law that I have mentioned, the earth reacts upon the moon. The form
which this reaction assumes expresses itself in a tendency to allow
the moon gradually to move further and further away from the earth
than the earth’s attraction would permit if our globe were a solid
mass void of all liquid capable of being distracted by tides. It
is, therefore, certain that the distance of the moon, which is at
present about two hundred and forty thousand miles, must be gradually
increasing; but we need not look for any appreciable change in the
moon’s distance arising from this cause when only an interval of
a few centuries is considered. We need not expect to measure the
difference due to tides between the size of the moon’s orbit this
month and the size of the orbit last month. In fact, there are so
many periodic causes of change in the dimensions of the moon’s orbit
that it becomes impossible to detect the tidal influence even in
the course of centuries. Here, again, we have to remember that in
dealing with the history of our earth we are to consider not merely
the thousands of years that include the human period, not merely the
millions of years that are required by the necessities of geology,
but also those unknown periods anterior to geological phenomena to
which we have already referred.
In the course of such vast ages the reaction of the earth on
the moon’s orbit has not only become perceptible, it has become
conspicuous; it has not only become conspicuous, but it has become
the chief determining agent in making the moon’s orbit as we find it
at the present day. We have seen that as we look into the past the
length of the day seems ever shorter and shorter; and concurrent with
this decline in the day is the diminution in the moon’s distance from
the earth. There was a time when the moon, instead of revolving at
a distance of two hundred and forty thousand miles, as it does at
present, revolved at a distance of only two hundred thousand miles.
As we think of epochs still earlier we discern the moon ever closer
and closer to the earth, until at last, at that critical time in
the history of the earth-moon system, when the earth was quickly
revolving in a period of a few hours, our satellite seems to have
been quite close to the earth; in fact, the two bodies were almost in
contact
The study of the tides has therefore conducted us to the knowledge
of a remarkable configuration exhibited in the primitive earth-moon
system. The earth was then spinning round rapidly in a day which
was only a few hours long, while close to the earth, or almost in
contact with it, the moon coursed around our globe, the period of
its revolution being shorter to such an extent that the satellite
completed its circuit in the same time as the earth required for one
turn round its axis.
We must remember that the materials destined to form the pair of
allied planets did not then form two solid bodies as they do at
present; they were both, in all probability, incandescent masses
glowing with fervor, and soft, if not actually molten, or incoherent,
or even gaseous. These aggregations were close together, and one of
them was whirling around the other in a period of a few hours, the
duration of that period being equal to the time in which the larger
mass revolved on its axis. In fact, the two objects, even though
distinct, seem to have revolved the one around the other as if they
had been bound together by rigid bonds. The rapid rotation with which
they were animated suggests a cause for this state of things. It is
well known that a fly-wheel, when driven at an unduly high speed,
is liable to break asunder in consequence of its rapid motion. If a
grindstone be urged around with excessive velocity the force tending
to rend the stone into fragments may overcome its cohesion, and it
will fly into pieces, often projected with such violence that fearful
accidents have been the consequence.
Viewing the earth as a rotating body, it must be subject to the law
that there is a speed which can not be exceeded with safety. With
the present period of rotation of once in every twenty-four hours
the tendency to disruption is but small and consequently the earth
retains its integrity, though no doubt the protuberance at the
equator is the result of the accommodation of the shape of the globe
to the circumstances attending its revolution. But let us suppose
that the length of the day was greatly diminished, or, what comes to
the same thing, that the speed with which the earth rotates on its
axis was greatly increased; it is then conceivable that the tension
thus arising might be too great for the coherence of the material
to withstand. We believe that the earth could turn round with double
the speed that it has at present before this tension approached the
point at which disruption would ensue. But supposing the day were to
be so much shortened that the period of rotation was only a very few
hours instead of twenty-four, there is then good reason to know that
the tension in the earth arising from this rapid rotation would be so
great that a rupture of the globe would be imminent.
