.
[Illustration: Fig. 4.]
It will be observed that the water in that wedge-shaped portion W C
W′ forming the incline cannot be in a state of static equilibrium.
A molecule of water at O, for example, will be pressed more in the
direction of C than in the direction of W′, and the amount of this
excess of pressure towards C will depend upon the height of W above
the line C W′. It is evident that the pressure tending to move the
molecule at O towards C will be far greater than the direct pull of
gravity tending to draw a molecule at O′ lying on the surface of the
incline towards C. The experiments of M. Dubuat prove that the direct
force of gravity will not move the molecule at O′—that is, cause it to
roll down the incline W C; but they do not prove that it may not yield
to pressure from above, or that the pressure of the column W W′ will
not move the molecule at O. The pressure is caused by gravity, and
cannot, of course, enable gravity to perform more work than what is
derived from the energy of gravity; it will enable gravity, however,
to overcome resistance, which it could not do by direct action. But
whether the pressure resulting from the greater height of the water
at the equator due to its higher temperature be actually sufficient
to produce displacement of the water is a question which I am wholly
unable to answer.
If we suppose 4 feet 6 inches to be the height of the equatorial
surface above the polar required to make the two columns balance
each other, the actual difference of level between the two columns
will certainly not be more than one-half that amount, because, if a
circulation exist, the weight of the polar column must always be in
excess of that of the equatorial. But this excess can only be obtained
at the expense of the surface-slope, as has already been shown at
length. The surface-slope probably will not be more than 2 feet or 2
feet 6 inches. Suppose the ocean to be of equal density from the poles
to the equator, and that by some means or other the surface of the
ocean at the equator is raised, say, 2 feet above that of the poles,
then there can be little doubt that in such a case the water would
soon regain its level; for the ocean at the equator being heavier than
at the poles by the weight of a layer 2 feet in thickness, it would
sink at the former place and rise at the latter until equilibrium was
restored, producing, of course, a very slight displacement of the
bottom-waters towards the poles. It will be observed, however, that
restoration of level in this case takes place by a simple yielding, as
it were, of the entire mass of the ocean without displacement of the
molecules of the water over each other to any great extent. In the case
of a slope produced by difference of temperature, however, the raised
portion of the ocean is not heavier but lighter than the depressed
portion, and consequently has no tendency to sink. Any movement which
the ocean as a mass makes in order to regain equilibrium tends, as we
have seen, rather to increase the difference of level than to reduce
it. Restoration of level can only be produced by the forces which are
in operation in the wedge-shaped mass W C W′, constituting the slope
itself. But it will be observed by a glance at the Figure that, in
order to the restoration of level, a large portion of the water W W′ at
the equator will require to flow to C, the pole.
According to the general _vertical_ oceanic circulation theory,
pressure from behind is not one of the forces employed in the
production of the flow from the equator to the poles. This is evident;
for there can be no pressure from behind acting on the water if there
be no slope existing between the equator and the poles. Dr. Carpenter
not only denies the actual existence of a slope, but denies the
necessity for its existence. But to deny the existence of a slope is to
deny the existence of pressure, and to deny the necessity for a slope
is to deny the necessity for pressure. That in Dr. Carpenter’s theory
the surface-water is supposed to be _drawn_ from the equator to the
poles, and not _pressed_ forward by a force from behind, is further
evident from the fact that he maintains that the force employed is not
_vis a tergo_ but _vis a fronte_.[83]
CHAPTER XI.
THE INADEQUACY OF THE GRAVITATION THEORY PROVED BY ANOTHER
METHOD.
Quantity of Heat which can be conveyed by the General Oceanic
Circulation trifling.—Tendency in the Advocates of the
Gravitation Theory to under-estimate the Volume of the
Gulf-stream.—Volume of the Stream as determined by the
_Challenger_.—Immense Volume of Warm Water discovered by
Captain Nares.—Condition of North Atlantic inconsistent with
the Gravitation Theory.—Dr. Carpenter’s Estimate of the
Thermal Work of the Gulf-stream.
I shall now proceed by another method to prove the inadequacy of such
a general oceanic circulation as that which Dr. Carpenter advocates.
By contrasting the quantity of heat carried by the Gulf-stream from
inter-tropical to temperate and polar regions with such amount as
can possibly be conveyed in the same direction by means of a general
oceanic circulation, it will become evident that the latter sinks into
utter insignificance before the former.
In my earlier papers on the amount of heat conveyed by the
Gulf-stream,[84] I estimated the volume of that stream as _equal
to that_ of a current 50 miles broad and 1,000 feet deep, flowing
(from the surface to the bottom) at 4 miles an hour. Of course I did
not mean, as Dr. Carpenter seems to suppose, that the stream at any
particular place is 50 miles broad and 1,000 feet deep, or that it
actually flows at the uniform rate of 4 miles an hour at surface and
bottom. All I meant was, that the Gulf-stream is _equal to that_ of
a current of the above size and velocity. But in my recent papers on
Ocean-currents, the substance of which appears in the present volume,
to obviate any objections on the grounds of having over-estimated the
volume, I have taken that at one half this estimate, viz., equal to
a current 50 miles broad and 1,000 feet deep flowing at the rate of
2 miles an hour. I have estimated the mean temperature of the stream
as it passes the Straits of Florida to be 65°, and have supposed that
the water in its course becomes ultimately cooled down on an average
to 40°. In this case each pound of water conveys 19,300 foot-pounds of
heat from the Gulf of Mexico, to be employed in warming temperate and
polar regions. Assuming these data to be correct, it follows that the
amount of heat transferred from the Gulf of Mexico by this stream per
day amounts to 77,479,650,000,000,000,000 foot-pounds. This enormous
quantity of heat is equal to one-fourth of all that is received from
the sun by the whole of the Atlantic Ocean from the Tropic of Cancer up
to the Arctic Circle.
This is the amount of heat conveyed from inter-tropical to temperate
and polar regions by the Gulf-stream. What now is the amount conveyed
by means of the General Oceanic Circulation?
According to this theory there ought to be as much warm water flowing
from inter-tropical regions towards the Antarctic as towards the Arctic
Circle. We may, therefore, in our calculations, consider that the heat
which is received in tropical regions to the south of the equator goes
to warm the southern hemisphere, and that received on the north side
of the equator to warm the northern hemisphere. The warm currents
found in the North Atlantic in temperate regions we may conclude came
from the regions lying to the north of the equator,—or, in other
words, from that part of the Atlantic lying between the equator and
the Tropic of Cancer. At least, according to the gravitation theory,
we have no reason to believe that the quantity of warm water flowing
from tropical to temperate and polar regions in the Atlantic is
greater than the area between the equator and the Tropic of Cancer
can supply—because it is affirmed that a very large proportion of the
cold water found in the North Atlantic comes, not from the arctic, but
from the antarctic regions. But if the North Atlantic is cooled by a
cold stream from the southern hemisphere, the southern hemisphere in
turn must be heated by a warm current from the North Atlantic—unless
we assume that the compensating current flowing from the Atlantic into
the southern hemisphere is as cold as the antarctic current, which is
very improbable. But Dr. Carpenter admits that the quantity of warm
water flowing from the Atlantic in equatorial regions towards the
south is even greater than that flowing northwards. “The unrestricted
communication,” he says, “which exists between the antarctic area and
the great Southern Ocean-basins would involve, if the doctrine of a
general oceanic circulation be admitted, a much more considerable
interchange of waters between the antarctic and the equatorial areas
than is possible in the northern hemisphere.”[85]
We have already seen that, were it not for the great mass of warm water
which finds its way to the polar regions, the temperature of these
regions would be enormously lower than they really are. It has been
shown likewise that the comparatively high temperature of north-western
Europe is due to the same cause. But if it be doubtful whether the
Gulf-stream reaches our shores, and if it be true that, even supposing
it did, it “could only affect the _most superficial_ stratum,” and that
the great mass of warm water found by Dr. Carpenter in his dredging
expeditions came directly from the equatorial regions, and not from
the Gulf-stream, then the principal part of the heating-effect must be
attributed, not to the Gulf-stream, but to the general flow of water
from the equatorial regions. It surely would not, then, be too much to
assume that the quantity of heat conveyed from equatorial regions by
this general flow of water into the North Atlantic is at least equal to
that conveyed by the Gulf-stream. If we assume this to be the amount of
heat conveyed by the two agencies into the Atlantic from inter-tropical
regions, it will, of course, be equal to twice that conveyed by the
Gulf-stream alone.
We shall now consider whether the area of the Atlantic to the north of
the equator is sufficient to supply the amount of heat demanded by Dr.
Carpenter’s theory.
The entire area of the Atlantic, extending from the equator to the
Tropic of Cancer, including the Caribbean Sea and the Gulf of Mexico,
is about 7,700,000 square miles.
The quantity of heat conveyed by the Gulf-stream through the Straits of
Florida is, as we have already endeavoured to show, equal to all the
heat received from the sun by 1,560,935 square miles at the equator.
The annual quantity of heat received from the sun by the torrid zone
per unit surface, taking the mean of the whole zone, is to that
received by the equator as 39 to 40, consequently the quantity of
heat conveyed by the Gulf-stream is equal to all the heat received by
1,600,960 square miles of the Atlantic in the torrid zone.
But if, according to Dr. Carpenter’s views, the quantity of heat
conveyed from the tropical regions is double that conveyed by the
Gulf-stream, the amount of heat in this case conveyed into the Atlantic
in temperate regions will be equal to all the heat received from the
sun by 3,201,920 square miles of the Atlantic between the equator and
the Tropic of Cancer. This is 32/77ths of all the heat received from
the sun by that area.
Taking the annual quantity received per unit surface at the equator at
1,000, the quantities received by the three zones would be respectively
as follows:—
Equator 1000
Torrid zone 975
Temperate zone 757
Frigid zone 454
Now, if we remove from the Atlantic in tropical regions 32/77ths of the
heat received from the sun, we remove 405 parts from every 975 received
from the sun, and consequently only 570 parts per unit surface remain.
It has been shown[86] that the quantity of heat conveyed by the
Gulf-stream from the equatorial regions into the temperate regions
is equal to 100/412ths of all the heat received by the Atlantic in
temperate regions. But according to the theory under consideration the
quantity removed is double this, or equal to 100/206ths of all the heat
received from the sun. But the amount received from the sun is equal
to 757 parts per unit surface; add then to this 100/206ths of 757, or
367, and we have 1,124 parts of heat per unit surface as the amount
possessed by the Atlantic in temperate regions. The Atlantic should in
this case be much warmer in temperate than in tropical regions; for
in temperate regions it would possess 1,124 parts of heat per unit
surface, whereas in tropical regions it would possess only 570 parts
per unit surface. Of course the heat conveyed from tropical regions
does not all remain in temperate regions; a very considerable portion
of it must pass into the arctic regions. Let us, then, assume that
one half goes to warm the Arctic Ocean, and the other half remains
in the temperate regions. In this case 183·5 parts would remain, and
consequently 757 + 183·5 = 940·5 parts would be the quantity possessed
by the Atlantic in temperate regions, a quantity which still exceeds by
no less than 370·5 parts the heat possessed by the Atlantic in tropical
regions.