Provided with this conception, let us think of the initial stage
when the moon was quite close to the earth. Our globe was then, as
we know, spinning round so rapidly that its materials were almost on
the point of breaking up in consequence of the strain produced by
the rotation. It is interesting to note that the tidal action of the
sun would also conduce to the rupture of our globe in the critical
circumstances we have supposed. It seems hardly possible to doubt
that such a separation of the glowing mass did actually take place, a
small fragment was discarded, and gradually drew itself by the mutual
attraction of its particles into a globular form and thus became the
moon.
We have seen that at the present moment the day is becoming gradually
longer and the moon is steadily receding further and further from the
earth. At present these changes take place with extreme slowness,
but in the primitive periods of which we have already spoken, the
changes in the length of the day, and the changes in the distance
of the moon, proceeded at a rate far more rapid than at present.
As the moon has receded further from the earth its efficiency as a
tide-producer has declined, and consequently the rate at which the
consequences of tidal action have proceeded is continually lessening.
It must therefore be expected that the progress of tidal evolution
in the future will be ever getting slower and slower, so that the
periods of time required for the further development of the phenomena
far exceed those which have elapsed in the course of the history
already given. We can, however, foreshadow what is to happen in the
following manner. The length of the day will slowly increase; and
we can indicate a state of things in the excessively remote future
toward which it may be said the system is tending. The day will grow
until it becomes not merely twenty-five or twenty-six hours, but
until it becomes as long as two or three of our present days. In
fact, as we stretch our imagination through ages so inconceivable
that I forbear to specify any figures which might characterize them,
we seem to discern that the length of the day may go on ever getting
longer and longer until at last a stage is reached when the day is
about fifty or sixty times as long as our present day.
All this time, in accordance with the general law of action and
reaction, the moon must be gradually retreating. As the orbit of
the moon is gradually enlarging, the time that the moon takes to
revolve around the earth must be continually on the increase; from
the present month of twenty-seven days the length of the month will
gradually augment as the ages roll by until at last when the moon
has reached a certain distance the period of its rotation will have
become double what it is at present, or indeed rather more than
double, and we shall have the day and the month equal, each being
about fourteen hundred hours long. When this state of things is
reached, the earth will always turn the same face toward the moon,
just as the moon at present always turns the same face toward the
earth.
We have already explained how the constant face of the moon can
be accounted for by the action of tides raised in the moon by the
attraction of the earth. Owing to the small size of the moon the
tides have already wrought all that they were capable of doing, and
have compelled the moon to succumb to the conditions they imposed.
Owing to the great mass of the earth and the comparatively small
mass of the moon the tides on the earth raised by the moon have
required a much longer period wherein to accomplish their effects
than was the case when the earth raised tides on the moon. But small
though our satellite may be, yet the tides raised on the earth have
incessantly tended to wear down the speed of our globe and reduce it
to conformity with the law that the two bodies shall bear the same
face toward each other. At present the earth turns round twenty-seven
times while the moon goes round once, so the tides have still a
gigantic task to accomplish. With unflagging energy, however, they
are incessantly engaged at the work, and they are constantly tending
to bring down the speed of the earth; constantly tending toward that
ultimate condition of things in which the earth and moon are destined
to revolve in a period of fourteen hundred hours as if they were
connected with invisible bonds.
If such a state of things as this were established then it is plain
that tides would no longer ebb and flow, that is, at least, if we
exclude from our consideration the intervention of any other body.
High tides must prevail at some parts of the earth, and low tides at
other parts, but the position of these tides will remain fixed. Where
it is high tide it will always be high tide; where it is low tide
it will always be low tide. When this state of things is reached,
the moon will be constantly visible in the same part of the sky from
one half of our globe, while the other half of our globe will never
be turned toward the moon. In fact, the moon would always appear
to us in a fixed position as the earth would always appear to be
if viewed by an observer stationed on the moon. If there were any
Lunarians whose residence was confined to the opposite side of the
moon, they could never see this earth at all, while those who lived
on this side of our satellite would always be able to see the earth
apparently fixed in the same part of the sky. An observatory located
at the middle of the moon’s disk, say near the crater Ptolemy, would
always have the earth in its zenith or very near thereto, while the
astronomer, let us say, in the Mare Crisium, would always find the
earth low down near his horizon.