As one half of the amount of heat conveyed from the tropical regions
is assumed to go into the Arctic Ocean, the quantity passing into
that ocean would therefore be equal to that which passes through the
Straits of Florida, an amount which, as we have found, is equal to all
the heat received from the sun by 3,436,900 square miles of the Arctic
Ocean.[87] The entire area covered by sea beyond the Arctic Circle is
under 5,000,000 square miles; but taking the Arctic Ocean in round
numbers at 5,000,000 square miles, the quantity of heat conveyed into
it by currents to that received from the sun would therefore be as
3,436,900 to 5,000,000.
The amount received on the unit surface of the arctic regions we have
seen to be 454 parts. The amount received from the currents would
therefore be 312 parts. This gives 766 parts of heat per unit surface
as the quantity possessed by the Arctic Ocean. Thus the Arctic Ocean
also would contain more heat than the Atlantic in tropical regions; for
the Atlantic in these regions would, in the case under consideration,
possess only 570 parts, while the Arctic Ocean would possess 766
parts. It is true that more rays are cut off in arctic regions than in
tropical; but still, after making due allowance for this, the Arctic
Ocean, if the theory we are considering were true, ought to be as warm
as, if not warmer than, the Atlantic in tropical regions. The relative
quantities of heat possessed by the three zones would therefore be as
follows:—
Atlantic, in torrid zone 570
〃 in temperate zone 940
〃 in frigid zone 766
It is here assumed, however, that none of the heat possessed by the
Gulf-stream is derived from the southern hemisphere, which, we know,
is not the case. But supposing that as much as one half of the heat
possessed by the stream came from the southern hemisphere, and that the
other half was obtained from the seas lying between the equator and the
Tropic of Cancer, the relative proportions of heat possessed by the
three zones per given area would be as follows:—
Atlantic, in torrid zone 671
〃 in temperate zone 940
〃 in frigid zone 766
This proves incontestably that, supposing there is such a general
oceanic circulation as is maintained, the quantity of heat conveyed by
means of it into the North Atlantic and Arctic Oceans must be trifling
in comparison with that conveyed by the Gulf-stream; for if it nearly
equalled that conveyed by the Gulf-stream, then not only the North
Atlantic in temperate regions, but even the Arctic Ocean itself would
be much warmer than the inter-tropical seas. In fact, so far as the
distribution of heat over the globe is concerned, it is a matter of
indifference whether there really is or is not such a thing as this
general oceanic circulation. The enormous amount of heat conveyed by
the Gulf-stream alone puts it beyond all doubt that ocean-currents are
the great agents employed in distributing over the globe the excess of
heat received by the sea in inter-tropical regions.
It is therefore, so far as concerns the theory of a General Oceanic
Circulation, of the utmost importance that the advocates of that
theory should prove that I have over-estimated the thermal power of
the Gulf-stream. This, however, can only be done by detecting some
error either in my computation or in the data on which it is based;
yet neither Dr. Carpenter nor any one else, as far as I know, has
challenged the accuracy of my figures. The question at issue is the
correctness of the data; but the only part of the data which can
possibly admit of being questioned is my estimate of the _volume_
and _temperature_ of the stream. Dr. Carpenter, however, does not
maintain that I have over-estimated the temperature of the stream; on
the contrary, he affirms that I have really under-estimated it. “If we
assume,” he remarks, “the limit of the stratum above 60° as that of
the real Gulf-stream current, we shall find its average temperature to
be somewhat higher than it has been stated by Mr. Croll, who seems to
have taken 65° as the average of the water flowing through the entire
channel. The average surface temperature of the Florida channel for
the whole year is 80°; and we may fairly set the average of the entire
outgoing stream, down to the plane of 60°, at 70°, instead of 65° as
estimated by Mr. Croll” (§ 141). It follows, then, that every pound of
water of the Gulf-stream actually conveys 5 units of heat more than
I have estimated it to do—the amount conveyed being 30 units instead
of 25 units as estimated by me. Consequently, if the Gulf-stream be
equal to that of a current of merely 41½ miles broad and 1,000 feet
deep, flowing at the rate of 2 miles an hour, it will still convey the
estimated quantity of heat. But this estimate of the volume of the
stream, let it be observed, barely exceeds _one-third_ of that given
by Herschel, Maury, and Colding,[88] and is little more than one-half
that assigned to it by Mr. Laughton, while it very little exceeds that
given by Mr. Findlay,[89] an author whom few will consider likely to
overrate either the volume or heating-power of the stream.
The important results obtained during the _Challenger_ expedition have
clearly proved that I have neither over-estimated the temperature nor
the volume of the Gulf-stream. Between Bermuda and Sandy Hook the
stream is 60 miles broad and 600 feet deep, with a maximum velocity of
from 3½ to 4 miles an hour. If the mean velocity of the entire section
amounts to 2¼ miles an hour, which it probably does, the volume of the
stream must equal that given in my estimate. But we have no evidence
that all the water flowing through the Straits of Florida passes
through the section examined by the officers of the _Challenger_. Be
this, however, as it may, the observations made between St. Thomas
and Sandy Hook reveal the existence of an immense flow of warm water,
2,300 feet deep, entirely distinct from the water included in the above
section of the Gulf-stream proper. As the thickest portion of this
immense body of water joins the warm water of the Gulf-stream, Captain
Nares considers that “it is evidently connected with it, and probably
as an offshoot.” At Sandy Hook, according to him, it extends 1,200
feet deeper than the Gulf-stream itself, but off Charleston, 600 miles
nearer the source, the same temperature is found at the same depth.
But whether it be an offshoot of the Gulf-stream or not, one thing is
certain, it can only come from the Gulf of Mexico or from the Caribbean
Sea. This mass of water, after flowing northwards for about 1,000
miles, turns to the right and crosses the Atlantic in the direction of
the Azores, where it appears to thin out.
If, therefore, we take into account the combined heat conveyed by
both streams, my estimate of the heat transferred from inter-tropical
regions into the North Atlantic will be found rather under than above
the truth.
_Dr. Carpenter’s Estimate of the Thermal Work of the Gulf-stream._—In
the appendix to an elaborate memoir on Oceanic Circulation lately
read before the Geographical Society, Dr. Carpenter endeavours to
show that I have over-estimated the thermal work of the Gulf-stream.
In that memoir[90] he has also favoured us with his own estimate of
the sectional area, rate of flow, and temperature of the stream. Even
adopting his data, however, I find myself unable to arrive at his
conclusions.
Let us consider first his estimate of the sectional area of the
stream. He admits that “it is impossible, in the present state of our
knowledge, to arrive at any exact estimate of the sectional area of the
stream; since it is for the most part only from the temperatures of
its different strata that we can judge whether they are, or are not,
in movement, and what is the direction of their movement.” Now it is
perfectly evident that our estimate of the sectional area of the stream
will depend upon what we assume to be its bottom temperature. If, for
example, we assume 70° to be the bottom temperature, we shall have a
small sectional area. Taking the temperature at 60°, the sectional
area will be larger, and if 50° be assumed to be the temperature, the
sectional area will be larger still, and so on. Now the small sectional
area obtained by Dr. Carpenter arises from the fact of his having
assumed the high temperature of 60° to be that of the bottom of the
stream. He concludes that all the water below 60° has an inward flow,
and that it is only that portion from 60° and upwards which constitutes
the Gulf-stream. I have been unable to find any satisfactory evidence
for assuming so high a temperature for the bottom of the stream. It
must be observed that the water underlying the Gulf-stream is not
the ordinary water of the Atlantic, but the cold current from the
arctic regions. In fact, it is the same water which reaches the
equator at almost every point with a temperature not much above the
freezing-point. It is therefore highly improbable that the under
surface of the Gulf-stream has a temperature so high as 60°.
Dr. Carpenter’s method of measuring the mean velocity of the
Gulf-stream is equally objectionable. He takes the mean annual rate at
the surface in the “Narrows” to be two miles an hour and the rate at
the bottom to be zero, and he concludes from this that the average rate
of the whole is one mile an hour—the arithmetical mean between these
two extremes. Now it will be observed that this conclusion only holds
true on the supposition that the breadth of the stream is as great at
the bottom as at the surface, which of course it is not. All admit that
the sides of the Gulf-stream are not perpendicular, but slope somewhat
in the manner of the banks of a river. The stream is broad at the
surface and narrows towards the bottom. It is therefore evident that
the upper half of the section has a much larger area than the lower;
the quantity of water flowing through the upper half with a greater
velocity than one mile an hour must be much larger than the quantity
flowing through the lower half with a less velocity than one mile an
hour.
His method of estimating the mean temperature of the stream is even
more objectionable. He says, “The average surface temperature of the
Florida Channel for the whole year is 80°, and we may set the average
of the entire outgoing stream down to the plane of 60° at 70°, instead
of 65°, as estimated by Mr. Croll.” If 80° be the surface and 60° be
the bottom temperature, temperature and rate of velocity being assumed
of course to decrease uniformly from the surface downwards, how is it
possible that 70° can be the average temperature? The amount of water
flowing through the upper half of the section, with a temperature above
70°, is far more than the amount flowing through the under half of the
section, with a temperature below 70°. Supposing the lower half of the
section to be as large as the upper half, which it is not, still the
quantity of water flowing through it would only equal one-third of
that flowing through the upper half, because the mean velocity of the
water in the lower half would be only half a mile per hour, whereas
the mean velocity of that in the upper half would be a mile and a half
an hour. But the area of the lower half is much less than that of the
upper half, consequently the amount of water whose temperature is under
70° must be even much under one-third of that, the temperature of which
is above 70°.
Had Dr. Carpenter taken the proper method of estimating the mean
temperature, he would have found that 75°, even according to his own
data, was much nearer the truth than 70°. I pointed out, several years
ago,[91] the fallacy of estimating the mean temperature of a stream in
this way.
So high a mean temperature as 75° for the Gulf-stream, even in the
Florida Channel, is manifestly absurd, but if 60° be the bottom
temperature of the stream, the mean temperature cannot possibly be much
under that amount. It is, of course, by under-estimating the sectional
area of the stream that its mean temperature is over-estimated. We
cannot reduce the mean temperature without increasing the sectional
area. If my estimate of 65° be taken as the mean temperature, which I
have little doubt will yet be found to be not far from the truth, Dr.
Carpenter’s estimate of the sectional area must be abandoned. For if
65° be the mean temperature of the stream, its bottom temperature must
be far under 60°, and if the bottom temperature be much under 60°, then
the sectional area must be greater than he estimates it to be.
Be this, however, as it may; even if we suppose that 60° will
eventually be found to be the actual bottom temperature of the
Gulf-stream, nevertheless, if the total quantity of heat conveyed by
the stream from inter-tropical regions be estimated in the proper way,
we shall still find that amount to be so enormous, that there is not
sufficient heat remaining in those regions to supply Dr. Carpenter’s
oceanic circulation with a quantity as great for distribution in the
North Atlantic.
It therefore follows (and so far as regards the theory of Secular
changes of climate, this is all that is worth contending for) that
Ocean-currents and not a General Oceanic Circulation resulting from
gravity, are the great agents employed in the distribution of heat over
the globe.
CHAPTER XII.