In order to facilitate our reasoning I have assumed that the moon
is the only tide-producing agent; this is, however, not the case.
No doubt the ebb and the flow around our coasts is generated mainly
by the attraction of the moon. It must not, however, be forgotten
that a portion of the tide is originated by the attraction of the
sun. These solar tides will still continue to ebb and flow quite
independently of the lunar tides, so that even if the accommodation
between the earth and the moon had been completed some further tidal
disturbance would not be wanting. The effect of the solar tides will
be to abate still further the velocity with which the earth turns
round on its axis, and consequently a time must ultimately arrive
when the length of the day will be longer than the time which the
moon takes to revolve around our earth.
THE GULF STREAM
--LORD KELVIN
I mean by the Gulf Stream that mass of heated water which pours from
the Strait of Florida across the North Atlantic, and likewise a wider
but less definite warm current, evidently forming part of the same
great movement of water, which curves northward to the eastward of
the West Indian Islands. I am myself inclined, without hesitation,
to regard this stream as simply the reflux of the equatorial
current, added to no doubt during its northeasterly course by the
surface-drift of the anti-trades which follows in the main the same
direction.
The scope and limit of the Gulf Stream will be better understood if
we inquire in the first place into its origin and cause. As is well
known--in two bands, one to the north and the other to the south
of the equator--the northeast and southeast trade-winds, reduced
to meridional directions by the eastward frictional impulse of the
earth’s rotation, drive before them a magnificent surface current of
hot water 4,000 miles long by 450 miles broad at an average rate of
thirty miles a day. Off the coast of Africa, near its starting-point
to the south of the Islands of St. Thomas and Anna Bon this
“equatorial current” has a speed of forty miles in the twenty-four
hours, and a temperature of 23° C.
Increasing quickly in bulk, and spreading out more and more on both
sides of the equator, it flows rapidly due west toward the coast
of South America. At the eastern point of South America, Cape St.
Roque, the equatorial current splits into two, and one portion
trends southward to deflect the isotherms of 21°, 15°.5, 10°, and
4°.5 C. into loops upon our maps, thus carrying a scrap of comfort
to the Falkland Islands and Cape Horn; while the northern portion
follows the northeast coast of South America, gaining continually
in temperature under the influence of the tropical sun. Its speed
has now increased to sixty-eight miles in twenty-four hours, and
by the union with it of the waters of the river Amazon, it rises
to one hundred miles (6.5 feet in a second), but it soon falls off
again when it gets into the Caribbean Sea. Flowing slowly through
the whole length of this sea, it reaches the Gulf of Mexico through
the Strait of Yucatan, when a part of it sweeps immediately round
Cuba; but the main stream, “having made the circuit of the Gulf of
Mexico, passes through the Strait of Florida; thence it issues
as the ‘Gulf Stream’ in a majestic current upward of thirty miles
broad, two thousand two hundred feet deep, with an average velocity
of four miles an hour, and a temperature of 86° Fahr. (30° C.).”
The hot water pours from the strait with a decided though slight
northeasterly impulse on account of its great initial velocity. Mr.
Croll calculates the Gulf Stream as equal to a stream of water fifty
miles broad and a thousand feet deep flowing at a rate of four miles
an hour; consequently conveying 5,575,680,000,000 cubic feet of water
per hour, or 133,816,320,000,000 cubic feet per day. This mass of
water has a mean temperature of 18° C. as it passes out of the gulf,
and on its northern journey it is cooled down to 4°.5, thus losing
heat to the amount of 13°.5 C. The total quantity of heat therefore
transferred from the equatorial regions per day amounts to something
like 154,959,300,000,000,000,000 foot-pounds.
This is nearly equal to the whole of the heat received from the
sun by the Arctic regions, and, reduced by a half to avoid all
possibility of exaggeration, it is still equal to one-fifth of the
whole amount received from the sun by the entire area of the North
Atlantic. The Gulf Stream, as it issues from the Strait of Florida
and expands into the ocean on its northward course, is probably the
most glorious natural phenomenon on the face of the earth. The water
is of a clear crystalline transparency and an intense blue, and long
after it has passed into the open sea it keeps itself apart, easily
distinguished by its warmth, its color, and its clearness; and with
its edges so sharply defined that a ship may have her stem in the
clear blue stream while her stern is still in the common water of the
ocean.
Setting aside the wider question of the possibility of a general
oceanic circulation arising from heat, cold, and evaporation, I
believe that Captain Maury and Dr. Carpenter are the only authorities
who of late years have disputed this source of the current which
we see and can gauge and measure as it passes out of the Strait
of Florida; for it is scarcely necessary to refer to the earlier
speculations that it is caused by the Mississippi River, or that it
flows downward by gravitation from a “head” of water produced by the
trade-winds in the Caribbean Sea.
Captain Maury writes that “the dynamical force that calls forth the
Gulf Stream is to be found in the difference as to specific gravity
of intertropical and polar waters.” “The dynamical forces which are
expressed by the Gulf Stream may with as much propriety be said to
reside in those northern waters as in the West India seas: for on
one side we have the Caribbean Sea and Gulf of Mexico with their
waters of brine; on the other the great polar basin, the Baltic and
the North Sea, the two latter with waters which are little more than
brackish. In one set of these sea-basins the water is heavy; in the
other it is light. Between them the ocean intervenes; but water is
bound to seek and to maintain its level; and here, therefore, we
unmask one of those agents concerned in causing the Gulf Stream. What
is the power of this agent? Is it greater than that of other agents?
and how much? We can not say how much; we only know it is one of the
chief agents concerned. Moreover, speculate as we may as to all the
agencies concerned in collecting these waters, that have supplied the
trade-winds with vapor, into the Caribbean Sea, and then in driving
them across the Atlantic, we are forced to conclude that the salt
which the trade-wind vapor leaves behind it in the tropics has to be
conveyed away from the trade-wind region, to be mixed up again in
due proportion with the other water of the sea--the Baltic Sea and
the Arctic Ocean included--and that these are some of the waters, at
least, which we see running off through the Gulf Stream. To convey
them away is doubtless one of the offices which in the economy of the
ocean has been assigned to it.”
Dr. Carpenter attributes all the great movements of ocean water to a
general convective circulation, and of this general circulation he
regards the Gulf Stream as a peculiarly modified case. Dr. Carpenter
states that “the Gulf Stream constitutes a peculiar case, modified by
local conditions,” of “a great general movement of equatorial water
toward the polar area.” I confess I feel myself compelled to take a
totally different view. It seems to me that the Gulf Stream is the
one natural physical phenomenon on the surface of the earth whose
origin and principal cause, the drift of the trade-winds, can be most
clearly and easily traced.
The further progress and extension of the Gulf Stream through the
North Atlantic in relation to influence upon climate has been,
however, a fruitful source of controversy. The first part of its
course, after leaving the strait, is sufficiently evident, for its
water long remains conspicuously different in color and temperature
from that of the ocean, and a current having a marked effect on
navigation is long perceptible in the peculiar Gulf Stream water.
“Narrow at first, it flows round the peninsula of Florida, and, with
a speed of about 70 or 80 miles, follows the coast at first in a
due north, afterward in a northeast direction. At the latitude of
Washington it leaves the North American coast altogether, keeping
its northeastward course; and to the south of the St. George’s and
Newfoundland banks it spreads its waters more and more over the
Atlantic Ocean, as far as the Azores. At these islands a part of it
turns southward again toward the African coast. The Gulf Stream has,
so long as its waters are kept together along the American coast, a
temperature of 26°.6 C.; but, even under north latitude 36°, Sabine
found it 23°.3 C. at the beginning of December, while the sea-water
beyond the stream showed only 16°.9 C. Under north latitude 40-41°
the water is, according to Humboldt, at 22°.5 C. within, and 17°.5 C.
without the stream.”