MR. A. G. FINDLAY’S OBJECTIONS CONSIDERED.
Mr. Findlay’s Estimate of the Volume of the Gulf-stream.—Mean
Temperature of a Cross Section less than Mean Temperature
of Stream.—Reason of such Diversity of Opinion regarding
Ocean-currents.—More rigid Method of Investigation necessary.
At the conclusion of the reading of Dr. Carpenter’s paper before the
Royal Geographical Society, on January 9th, 1871, Mr. Findlay made the
following remarks:—
“When, by the direction of the United States Government, ten or eleven
years ago, the narrowest part of the Gulf-stream was examined, figures
were obtained which shut out all idea of its ever reaching our shores
as a heat-bearing current. In the narrowest part, certainly not more
than from 250 to 300 cubic miles of water pass per diem. Six months
afterwards that water reaches the banks of Newfoundland, and nine or
twelve months afterwards the coast of England, by which time it is
popularly supposed to cover an area of 1,500,000 square miles. The
proportion of the water that passes through the Gulf of Florida will
not make a layer of water more than 6 inches thick per diem over such
a space. Every one knows how soon a cup of tea cools; and yet it is
commonly imagined that a film of only a few inches in depth, after the
lapse of so long a time, has an effect upon our climate. There is no
need for calculations; the thing is self-evident.”[92]
About five years ago, Mr. Findlay objected to the conclusions which I
had arrived at regarding the enormous heating-power of the Gulf-stream
on the ground that I had over-estimated the volume of the stream. He
stated that its volume was only about the half of what I had estimated
it to be. To obviate this objection, I subsequently reduced the volume
to one-half of my former estimate.[93] But taking the volume at this
low estimate, it was nevertheless found that the quantity of heat
conveyed into the Atlantic through the Straits of Florida by means of
the stream was equal to about _one-fourth_ of all the heat received
from the sun by the Atlantic from the latitude of the Strait of Florida
up to the Arctic Circle.
Mr. Findlay, in his paper read before the British Association, affirmed
that the volume of the stream is somewhere from 294 to 333 cubic miles
per day; but in his remarks at the close of Dr. Carpenter’s address, he
stated it to be not greater than from 250 to 300 cubic miles per day. I
am unable to reconcile any of those figures with the data from which he
appears to have derived them. In his paper to the British Association,
he remarks that “the Gulf-stream at its outset is not more than 39½
miles wide, and 1,200 feet deep.” From all attainable data, he computes
the mean annual rate of motion to be 65·4 miles per day; but as the
rate decreases with the depth, the mean velocity of the whole mass does
not exceed 49·4 miles per day. When he speaks of the mean velocity of
the Gulf-stream being so and so, he must refer to the mean velocity at
some particular place. This is evident; for the mean velocity entirely
depends upon the sectional area of the stream. The place where the
mean velocity is 49·4 miles per day must be the place where it is 39½
miles broad and 1,200 feet deep; for he is here endeavouring to show us
how small the volume of the stream actually is. Now, unless the mean
velocity refers to the place where he gives us the breadth and depth
of the stream, his figures have no bearing on the point in question.
But a stream 39½ miles broad and 1,200 feet deep has a sectional area
of 8·97 square miles, and this, with a mean velocity of 49·4 miles
per day, will give 443 cubic miles of water. The amount, according to
my estimate, is 459 cubic miles per day; it therefore exceeds Mr.
Findlay’s estimate by only 16 cubic miles.
Mr. Findlay does not, as far as I know, consider that I have
over-estimated the mean temperature of the stream. He states[94] that
between Sand Key and Havana the Gulf-stream is about 1,200 feet deep,
and that it does not reach the summit of a submarine ridge, which he
states has a temperature of 60°. It is evident, then, that the bottom
of the stream has a temperature of at least 60°, which is within 5° of
what I regard as the mean temperature of the mass. But the surface of
the stream is at least 17° above this mean. Now, when we consider that
it is at the upper parts of the stream, the place where the temperature
is so much above 65°, that the motion is greatest, it is evident that
the mean temperature of the entire moving mass must, according to Mr.
Findlay, be considerably over 65°. It therefore follows, according
to his own data, that the Gulf-stream conveys into the Atlantic an
amount of heat equal to one-fourth of all the heat which the Atlantic,
from the latitude of the Straits of Florida up to the arctic regions,
derives from the sun.
But it must be borne in mind that although the mean temperature of the
cross section should be below 65°, it does not therefore follow that
the mean temperature of the _water flowing through this cross section_
must be below that temperature, for it is perfectly obvious that the
mean temperature of the mass of water flowing through the cross section
in a given time must be much higher than that of the cross section
itself. The reason is very simple. It is in the upper half of the
section where the high temperature exists; but as the velocity of the
stream is much greater in its upper than in its lower half, the greater
portion of the water passing through this cross section is water of
high temperature.
But even supposing we were to halve Mr. Findlay’s own estimate, and
assume that the volume of the stream is equal to only 222 cubic miles
of water per day instead of 443, still the amount of heat conveyed
would be equal to one-eighth part of the heat received from the sun by
the Atlantic. But would not the withdrawal of an amount of heat equal
to one-eighth of that received from the sun greatly affect the climate
of the Atlantic? Supposing we take the mean temperature of the Atlantic
at, say, 56°; this will make its temperature 295° above that of space.
Extinguish the sun and stop the Gulf-stream, and the temperature ought
to sink 295°. How far, then, ought the temperature to sink, supposing
the sun to remain and the Gulf-stream to stop? Would not the withdrawal
of the stream cause the temperature to sink some 30°? Of course, if
the Gulf-stream were withdrawn and everything else were to remain the
same, the temperature of the Atlantic would not actually remain 30°
lower than at present; for heat would flow in from all sides and partly
make up for the loss of the stream. But nevertheless 30° represents the
amount of temperature maintained by means of the heat from the stream.
And this, be it observed, is taking the volume of the stream at a lower
estimate than even Mr. Findlay himself would be willing to admit. Mr.
Findlay says that, by the time the Gulf-stream reaches the shores of
England, it is supposed to cover a space of 1,500,000 square miles.
“The proportion of water that passes through the Straits of Florida
will not make,” according to him, “a layer of water more than 6 inches
thick per diem over such a space.” But a layer of water 6 inches thick
cooling 25° will give out 579,000 foot-pounds of heat per square foot.
If, therefore, the Gulf-stream, as he asserts, supplies 6 inches per
day to that area, then every square foot of the area gives off per
day 579,000 foot-pounds of heat. The amount of heat received from the
sun per square foot in latitude 55°, which is not much above the mean
latitude of Great Britain, is 1,047,730 foot-pounds per day, taking, of
course, the mean of the whole year; _consequently this layer of water
gives out an amount of heat equal to more than_ one-half _of all that
is received from the sun_. But assuming that the stream should leave
the half of its heat on the American shores and carry to the shores of
Britain only 12½° of heat, still we should have 289,500 foot-pounds per
square foot, which notwithstanding _is more than equal to_ one-fourth
_of that received from the sun_. If an amount of heat so enormous
cannot affect climate, what can?
I shall just allude to one other erroneous notion which prevails in
regard to the Gulf-stream; but it is an error which I by no means
attribute either to Mr. Findlay or to Dr. Carpenter. The error to which
I refer is that of supposing that when the Gulf-stream widens out to
hundreds of miles, as it does before it reaches our shores, its depth
must on this account be much less than when it issues from the Gulf of
Mexico. Although the stream may be hundreds of miles in breadth, there
is no necessity why it should be only 6 inches, or 6 feet, or 60 feet,
or even 600 feet in depth. It may just as likely be 6,000 feet deep as
6 inches.
_The Reason why such Diversity of Opinion prevails in Regard to
Ocean-currents._—In conclusion I venture to remark that more than
nine-tenths of all the error and uncertainty which prevail, both
in regard to the cause of ocean-currents and to their influence on
climate, is due, not, as is generally supposed, to the intrinsic
difficulties of the subject, but rather to the defective methods
which have hitherto been employed in its investigation—that is, in
not treating the subject according to the rigid methods adopted in
other departments of physics. What I most particularly allude to is
the disregard paid to the modern method of determining the amount of
effects in _absolute measure_.
But let me not be misunderstood on this point. I by no means suppose
that the _absolute quantity_ is the thing always required for its
own sake. It is in most cases required simply as a means to an end;
and very often that end is the knowledge of the _relative_ quantity.
Take, for example, the Gulf-stream. Suppose the question is asked,
to what extent does the heat conveyed by that stream influence the
climate of the North Atlantic? In order to the proper answering of this
question, the principal thing required is to know what proportion the
amount of heat conveyed by the stream into the Atlantic bears to that
received from the sun by that area. We want the _relative proportions_
of these two quantities. But how are we to obtain them? We can only
do so by determining first the _absolute_ quantity of each. We must
first measure each before we can know how much the one is greater
than the other, or, in other words, before we can know their relative
proportions. We have the means of determining the absolute amount
of heat received from the sun by a given area at any latitude with
tolerable accuracy; but the same cannot be done with equal accuracy in
regard to the amount of heat conveyed by the Gulf-stream, because the
volume and mean temperature of the stream are not known with certainty.
Nevertheless we have sufficient data to enable us to fix upon such a
maximum and minimum value to these quantities as will induce us to
admit that the truth must lie somewhere between them. In order to give
full justice to those who maintain that the Gulf-stream exercises
but little influence on climate, and to put an end to all further
objections as to the uncertainty of my data, I shall take a minimum
to which none of them surely can reasonably object, viz. that the
volume of the stream is not over 230 cubic miles per day, and the heat
conveyed per pound of water not over 12½ units. Calculating from these
data, we find that the amount of heat carried into the North Atlantic
is equal to one-sixteenth of all the heat received from the sun by that
area. There are, I presume, few who will not admit that the actual
proportion is much higher than this, probably as high as 1 to 3, or 1
to 4. But, who, without adopting the method I have pursued, could ever
have come to the conclusion that the proportion was even 1 to 16? He
might have guessed it to be 1 to 100 or 1 to 1000, but he never would
have guessed it to be 1 to 16. Hence the reason why the great influence
of the Gulf-stream as a heating agent has been so much under-estimated.
The same remarks apply to the gravitation theory of the cause of
currents. Viewed simply as a theory it looks very reasonable. There is
no one acquainted with physics but will admit that the tendency of the
difference of temperature between the equator and the poles is to cause
a surface current from the equator towards the poles, and an under
current from the poles to the equator. But before we can prove that
this tendency does actually produce such currents, another question
must be settled, viz. is this force sufficiently great to produce
the required motion? Now when we apply the method to which I refer,
and determine the absolute amount of the force resulting from the
difference of specific gravity, we discover that not to be the powerful
agent which the advocates of the gravitation theory suppose, but a
force so infinitesimal as not to be worthy of being taken into account
when considering the causes by which currents are produced.
CHAPTER XIII.
THE WIND THEORY OF OCEANIC CIRCULATION.