Opposite Tortugas, passing along the Cuban coast, the stream is
unbroken and the current feeble; the temperature at the surface is
about 26°.7 C. Issuing from the Strait of Bemini the current is
turned nearly directly northward by the form of the land; a little
to the north of the strait, the rate is from three to five miles
an hour. The depth is only 325 fathoms, and the bottom, which in
the Strait of Florida was a simple slope and counter-slope, is now
corrugated. The surface temperature is about 26°.5 C., while the
bottom temperature is 4°.5; so that in the moderate depth of 325
fathoms the equatorial current above and the polar counter-current
beneath have room to pass one another, the current from the north
being evidently tempered considerably by mixture. North of Mosquito
inlet the stream trends to the eastward of north, and off St.
Augustine it has a decided set to the eastward. Between St. Augustine
and Cape Hatteras the set of the stream and the trend of the coast
differ but little, making 5° of easting in 5° of northing. At
Hatteras it curves to the northward, and then runs easterly. In the
latitude of Cape Charles it turns quite to the eastward, having a
velocity of from a mile to a mile and a half in the hour.
A brief account of one of the sections will best explain the general
phenomena of the stream off the coast of America. I will take the
section following a line at right angles to the coast off Sandy Hook.
From the shore out, for a distance of about 250 miles, the surface
temperature gradually rises from 21° to 24° C.; at 10 fathoms it
rises from 19° to 22° C.; and at 20 fathoms it maintains, with a few
irregularities, a temperature of 19° C. throughout the whole space;
while at 100, 200, 300, and 400 fathoms it maintains in like manner
the respective temperatures of 8°.8, 5°.7, 4°.5, and 2°.5 C. This
space is, therefore, occupied by cold water, and observation has
sufficiently proved that the low temperature is due to a branch of
the Labrador current creeping down along the coast in a direction
opposite to that of the Gulf Stream. In the Strait of Florida this
cold stream divides--one portion of it passing under the hot Gulf
Stream water into the Gulf of Mexico, while the remainder courses
round the western end of Cuba. Two hundred and forty miles from the
shore the whole mass of water takes a sudden rise of about 10° C.
within 25 miles, a rise affecting nearly equally the water at all
depths, and thus producing the singular phenomenon of two masses of
water in contact--one passing slowly southward and the other more
rapidly northward, at widely different temperatures at the same
levels. This abutting of the side of the cold current against that
of the Gulf Stream is so abrupt that it has been aptly called by
Lieutenant George M. Bache the “cold wall.” Passing the cold wall,
we reach the Gulf Stream, presenting all its special characters of
color and transparency and of temperature. In the section which we
have chosen as an example, upward of 300 miles in length, the surface
temperature is about 26°.5 C., but the heat is not uniform across
the stream, for we find that throughout its entire length, as far
south as the Cape Canaveral section, the stream is broken up into
longitudinal alternating bands of warmer and cooler water. Off Sandy
Hook, beyond the cold wall, the stream rises to a maximum of 27°.8
C., and this warm band extends for about 60 miles. The temperature
then falls to a minimum of 26°.5 C., which it retains for about
30 miles, when a second maximum of 27°.4 succeeds, which includes
the axis of the Gulf Stream, and is about 170 miles wide. This
is followed by a second minimum of 25°.5 C., and this by a third
maximum, when the bands become indistinct. It is singular that the
minimum bands correspond with valley-like depressions in the bottom,
which follow in succession the outline of the coast and lodge deep
southward extensions of the polar indraught.
The last section of the Gulf Stream surveyed by the American
hydrographers extends in a southeasterly direction from Cape Cod,
lat. 41° N., and traces the Gulf Stream, still broken up by its
bands of unequal temperature, spreading directly eastward across the
Atlantic; its velocity has, however, now become inconsiderable, and
its limits are best traced by the thermometer.