Ocean-currents not due alone to the Trade-winds.—An Objection
by Maury.—Trade-winds do not explain the Great Antarctic
Current.—Ocean-currents due to the System of Winds.—The
System of Currents agrees with the System of the
Winds.—Chart showing the Agreement between the System
of Currents and System of Winds.—Cause of the Gibraltar
Current.—North Atlantic an immense Whirlpool.—Theory of Under
Currents.—Difficulty regarding Under Currents obviated.—Work
performed by the Wind in impelling the Water forward.—The
_Challenger’s_ crucial Test of the Wind and Gravitation
Theories.—North Atlantic above the Level of Equator.—Thermal
Condition of the Southern Ocean irreconcilable with the
Gravitation Theory.
_Ocean-currents not due alone to the Trade-winds._—The generally
received opinion amongst the advocates of the wind theory of oceanic
circulation is that the Gulf-stream and other currents of the ocean are
due to the impulse of the trade-winds. The tendency of the trade-winds
is to impel the inter-tropical waters along the line of the equator
from east to west; and were those regions not occupied in some places
by land, this equatorial current would flow directly round the
globe. Its westward progress, however, is arrested by the two great
continents, the old and the new. On approaching the land the current
bifurcates, one portion trending northwards and the other southwards.
The northern branch of the equatorial current of the Atlantic passes
into the Caribbean Sea, and after making a circuit of the Gulf of
Mexico, flows northward and continues its course into the Arctic
Ocean. The southern branch, on the other hand, is deflected along the
South-American coast, constituting what is known as the Brazilian
current. In the Pacific a similar deflection occurs against the
Asiatic coast, forming a current somewhat resembling the Gulf-stream,
a portion of which (Kamtschatka current) in like manner passes into
the arctic regions. In reference to all these various currents, the
impelling cause is supposed to be the force of the trade-winds.
It is, however, urged as an objection by Maury and other advocates of
the gravitation theory, that a current like the Gulf-stream, extending
as far as the arctic regions, could not possibly be impelled and
maintained by a force acting at the equatorial regions. But this is
a somewhat weak objection. It seems to be based upon a misconception
of the magnitude of the force in operation. It does not take into
account that this force acts on nearly the whole area of the ocean in
inter-tropical regions. If, in a basin of water, say three feet in
diameter, a force is applied sufficient to produce a surface-flow one
foot broad across the centre of the basin, the water impelled against
the side will be deflected to the extremes of the vessel. And this
result does not in any way depend upon the size of the basin. The
same effect which occurs in a small basin will occur in a large one,
provided the proportion between the breadth of the belt of water put in
motion and the size of the vessel be the same in both cases. It does
not matter, therefore, whether the diameter of the basin be supposed to
be three feet, or three thousand miles, or ten thousand miles.
There is a more formidable objection, however, to the theory.
The trade-winds will account for the Gulf-stream, Brazil, Japan,
Mozambique, and many other currents; but there are currents, such as
some of the polar currents, which cannot be so accounted for. Take,
for example, the great antarctic current flowing northward into the
Pacific. This current does not bend to the left under the influence
of the earth’s rotation and continue its course in a north-westerly
direction, but actually bends round to the right and flows eastward
against the South-American coast, in direct opposition both to the
influence of rotation and to the trade-winds. The trade-wind theory,
therefore, is insufficient to account for all the facts. But there is
yet another explanation, which satisfactorily solves our difficulties.
The currents of the ocean owe their origin, not to the trade-winds
alone, but to the _prevailing_ winds of the globe (including, of
course, the trade-winds).
_Ocean-currents due to the System of Winds._—If we leave out of account
a few small inland sheets of water, the globe may be said to have but
one sea, just as it possesses only one atmosphere. We have accustomed
ourselves, however, to speak of parts or geographical divisions of
the one great ocean, such as the Atlantic and the Pacific, as if they
were so many separate oceans. And we have likewise come to regard the
currents of the ocean as separate and independent of one another. This
notion has no doubt to a considerable extent militated against the
acceptance of the theory that the currents are caused by the winds, and
not by difference of specific gravity; for it leads to the conclusion
that currents in a sea must flow in the direction of the prevailing
winds blowing over that particular sea. The proper view of the matter,
as I hope to be able to show, is that which regards the various
currents merely as members of one grand system of circulation produced,
not by the trade-winds alone, nor by the prevailing winds proper alone,
but by the combined action of all the prevailing winds of the globe,
regarded as one system of circulation.
If the winds be the impelling cause of currents, the _direction_ of the
currents will depend upon two circumstances, viz.:—(1) the direction
of the prevailing winds of the globe, including, of course, under this
term the prevailing winds proper and the trade-winds; and (2) the
conformation of land and sea. It follows, therefore, that as a current
in any given sea is but a member of a general system of circulation,
its direction is determined, not alone by the prevailing winds blowing
over the sea in question, but by the general system of prevailing
winds. It may consequently sometimes happen that the general system
of winds may produce a current directly opposite to the prevailing
wind blowing over the current. The accompanying Chart (Plate I.) shows
how exactly the system of ocean-currents agrees with the system of
the prevailing winds. The fine lines indicate the paths of the
prevailing winds, and the fine arrows the direction in which the wind
blows along those paths. The large arrows show the direction of the
principal ocean-currents.
[Illustration: PLATE I.
CHART SHOWING THE GENERAL AGREEMENT BETWEEN THE SYSTEM OF OCEAN
CURRENTS AND WINDS.
W. & A. K. Johnston, Edinb^r. and London.]
The directions and paths of the prevailing winds have been taken from
Messrs. Johnston’s small physical Atlas, which, I find, agrees exactly
with the direction of the prevailing winds as deduced from the four
quarterly wind charts lately published by the Hydrographic Department
of the Admiralty. The direction of the ocean-currents has been taken
from the Current-chart published by the Admiralty.
In every case, without exception, the direction of the main currents of
the globe agrees exactly with the direction of the prevailing winds.
There could not possibly be a more convincing proof that those winds
are the cause of the ocean-currents than this general agreement of the
two systems as indicated by the chart. Take, for example, the North
Atlantic. The Gulf-stream follows exactly the path of the prevailing
winds. The Gulf-stream bifurcates in mid-Atlantic; so does the wind.
The left branch of the stream passes north-eastwards into the arctic
regions, and the right branch south-eastwards by the Azores; so does
the wind. The south-eastern branch of the stream, after passing the
Canaries, re-enters the equatorial current and flows into the Gulf
of Mexico; the same, it will be observed, holds true of the wind. A
like remarkable agreement exists in reference to all the other leading
currents of the ocean. This is particularly seen in the case of the
great antarctic current between long. 140° W. and 160° W. This current,
flowing northwards from the antarctic regions, instead of bending to
the left under the influence of rotation, turns to the right when it
enters the regions of the westerly winds, and flows eastwards towards
the South-American shores. In fact, all the currents in this region of
strong westerly winds flow in an easterly or north-easterly direction.
Taking into account the effects resulting from the conformation of
sea and land, the system of ocean-currents agrees precisely with
the system of the winds. All the principal currents of the globe are
in fact moving in the exact direction in which they ought to move,
assuming the winds to be the sole impelling cause. In short, so perfect
is the agreement between the two systems, that, given the system of
winds and the conformation of sea and land, and the direction of all
the currents of the ocean, or more properly the system of oceanic
circulation, might be determined _à priori_. Or given the system of the
ocean-currents together with the conformation of sea and land, and the
direction of the prevailing winds could also be determined _à priori_.
Or, thirdly, given the system of winds and the system of currents,
and the conformation of sea and land might be roughly determined. For
example, it can be shown by this means that the antarctic regions
are probably occupied by a continent and not by a number of separate
islands, nor by sea.
While holding that the currents of the ocean form one system of
circulation, we must not be supposed to mean that the various currents
are connected end to end, having the same water flowing through them
all in succession like that in a heating apparatus. All that is
maintained is simply this, that the currents are so mutually related
that any great change in one would modify the conditions of all the
others. For example, a great increase or decrease in the easterly flow
of antarctic water in the Southern Ocean would decrease or increase,
as the case might be, the strength of the West Australian current;
and this change would modify the equatorial current of the Indian
Ocean, a modification which in like manner would affect the Agulhas
current and the Southern Atlantic current—this last leading in turn
to a modification of the equatorial current of the Atlantic, and
consequently of the Brazilian current and the Gulf-stream. Furthermore,
since a current impelled by the winds, as Mr. Laughton in his excellent
paper on Ocean-currents justly remarks, tends to leave a vacancy
behind, it follows that a decrease or increase in the Gulf-stream would
affect the equatorial current, the Agulhas current, and all the other
currents back to the antarctic currents. Again, a large modification
in the great antarctic drift-current would in like manner affect all
the currents of the Pacific. On the other hand, any great change in
the currents of the Pacific would ultimately affect the currents of
the Atlantic and Indian Oceans, through its influence on the Cape Horn
current, the South Australian current, and the current passing through
the Asiatic archipelago; and _vice versâ_, any changes in the currents
of the Atlantic or Indian Oceans would modify the currents of the
Pacific.
_Cause of Gibraltar Current._—I may now consider the cause of the
Gibraltar current. There can be little doubt that this current owes its
origin (as Mr. Laughton points out) to the Gulf-stream. “I conceive,”
that author remarks, “that the Gibraltar current is distinctly a stream
formed by easterly drift of the North Atlantic, which, although it
forms a southerly current on the coast of Portugal, is still strongly
pressed to the eastward and seeks the first escape it can find. So
great indeed does this pressure seem to be, that more water is forced
through the Straits than the Mediterranean can receive, and a part
of it is ejected in reverse currents, some as lateral currents on
the surface, some, it appears, as an under current at a considerable
depth.”[95] The funnel-shaped nature of the strait through which the
water is impelled helps to explain the existence of the under current.
The water being pressed into the narrow neck of the channel tends to
produce a slight banking up; and as the pressure urging the water
forward is greatest at the surface and diminishes rapidly downwards,
the tendency to the restoration of level will cause an underflow
towards the Atlantic, because below the surface the water will find the
path of least resistance. It is evident indeed that this underflow will
not take place toward the Mediterranean, from the fact that that sea is
already filled to overflowing by the current received from the outside
ocean.
If we examine the Current-chart published by the Hydrographic
Department of the Admiralty, we shall find the Gibraltar current
represented as merely a continuation of the S.E. flow of Gulf-stream
water. Now, if the arrows shown upon this chart indicate correctly the
direction of the flow, we must become convinced that the Gulf-stream
water cannot possibly avoid passing through the Gibraltar Strait. Of
course the excess of evaporation over that of precipitation within
the Mediterranean area would alone suffice to produce a considerable
current through the Strait; but this of itself would not fill that
inland sea to overflowing.[96]
The Atlantic may, in fact, be regarded as an immense whirlpool with the
Saragossa Sea as its vortex; and although it is true, as will be seen
from an inspection of the Chart, that the wind blows round the Atlantic
along the very path taken by the water, impelling the water forward
along every inch of its course, yet nevertheless it must hold equally
true that the water has a tendency to flow off in a straight line at
a tangent to the circular course in which it is moving. But the water
is so hemmed in on all sides that it cannot leave this circular path
except only at two points; and at these two points it actually does
flow outwards. On the east and west sides the land prevents any such
outflow. Similarly, in the south the escape of the water is frustrated
by the pressure of the opposing currents flowing from that quarter;
while in the north it is prevented by the pressure exerted by polar
currents from Davis Strait and the Arctic Ocean. But in the Strait of
Gibraltar and in the north-eastern portion of the Atlantic between
Iceland and the north-eastern shores of Europe there is no resistance
offered: and at these two points an outflow does actually take place.
In both cases, however, especially the latter, the outflow is greatly
aided by the impulse of the prevailing winds.
No one, who will glance at the accompanying chart (Plate I.) showing
how the north-eastern branch of the Gulf-stream bends round and, of
course, necessarily presses against the coast, can fail to understand
how the Atlantic water should be impelled into the Gibraltar Strait,
even although the loss sustained by the Mediterranean from evaporation
did not exceed the gain from rain and rivers.
_Theory of Under Currents._—The consideration that ocean-currents are
simply parts of a system of circulation produced by the system of
prevailing winds, and not by the impulse of the trade-winds alone,
helps to remove the difficulty which some have in accounting for the
existence of under currents without referring them to difference of
specific gravity. Take the case of the Gulf-stream, which passes
under the polar stream on the west of Spitzbergen, this latter stream
passing in turn under the Gulf-stream a little beyond Bear Island. The
polar streams have their origin in the region of prevailing northerly
winds, which no doubt extends to the pole. The current flowing past
the western shores of Spitzbergen, throughout its entire course up
to near the point where it disappears under the warm waters of the
Gulf-stream, lies in the region of these same northerly winds. Now why
should this current cease to be a surface current as soon as it passes
out of the region of northerly into that of south-westerly winds? The
explanation seems to be this: when the stream enters the region of
prevailing south-westerly winds, its progress southwards along the
surface of the ocean is retarded both by the wind and by the surface
water moving in opposition to its course; but being continually pressed
forward by the impulse of the northerly winds acting along its whole
course back almost to the pole, perhaps, or as far north at least as
the sea is not wholly covered with ice, the polar current cannot stop
when it enters the region of opposing winds and currents; it must move
forward. But the water thus pressed from behind will naturally take
the _path of least resistance_. Now in the present case this path will
necessarily lie at a considerable distance below the surface. Had the
polar stream simply to contend with the Gulf-stream flowing in the
opposite direction, it would probably keep the surface and continue its
course along the side of that stream; but it is opposed by the winds,
from which it cannot escape except by dipping down under the surface;
and the depth to which it will descend will depend upon the depth of
the surface current flowing in the opposite direction. There is no
necessity for supposing a heaping up of the water in order to produce
by pressure a force sufficient to impel the under current. The pressure
of the water from behind is of itself enough. The same explanation, of
course, applies to the case of the Gulf-stream passing under the polar
stream. And if we reflect that these under currents are but parts of
the general system of circulation, and that in most cases they are
currents compensating for water drained off at some other quarter, we
need not wonder at the distance which they may in some cases flow, as,
for example, from the banks of Newfoundland to the Gulf of Mexico.
The under currents of the Gulf-stream are necessary to compensate for
the water impelled southwards by the northerly winds; and again, the
polar under currents are necessary to compensate for the water impelled
northward by the south and south-westerly winds.
But it may be asked, how do the opposing currents succeed in crossing
each other? It is evident that the Gulf-stream must plunge through
the whole thickness of the polar stream before it can become an
under current, and so likewise must the cold water of the polar-flow
pass through the genial water of the Gulf-stream in order to get
underneath it and continue on its course towards the south. The
accompanying diagram (Plate II., Fig. 1) will render this sufficiently
intelligible.
[Illustration: _Fig. 3_ PLATE II.
_Map shewing meeting of the Gulf-stream and Polar Current (from
D^r. Petermann’s Geographische Mittheilungen._) _The curved lines
are Isotherms; temperatures are in Fahrenheit._]
[Illustration: _Fig. 1_
_Diagram to shew how two opposing currents intersect each other_]
[Illustration: _Surface Plan to shew how two opposing currents meet
each other_
W. & A. K. Johnston, Edinb^r. and London.
_Fig. 2_]
Now these two great ocean-currents are so compelled to intersect each
other for the simple reason that they cannot turn aside, the one to the
left and the other to the right. When two broad streams like those in
question are pressed up against each other, they succeed in mutually
intersecting each other’s path by breaking up into bands or belts—the
cold water being invaded and pierced as it were by long tongues of
warm water, while at the same time the latter is similarly intersected
by corresponding protrusions of cold water. The two streams become
in a manner interlocked, and the one passes through the other very
much as we pass the fingers of one hand between the fingers of the
other. The diagram (Plate II., Fig. 2), representing the surface of
the ocean at the place of meeting of two opposing currents, will show
this better than description. At the surface the bands necessarily
assume the tongue-shaped appearance represented in the diagram, but
when they have succeeded in mutually passing down through the whole
thickness of the opposing currents, they then unite and form two
definite under currents, flowing in opposite directions. The polar
bands, after penetrating the Gulf-stream, unite below to form a
southward-flowing under current, and in the same way the Gulf-stream
bands, uniting underneath the polar current, continue in their
northerly course as a broad under current of warm water. That this is
a correct representation of what actually occurs in nature becomes
evident from an inspection of the current charts. Thus in the chart
of the North Atlantic which accompanies Dr. Petermann’s Memoir on the
Gulf-stream, we observe that south of Spitzbergen the polar current and
the Gulf-stream are mutually interpenetrated—long tongues invading and
dipping down underneath the Gulf-stream, while in like manner the polar
current becomes similarly intersected by well-marked protrusions of
warm water flowing from the south. (See Plate II., Fig. 3.)
No accurate observations, as far as I know, have been made regarding
the amount of work performed by the wind in impelling the water
forward; but when we consider the great retarding effect of objects
on the earth’s surface, it is quite apparent that the amount of work
performed on the surface of the ocean must be far greater than is
generally supposed. For example, Mr. Buchan, Secretary to the Scottish
Meteorological Society, has shown[97] that a fence made of slabs of
wood three inches in width and three inches apart from each other is a
protection even during high winds to objects on the lee side of it, and
that a wire screen with meshes about an inch apart affords protection
during a gale to flower-pots. The same writer was informed by Mr. Addie
that such a screen put up at Rockville was torn to pieces by a storm of
wind, the wire screen giving way much in the same way as sails during a
hurricane at sea.
_The “Challenger’s” Crucial Test of the Wind and Gravitation Theories
of Oceanic Circulation._—It has been shown in former chapters that all
the facts which have been adduced in support of the gravitation theory
are equally well explained by the wind theory. We may now consider a
class of facts which do not appear to harmonize with either theory. The
recent investigations of the _Challenger_ Expedition into the thermal
state of the ocean reveal a condition of things which appears to me
utterly irreconcilable with the gravitation theory.
It is a condition absolutely essential to the gravitation theory that
the surface of the ocean should be highest in equatorial regions and
slope downwards to either pole. Were water absolutely frictionless, an
incline, however small, would be sufficient to produce a surface-flow
from the equator to the poles, but to induce such an effect some slope
there must be, or gravitation could exercise no power in drawing the
surface-water polewards.
The researches of the _Challenger_ Expedition bring to light the
striking and important fact that the general surface of the North
Atlantic in order to produce equilibrium must stand at a higher level
than at the equator. In other words the surface of the Atlantic is
lowest at the equator, and rises with a gentle slope to well-nigh the
latitude of England. If this be the case, then it is mechanically
impossible that, as far as the North Atlantic is concerned, there can
be any such general movement as Dr. Carpenter believes. Gravitation can
no more cause the surface-water of the Atlantic to flow towards the
arctic regions than it can compel the waters of the Gulf of Mexico up
the Mississippi into the Missouri. The impossibility is equally great
in both cases.
In order to prove what has been stated, let us take a section of the
mid-Atlantic, north and south, across the equator; and, to give the
gravitation theory every advantage, let us select that particular
section adopted by Dr. Carpenter as the one of all others most
favourable to his theory, viz., Section marked No. VIII. in his memoir
lately read before the Royal Geographical Society.[98]
The fact that the polar cold water comes so near the surface at the
equator is regarded by Dr. Carpenter as evidence in favour of the
gravitation theory. On first looking at Dr. Carpenter’s section it
forcibly struck me that if it was accurately drawn, the ocean to be
in equilibrium would require to stand at a higher level in the North
Atlantic than at the equator. In order, therefore, to determine
whether this is the case or not I asked the hydrographer of the
Admiralty to favour me with the temperature soundings indicated in the
section, a favour which was most obligingly granted. The following
are the temperature soundings at the three stations A, B, and C. The
temperature of C are the mean of six soundings taken along near the
equator:—
+--------+----------------+----------------+----------------------+
| | A | B | C |
| | | | |
| Depth |Lat. 37° 54′ N. |Lat. 23° 10′ N. | Mean of six |
| in |Long. 41° 44′ W.|Long. 38° 42′ W.|temperature soundings |
|Fathoms.| | | near equator. |
| | | +---------+------------+
| | Temperature. | Temperature. | Depth in|Temperature.|
| | | | Fathoms.| |
+--------+----------------+----------------+---------+------------+
| | ° | ° | | ° |
|Surface.| 70·0 | 72·0 |Surface. | 77·9 |
| 100 | 63·5 | 67·0 | 10 | 77·2 |
| 200 | 60·6 | 57·6 | 20 | 77·1 |
| 300 | 60·0 | 52·5 | 30 | 76·9 |
| 400 | 54·8 | 47·7 | 40 | 71·7 |
| 500 | 46·7 | 43·7 | 50 | 64·0 |
| 600 | 41·6 | 41·7 | 60 | 60·4 |
| 700 | 40·6 | 40·6 | 70 | 59·4 |
| 800 | 38·1 | 39·4 | 80 | 58·0 |
| 900 | 37·8 | 39·2 | 90 | 58·0 |
| 1000 | 37·9 | 38·3 | 100 | 55·6 |
| 1100 | 37·1 | 38·0 | 150 | 51·0 |
| 1200 | 37·1 | 37·6 | 200 | 46·6 |
| 1300 | 37·2 | 36·7 | 300 | 42·2 |
| 1400 | 37·1 | 36·9 | 400 | 40·3 |
| 1500 | .. | 36·7 | 500 | 38·9 |
| 2700 | 35·2 | .. | 600 | 39·2 |
| 2720 | .. | 35·4 | 700 | 39·0 |
| | | | 800 | 39·1 |
| | | | 900 | 38·2 |
| | | | 1000 | 36·9 |
| | | | 1100 | 37·6 |
| | | | 1200 | 36·7 |
| | | | 1300 | 35·8 |
| | | | 1400 | 36·4 |
| | | | 1500 | 36·1 |
| | | | Bottom. | 34·7 |
+--------+----------------+----------------+---------+------------+
On computing the extent to which the three columns A, B, and C are each
expanded by heat according to Muncke’s table of the expansion of sea
water for every degree Fahrenheit, I found that column B, in order to
be in equilibrium with C (the equatorial column), would require to have
its surface standing fully 2 feet 6 inches above the level of column C,
and column A fully 3 feet 6 inches above that column. In short, it is
evident that there must be a gradual rise from the equator to latitude
38° N. of 3½ feet. Any one can verify the accuracy of these results by
making the necessary computations for himself.[99]
[Illustration: PLATE III.
W. & A. K. Johnston, Edinb^r. and London.
SECTION OF THE ATLANTIC nearly North and South, between LAT. 38° N.
& LAT. 38° S.]
I may observe that, had column C extended to the same depth as columns
A and B, the difference of level would be considerably greater, for
column C requires to balance only that portion of columns A and B
which lies above the level of its base. Suppose a depth of ocean equal
to that of column C to extend to the north pole, and the polar water
to have a uniform temperature of 32° from the surface to the bottom,
then, in order to produce equilibrium, the surface of the ocean at
the equator would require to be 4 feet 6 inches above that at the
pole. But the surface of the ocean at B would be 7 feet, and at A 8
feet, above the poles. Gravitation never could have caused the ocean
to assume this form. It is impossible that this immense mass of warm
water, extending to such a depth in the North Atlantic, could have been
brought from equatorial regions by means of gravitation. And, even
if we suppose this accumulation of warm water can be accounted for
by some other means, still its presence precludes the possibility of
any such surface-flow as that advocated by Dr. Carpenter. For so long
as the North Atlantic stands 3½ feet above the level of the equator,
gravitation can never move the equatorial waters polewards.
There is another feature of this section irreconcilable with the
gravitation theory. It will be observed that the accumulation of warm
water is all in the North Atlantic, and that there is little or none
in the south. But according to the gravitation theory it ought to
have been the reverse. For owing to the unrestricted communication
between the equatorial and antarctic regions, the general flow of
water towards the south pole is, according to that theory, supposed to
be greater than towards the north, and consequently the quantity of
warm equatorial water in the South Atlantic ought also to be greater.
Dr. Carpenter himself seems to be aware of this difficulty besetting
the theory, and meets it by stating that “the upper stratum of the
North Atlantic is not nearly as much cooled down by its limited polar
underflow, as that of the South Atlantic is by the vast movement of
antarctic water which is constantly taking place towards the equator.”
But this “vast movement of antarctic water” necessarily implies a vast
counter-movement of warm surface-water. So that if there is more polar
water in the South Atlantic to produce the cooling effect, there should
likewise be more warm water to be cooled.
According to the wind theory of oceanic circulation the explanation of
the whole phenomena is simple and obvious. It has already been shown
that owing to the fact that the S. E. trades are stronger than the N.
E., and blow constantly over upon the northern hemisphere, the warm
surface-water of the South Atlantic is drifted across the equator. It
is then carried by the equatorial current into the Gulf of Mexico, and
afterwards of course forms a part of the Gulf-stream.
The North Atlantic, on the other hand, not only does not lose its
surface heat like the equatorial and South Atlantic, but it receives
from the Gulf-stream in the form of warm water an amount of heat, as we
have seen, equal to one-fourth of all the heat which it receives from
the sun. The reason why the warm surface strata are so much thicker
on the North Atlantic than on the equatorial regions is perfectly
obvious. The surface-water at the equator is swept into the Gulf of
Mexico by the trade-winds and the equatorial current, as rapidly as it
is heated by the sun, so that it has not time to gather to any great
depth. But all this warm water is carried by the Gulf-stream into the
North Atlantic, where it accumulates. That this great depth of warm
water in the North Atlantic, represented in the section, is derived
from the Gulf-stream, and not from a direct flow from the equator due
to gravitation, is further evident from the fact that temperature
sounding A in latitude 38° N. is made through that immense body of warm
water, upwards of 300 fathoms thick, extending from Bermuda to near the
Azores, discovered by the _Challenger_ Expedition, and justly regarded
by Captain Nares as an offshoot of the Gulf-stream. This, in Captain
Nares’s Report, is No. 8 “temperature sounding,” between Bermuda and
the Azores; sounding B is No. 6 “temperature curve,” between Teneriffe
and St. Thomas.
There is an additional reason to the one already stated why the
surface temperature of the South Atlantic should be so much below
that of the North. It is perfectly true that whatever amount of water
is transferred from the southern hemisphere to the northern must be
compensated by an equal amount from the northern to the southern
hemisphere, nevertheless the warm water which is carried off the South
Atlantic by the winds is not directly compensated by water from the
north, but by that cold antarctic current whose existence is so well
known to mariners from the immense masses of ice which it brings from
the Southern Ocean.
_Thermal Condition of Southern Ocean._——The thermal condition of the
Southern Ocean, as ascertained by the _Challenger_ Expedition, appears
to me to be also irreconcilable with the gravitation theory. Between
the parallels of latitude 65° 42′ S. and 50° 1′ S., the ocean, with
the exception of a thin stratum at the surface heated by the sun’s
rays, was found, down to the depth of about 200 fathoms, to be several
degrees colder than the water underneath.[100] The cold upper stratum
is evidently an antarctic current, and the warm underlying water an
equatorial under current. But, according to the gravitation theory, the
colder water should be underneath.
The very fact of a mass of water, 200 fathoms deep and extending over
fifteen degrees of latitude, remaining above water of three or four
degrees higher temperature shows how little influence difference of
temperature has in producing motion. If it had the potency which some
attribute to it, one would suppose that this cold stratum should sink
down and displace the warm water underneath. If difference of density
is sufficient to move the water horizontally, surely it must be more
than sufficient to cause it to sink vertically.
CHAPTER XIV.
THE WIND THEORY OF OCEANIC CIRCULATION IN RELATION TO CHANGE
OF CLIMATE.
Direction of Currents depends on Direction of the Winds.—Causes
which affect the Direction of Currents will affect
Climate.—How Change of Eccentricity affects the Mode
of Distribution of the Winds.—Mutual Reaction of Cause
and Effect.—Displacement of the Great Equatorial
Current.—Displacement of the Median Line between the Trades,
and its Effect on Currents.—Ocean-currents in Relation to the
Distribution of Plants and Animals.—Alternate Cold and Warm
Periods in North and South.—Mr. Darwin’s Views quoted.—How
Glaciers at the Equator may be accounted for.—Migration
across the Equator.
_Ocean-currents in Relation to Change of Climate._—In my attempts to
prove that oceanic circulation is produced by the winds and not by
difference of specific gravity, and that ocean-currents are the great
distributors of heat over the globe, my chief aim has been to show
the bearing which these points have on the grand question of secular
changes of climate during geological epochs, more particularly in
reference to that mystery the cause of the glacial epoch.
In concluding this discussion regarding oceanic circulation, I may
therefore be allowed briefly to recapitulate those points connected
with the subject which seem to shed most light on the question of
changes of climate.
The complete agreement between the systems of ocean-currents and
winds not only shows that the winds are the impelling cause of the
currents, but it also indicates to what an extent the _directions_ of
the currents are determined by the winds, or, more properly, to what an
extent their directions are determined by the _direction_ of the winds.
We have seen in Chapter II. to what an enormous extent the climatic
conditions of the globe are dependent on the distribution of heat
effected by means of ocean-currents. It has been there pointed out
that, if the heat conveyed from inter-tropical to temperate and polar
regions by oceanic circulation were restored to the former, the
equatorial regions would then have a temperature about 55° warmer,
and the high polar regions a climate 83° colder than at present. It
follows, therefore, that any cause which will greatly affect the
currents or greatly change their paths and mode of distribution, will
of necessity seriously affect the climatic condition of the globe. But
as the existence of these currents depends on the winds, and their
direction and form of distribution depend upon the direction and form
of distribution of the winds, any cause which will greatly affect the
winds will also greatly affect the currents, and consequently will
influence the climatic condition of the globe. Again, as the existence
of the winds depends mainly on the difference of temperature between
equatorial and polar regions, any cause which will greatly affect this
difference of temperature will likewise greatly affect the winds; and
these will just as surely react on the currents and climatic conditions
of the globe. A simple increase or decrease in the difference of
temperature between equatorial and polar regions, though it would
certainly produce an increase or a decrease, as the case might be, in
the strength of the winds, and consequently in the strength of the
currents, would not, however, greatly affect the mode of _distribution_
of the winds, nor, as a consequence, the mode of _distribution_ of
the currents. But although a simple change in the difference of
temperature between the equator and the poles would not produce a
different _distribution_ of aërial, and consequently of ocean-currents,
nevertheless a _difference in the difference_ of temperature between
the equator and the two poles would do so; that is to say, any cause
that should increase the difference of temperature between the equator
and the pole on the one hemisphere, and decrease that difference on
the other, would effect a change in the distribution of the aërial
currents, which change would in turn produce a corresponding change in
the distribution of ocean-currents.
It has been shown[101] that an increase in the eccentricity of the
earth’s orbit tends to lower the temperature of the one hemisphere and
to raise the temperature of the other. It is true that an increase of
eccentricity does not afford more heat to the one hemisphere than to
the other; nevertheless it brings about a condition of things which
tends to lower the temperature of the one hemisphere and to raise the
temperature of the other. Let us imagine the eccentricity to be at its
superior limit, 0·07775, and the winter solstice in the aphelion. The
midwinter temperature, owing to the increased distance of the sun,
would be lowered enormously; and the effect of this would be to cause
all the moisture which now falls as rain during winter in temperate
regions to fall as snow. Nor is this all; the winters would not merely
be colder than now, but they would also be much longer. At present the
summer half-year exceeds the winter half year by nearly eight days; but
at the period in question the winters would be longer than the summers
by upwards of thirty-six days. The heat of the sun during the short
summer, for reasons which have already been explained, would not be
sufficient to melt the snow of winter; so that gradually, year by year,
the snow would continue to accumulate on the ground.
On the southern hemisphere the opposite condition of things would
obtain. Owing to the nearness of the sun during the winter of that
hemisphere, the moisture of the air would be precipitated as rain in
regions where at present it falls as snow. This and the shortness of
the winter would tend to produce a decrease in the quantity of snow.
The difference of temperature between the equatorial and the temperate
and polar regions would therefore be greater on the northern than on
the southern hemisphere; and, as a consequence, the aërial currents
of the former hemisphere would be stronger than those of the latter.
This would be more especially the case with the trade-winds. The
N.E. trades being stronger than the S.E. trades would blow across the
equator, and the median line between them would therefore be at some
distance to the south of the equator. Thus the equatorial waters would
be impelled more to the southern than to the northern hemisphere; and
the warm water carried over in this manner to the southern hemisphere
would tend to increase the difference of temperature between the two
hemispheres. This change, again, would in turn tend to strengthen the
N.E. and to weaken the S.E. trades, and would thus induce a still
greater flow of equatorial waters into the southern hemisphere—a
result which would still more increase the difference of temperature
between the northern and southern hemisphere, and so on—the one cause
so reacting on the other as to increase its effects, as was shown at
length in Chapter IV.
It was this mutual reaction of those physical agents which led, as was
pointed out in Chapter IV., to that extraordinary condition of climate
which prevailed during the glacial epoch.
There is another circumstance to be considered which perhaps more
than any thing else would tend to lower the temperature of the one
hemisphere and to raise the temperature of the other; and this is
the _displacement of the great equatorial current_. During a glacial
period in the northern hemisphere the median line between the trades
would be shifted very considerably south of the equator; and the same
would necessarily be the case with the great equatorial currents, the
only difference being that the equatorial currents, other things being
equal, would be deflected farther south than the median line. For the
water impelled by the strong N.E. trades would be moving with greater
velocity than the waters impelled by the weaker S.E. trades, and, of
course, would cross the median line of the trades before its progress
southwards could be arrested by the counteracting influence of the S.E.
trades. Let us glance briefly at the results which would follow from
such a condition of things. In the first place, as was shown on former
occasions,[102] were the equatorial current of the Atlantic (the feeder
of the Gulf-stream) shifted considerably south of its present position,
it would not bifurcate, as it now does, off Cape St. Roque, owing to
the fact that the whole of the waters would strike obliquely against
the Brazilian coast and thus be deflected into the Southern Ocean. The
effect produced on the climate of the North Atlantic and North-Western
Europe by the withdrawal of the water forming the Gulf-stream, may be
conceived from what has already been stated concerning the amount of
heat conveyed by that stream. The heat thus withdrawn from the North
Atlantic would go to raise the temperature of the Southern Ocean and
antarctic regions. A similar result would take place in the Pacific
Ocean. Were the equatorial current of that ocean removed greatly to
the south of its present position, it would not then impinge and be
deflected upon the Asiatic coast, but upon the continent of Australia;
and the greater portion of its waters would then pass southward into
the Southern Ocean, while that portion passing round the north of
Australia (owing to the great strength of the N.E. trades) would rather
flow into the Indian Ocean than turn round, as now, along the east
coast of Asia by the Japan Islands. The stoppage of the Japan current,
combined with the displacement of the equatorial current to the south
of the equator, would greatly lower the temperature of the whole of the
North Pacific and adjoining continents, and raise to a corresponding
degree the temperature of the South Pacific and Southern Ocean. Again,
the waters of the equatorial current of the Indian Ocean (owing to the
opposing N.E. trades), would not, as at present, find their way round
the Cape of Good Hope into the North Atlantic, but would be deflected
southwards into the Antarctic Sea.
We have in the present state of things a striking example of the extent
to which the median line between the two trades may be shifted, and the
position of the great equatorial currents of the ocean may be affected,
by a slight difference in the relative strength of the two aërial
currents. The S.E. trades are at present a little stronger than the
N.E.; and the consequence is that they blow across the equator into the
northern hemisphere to a distance sometimes of 10 or 15°, so that the
mean position of the median line lies at least 6 or 7 degrees north of
the equator.
And it is doubtless owing to the superior strength of the S.E. trades
that so much warm water crosses the equator from the South to the North
Atlantic, and that the main portion of the equatorial current flows
into the Caribbean Sea rather than along the Brazilian coast. Were the
two trades of equal strength, the transference of heat into the North
Atlantic from the southern hemisphere by means of the Southern Atlantic
and equatorial currents would be much less than at present. The same
would also hold true in regard to the Pacific.
_Ocean-currents in Relation to the Distribution of Plants and
Animals._—In the fifth and last editions of the “Origin of Species,”
Mr. Darwin has done me the honour to express his belief that the
foregoing view regarding alternate cold and warm periods in north
and south during the glacial epoch explains a great many facts in
connection with the distribution of plants and animals which have
always been regarded as exceedingly puzzling.
There are certain species of plants which occur alike in the temperate
regions of the southern and northern hemispheres. At the equator these
same temperate forms are found on elevated mountains, but not on the
lowlands. How, then, did these temperate forms manage to cross the
equator from the northern temperate regions to the southern, and _vice
versâ_? Mr. Darwin’s solution of the problem is (in his own words) as
follows:—
“As the cold became more and more intense, we know that arctic forms
invaded the temperate regions; and from the facts just given, there
can hardly be a doubt that some of the more vigorous, dominant, and
widest-spreading temperate forms invaded the equatorial lowlands.
The inhabitants of these hot lowlands would at the same time have
migrated to the tropical and subtropical regions of the south; for the
southern hemisphere was at this period warmer. On the decline of the
glacial period, as both hemispheres gradually recovered their former
temperatures, the northern temperate forms living on the lowlands under
the equator would have been driven to their former homes or have been
destroyed, being replaced by the equatorial forms returning from the
south. Some, however, of the northern temperate forms would almost
certainly have ascended any adjoining high land, where, if sufficiently
lofty, they would have long survived like the arctic forms on the
mountains of Europe.”
“In the regular course of events the southern hemisphere would in
its turn be subjected to a severe glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate forms
would invade the equatorial lowlands. The northern forms which had
before been left on the mountains would now descend and mingle with the
southern forms. These latter, when the warmth returned, would return
to their former homes, leaving some few species on the mountains, and
carrying southward with them some of the northern temperate forms which
had descended from their mountain fastnesses. Thus we should have some
few species identically the same in the northern and southern temperate
zones and on the mountains of the intermediate tropical regions” (p.
339, sixth edition).
Additional light is cast on this subject by the results already stated
in regard to the enormous extent to which the temperature of the
equator is affected by ocean-currents. Were there no transferrence of
heat from equatorial to temperate and polar regions, the temperature
of the equator, as has been remarked, would probably be about 55°
warmer than at present. In such a case no plant existing on the face of
the globe could live at the equator unless on some elevated mountain
region. On the other hand, were the quantity of warm water which is
being transferred from the equator to be very much increased, the
temperature of inter-tropical latitudes might be so lowered as easily
to admit of temperate species of plants growing at the equator. A
lowering of the temperature at the equator some 20° or 30° is all that
would be required; and only a moderate increase in the volume of the
currents proceeding from the equator, taken in connection with the
effects flowing from the following considerations, might suffice to
produce that result. During the glacial epoch, when the one hemisphere
was under ice and the other enjoying a warm and equable climate, the
median line between the trades may have been shifted to almost the
tropical line of the warm hemisphere. Under such a condition of things
the warmest part would probably be somewhere about the tropic of the
warm hemisphere, and not, as now, at the equator; for since all, or
nearly all, the surface-water of the equator would then be impelled
over to the warm hemisphere, the tropical regions of that hemisphere
would be receiving nearly double their present amount of warm water.
Again, as the equatorial current at this time would be shifted towards
the tropic of the warm hemisphere, the surface-water would not, as at
present, be flowing in equatorial regions parallel to the equator,
but obliquely across it from the cold to the warm hemisphere. This of
itself would tend greatly to lower the temperature of the equator.
It follows, therefore, as a necessary consequence, that during the
glacial epoch, when the one hemisphere was under snow and ice and
the other enjoying a warm and equable climate, the temperature of
the equator would be lower than at present. But when the glaciated
hemisphere (which we may assume to be the northern) began to grow
warmer and the climate of the southern or warm hemisphere to get
colder, the median line of the trades and the equatorial currents
of the ocean also would begin to move back from the southern tropic
towards the equator. This would cause the temperature of the equator
to rise and to continue rising until the equatorial currents reached
their normal position. When the snow began to accumulate on the
southern hemisphere and to disappear on the northern, the median line
of the trades and the equatorial currents of the ocean would then
begin to move towards the northern tropic as they had formerly towards
the southern. The temperature of the equator would then again begin
to sink, and continue to do so until the glaciation of the southern
hemisphere reached its maximum. This oscillation of the thermal equator
to and fro across the geographical equator would continue so long as
the alternate glaciation of the two hemispheres continued.
This lowering of the temperature of the equator during the severest
part of the glacial epoch will help to explain the former existence of
glaciers in inter-tropical regions at no very great elevation above the
sea-level, evidence of which appears recently to have been found by
Professor Agassiz, Mr. Belt, and others.
The glacial _epoch_ may be considered as contemporaneous in both
hemispheres. But the epoch consisted of a succession of cold and warm
_periods_, the cold periods of one hemisphere coinciding with the warm
periods of the other, and _vice versâ_.
_Migration across the Equator._—Mr. Belt[103] and others have felt
some difficulty in understanding how, according to theory, the plants
and animals of temperate regions could manage to migrate from one
hemisphere to the other, seeing that in their passage they would have
to cross the thermal equator. The oscillation to and fro of the thermal
equator across the geographical, removes every difficulty in regard to
how the migration takes place. When, for example, a cold period on the
northern hemisphere and the corresponding warm one on the southern were
at their maximum, the thermal equator would by this time have probably
passed beyond the Tropic of Capricorn. The geographical equator would
then be enjoying a subtropical, if not a temperate condition of
climate, and the plants and animals of the northern hemisphere would
manage then to reach the equator. When the cold began to abate on
the northern and to increase on the southern hemisphere, the thermal
equator would commence its retreat towards the geographical. The plants
and animals from the north, in order to escape the increasing heat as
the thermal equator approached them, would begin to ascend the mountain
heights; and when that equator had passed to its northern limit, and
the geographical equator was again enjoying a subtropical condition of
climate, the plants and animals would begin to descend and pursue their
journey southwards as the cold abated on the southern hemisphere.
CHAPTER XV.
WARM INTER-GLACIAL PERIODS.
Alternate Cold and Warm Periods.—Warm Inter-glacial Periods
a Test of Theories.—Reason why their Occurrence has not
been hitherto recognised.—Instances of Warm Inter-glacial
Periods.—Dranse, Dürnten, Hoxne, Chapelhall, Craiglockhart,
Leith Walk, Redhall Quarry, Beith, Crofthead, Kilmaurs,
Sweden, Ohio, Cromer, Mundesley, &c., &c.—Cave and River
Deposits.—Occurrence of Arctic and Warm Animals in some Beds
accounted for.—Mr. Boyd Dawkins’s Objections.—Occurrence
of Southern Shells in Glacial Deposits.—Evidence of Warm
Inter-glacial Periods from Mineral Borings.—Striated
Pavements.—Reason why Inter-glacial Land-surfaces are so rare.
_Alternate Cold and Warm Periods._—If the theory developed in the
foregoing chapters in reference to the cause of secular changes of
climate be correct, it follows that that long age known as the glacial
epoch did not, as has hitherto been generally supposed, consist of one
long unbroken period of cold and ice. Neither did it consist, as some
have concluded, of two long periods of ice with an intervening mild
period, but it must have consisted of a long succession of cold and
warm periods; the warm periods of the one hemisphere corresponding in
time with the cold periods of the other and _vice versâ_. It follows
also from theory that as the cold periods became more and more severe,
the warm intervening periods would become more and more warm and
equable. As the ice began to accumulate during the cold periods in
subarctic and temperate regions in places where it previously did not
exist, so in like manner during the corresponding warm periods it would
begin to disappear in arctic regions where it had held enduring sway
throughout the now closing cycle. As the cold periods in the southern
hemisphere became more and more severe, the ice would continue to
advance northwards in the temperate regions; but at that very same
time the intervening warm periods in the northern hemisphere would
become warmer and warmer and more equable, and the ice of the arctic
regions would continue to disappear farther and farther to the north,
till by the time that the ice had reached a maximum during the cold
antarctic periods, Greenland and the arctic regions would, during the
warm intervening periods, be probably free of ice and enjoying a mild
and equable climate. Or we may say that as the one hemisphere became
cold the other became warm, and when the cold reached a maximum in the
one hemisphere, the warmth would reach a maximum in the other. The time
when the ice had reached its greatest extension on the one hemisphere
would be the time when it had disappeared from the other.
_Inter-glacial Periods a Test of Theories._—Here we have the grand
crucial test of the truth of the foregoing theory of the cause of
the glacial epoch. That the glacial epoch should have consisted of a
succession of cold and warm periods is utterly inconsistent with all
previous theories which have been advanced to account for it. What,
then, is the evidence of geology on this subject? If the glacial epoch
can be proved from geological evidence to have consisted of such a
succession of cold and warm periods, then I have little doubt but the
theory will soon be generally accepted. But at the very outset an
objection meets us, viz., why call an epoch, which consisted as much of
warm periods as of cold, a glacial epoch, or an “Ice Age,” as Mr. James
Geikie tersely expresses it? Why not as well call it a warm epoch as a
cold one, seeing that, according to theory, it was just as much a warm
as a cold epoch? The answer to this objection will be fully discussed
in the chapter on the Reason of the Imperfection of Geological Records.
But in the meantime, I may remark that it will be shown that the epoch
known as the glacial has been justly called the glacial epoch or “Ice
Age,” because the geological evidences of the cold periods remain in
a remarkably perfect state, whilst the evidences of the warm periods
have to a great extent disappeared. The reason of this difference
in the two cases will be discussed in the chapter to which I have
referred. Besides, the condition of things during the cold periods was
so extraordinary, so exceptional, so totally different from those now
prevailing, that even supposing the geological records of the warm
periods had been as well preserved as those of the cold, nevertheless
we should have termed the epoch in question a glacial epoch. There
is yet another reason, however, for our limited knowledge of warm
inter-glacial periods. Till very lately, little or no attention was
paid by geologists to this part of the subject in the way of keeping
records of cases of inter-glacial deposits which, from time to time,
have been observed. Few geologists ever dreamt of such a thing as
warm periods during the age of ice, so that when intercalated beds of
sand and gravel, beds of peat, roots, branches, trunks, leaves, and
fruits of trees were found in the boulder clay, no physical importance
was attached to them, and consequently no description or record of
them ever kept. In fact, all such examples were regarded as purely
accidental and exceptional, and were considered not worthy of any
special attention. A case which came under my own observation will
illustrate my meaning. An intelligent geologist, some years ago, read a
paper before one of our local geological societies, giving an account
of a fossiliferous bed of clay found intercalated between two distinct
beds of till. In this intercalated bed were found rootlets and stems of
trees, nuts, and other remains, showing that it had evidently been an
old inter-glacial land surface. In the transactions of the society a
description of the two beds of till was given, but no mention whatever
was made of the intercalated bed containing the organic remains,
although this was the only point of any real importance.
Since the theory that the glacial epoch resulted from a high state
of eccentricity of the earth’s orbit began to receive some little
acceptance, geologists have paid a good deal of attention to cases of
intercalated beds in the till containing organic remains, and the
result is that we have already a great body of evidence of a geological
nature in favour of warm inter-glacial periods, and I have little
doubt that in the course of a few years the former occurrence of warm
inter-glacial periods will be universally admitted.
I shall now proceed to give a very brief outline of the evidence
bearing on the subject. But the cases to which I shall have to refer
are much too numerous to allow me to enter into details.
_Inter-glacial Beds of Switzerland._—The first geologist, so far as I
am aware, who directed attention to evidence of a break in the cold of
the glacial epoch was M. Morlot. It is now twenty years ago since he
announced the existence of a warm period during the glacial epoch from
geological evidence connected with the glacial drift of the Alps.[104]
The rivers of Switzerland, he found, show on their banks three
well-marked terraces of regularly stratified and well-rounded shingle,
identical with the modern deposits of the rivers. They stand at 50,
100, and 150 feet above the present level of the rivers. These terraces
were evidently formed by the present system of rivers when these flowed
at a higher level, and extend up the Alps to a height of from 3,000 to
4,000 feet above the level of the sea. There is a terrace bordering the
Rhine at Camischollas, above Disentis, 4,400 feet above the level of
the sea, proving that during the period of its formation the Alps were
free of ice up to the height of 4,400 feet above the sea-level. It is
well known that a glacial period must have succeeded the formation of
these drifts, for they are in many places covered with erratics. At
Geneva, for example, an erratic drift nearly 50 feet thick is seen to
rest on the drift of the middle terrace, which rises 100 feet above the
level of the lake. But it is also evident that a glacial period must
have preceded the formation of the drift beds, for they are found to
lie in many places upon the unstratified boulder clay or _till_. M.
Morlot observed in the neighbourhood of Clareus, from 7 to 9 feet of
drift resting upon a bed of true till 40 feet thick; the latter was
composed of a compact blue clay, containing worn and scratched alpine
boulders and without any trace of stratification. In the gorge of
Dranse, near Thoron, M. Morlot found the whole three formations in a
direct superimposed series. At the bottom was a mass of compact till or
boulder clay, 12 feet thick, containing boulders of alpine limestone.
Over this mass came regularly stratified beds 150 feet thick, made
up of rounded pebbles in horizontal beds. Above this again lay a
second formation of unstratified boulder clay, with erratic blocks and
striated pebbles, which constituted the left lateral moraine of the
great glacier of the Rhone, when it advanced for the second time to the
Lake of Geneva. A condition of things somewhat similar was observed by
M. Ischer in the neighbourhood of Berne.
These facts, M. Morlot justly considers, prove the existence of two
glacial periods separated by an intermediate one, during which the
ice, which had not only covered Switzerland, but the greater part of
Europe, disappeared even in the principal valleys of the Alps to a
height of more than 4,400 feet above the present level of the sea. This
warm period, after continuing for long ages, was succeeded by a second
glacial period, during which the country was again covered with ice as
before. M. Morlot even suggests the possibility of these alternations
of cold and warm periods depending upon a cosmical cause. “Wild as it
may have appeared,” he says, “when first started, the idea of general
and periodical eras of refrigeration for our planet, connected perhaps
with some cosmic agency, may eventually prove correct.”[105]
Shortly afterwards, evidence of a far more remarkable character was
found in the glacial drift of Switzerland, namely, the famous lignite
beds of Dürnten. In the vicinity of Utznach and Dürnten, on the Lake of
Zurich, and near Mörschwyl, on the Lake of Constance, there are beds of
coal or lignite, nearly 12 feet thick, lying directly on the boulder
clay. Overlying these beds is another mass of drift and clay 30 feet
in thickness, with rounded blocks, and on the top of this upper drift
lie long angular erratics, which evidently have been transported on
the back of glaciers.[106] Professor Vogt attributes their transport
to floating ice; but he evidently does so to avoid the hypothesis of a
warm period during the glacial epoch.
Here we have proof not merely of the disappearance of the ice during
the glacial epoch, but of its absence during a period of sufficient
length to allow of the growth of 10 or 12 feet of coal. Professor Heer
thinks that this coal-bed, when in the condition of peat, must have
been 60 feet thick; and assuming that one foot of peat would be formed
in a century, he concludes that 6,000 years must have been required
for the growth of the coal plants. According to Liebig, 9,600 years
would be required. This, as we have already seen, is about the average
duration of a warm period.
In these beds have been found the bones of the elephant (_E. Merkii_),
stag, cave-bear, and other animals. Numerous insects have also been met
with, which further prove the warm, mild condition of climate which
must have prevailed at the time of the formation of the lignite.
At Hoxne, near Diss, in Suffolk, a black peaty mass several feet thick,
containing fragments of wood of the oak, yew, and fir, was found,
overlying the boulder clay.[107] Professor Vogt believes that this peat
bed is of the same age as the lignite beds of Switzerland.
In the glacial drift of North America, particularly about Lake
Champlain and the valley of the St. Lawrence, there is similar evidence
of two glacial periods with an intervening non-glacial or warm
period.[108]
_Glacial and Inter-glacial Periods of the Southern Hemisphere_—(_South
Africa_).—Mr. G. W. Stow, in a paper on the “Geology of South
Africa,”[109] describes a recent glaciation extending over a large
portion of Natal, British Kaffraria, the Kaga and Krome mountains,
which he attributes to the action of land-ice. He sums up the phenomena
as follows:—“The rounding off of the hills in the interiors of the
ancient basins; the numerous dome-shaped (_roches moutonnée_) rocks;
the enormous erratic boulders in positions where water could not have
carried them; the frequency of unstratified clays—clays with imbedded
angular boulders; drift and lofty mounds of boulders; large tracts of
country thickly spread over with unstratified clays and superimposed
fragments of rock; the Oliphant’s-Hoek clay, and the vast piles of Enon
conglomerate.” In addition to these results of ice-action, he records
the discovery by himself of distinct ice-scratches or groovings on the
surface of the rocks at Reit-Poort in the Tarka, and subsequently[110]
the discovery by Mr. G. Gilfillan of a large boulder at Pniel with
_striæ_ distinctly marked upon it, and also that the same observer
found that almost every boulder in the gravel at “Moonlight Rush” had
unmistakable striæ on one or more sides.
In South Africa there is evidence not only of a glacial condition
during the Pliocene period, but also of a warmer climate than now
prevails in that region. “The evidence,” says Mr. Stow, “of the
Pliocene shells of the superficial limestone of the Zwartkops heights,
and elsewhere, leads us to believe that the climate of South Africa
must have been of a far more tropical character than at present.
“Take, for instance, the characteristic _Venericardia_ of that
limestone. This has migrated along the coast some 29° or 30° and is now
found within a few degrees of the equator, near Zanzibar, gradually
driven, as I presume it must have been, further and further north by a
gradual lowering of the temperature of the more southern parts of this
coast since the limestone was deposited.”
“During the formation of the shell-banks in the Zwartkops estuary,
younger than the Pliocene limestone, the immense number of certain
species of shells, which have as yet been found living only in
latitudes nearer the equator, point to a somewhat similar though a more
modified change of temperature.”
_Inter-glacial Beds of Scotland._—Upwards of a dozen years ago,
Professor Geikie arrived, from his own observations of the glacial
drift of Scotland, at a similar conclusion to that of M. Morlot
regarding the intercalation of warm periods during the glacial epoch;
and the facts on which Professor Geikie’s conclusions were based are
briefly as follows. In a cliff of boulder clay on the banks of the
Slitrig Water, near the town of Hawick, he observed a bed of stones
or shingle. Over the lower stratum of stones lay a few inches of
well-stratified sand, silt, and clay, some of the layers being black
and peaty, _with enclosed vegetable fibres_ in a crumbling state.[111]
There were some 30 or 40 feet of boulder clay above these stratified
beds, and 15 or 20 feet under them. The stones in the shingle band
were identical with those of the boulder clay, but they showed no
striations, and were more rounded and water-worn, and resembled in
every respect the stones now lying in the bed of the Slitrig. The
section of the cliff stood as under:—
1. Vegetable soil.
2. Boulder clay, thirty to forty feet.
{ 3. Yellowish gravelly sand.
Stratified beds { 4. Peaty silt and clay.
{ 5. Fine ferruginous sand.
{ 6. Coarse shingle, two to three feet.
7. Coarse, stiff boulder clay, fifteen to twenty feet.
A few more cases of intercalation of stratified materials in the true
till were also found in the same valley.