the famous one of Halley. You know something of the history of this.
Its period is 75-1/2 years. Halley saw it in 1682, and predicted its
return in 1758 or 1759--the first cometary prediction. Clairaut
calculated its return right within a month (p. 219). It has been back
once more, in 1835; and this time its date was correctly predicted
within three days, because Uranus was now known. It was away at its
furthest point in 1873. It will be back again in 1911.
[Illustration: FIG. 99.--Orbit of Halley's comet.]
Coming to recent times, we have the great comets of 1843 and of 1858,
the history of neither being known. Quite possibly they arrived then for
the first time. Possibly the second will appear again in 3808. But
besides these great comets, there are a multitude of telescopic ones,
which do not show these striking features, and have no gigantic tail.
Some have no tail at all, others have at best a few insignificant
streamers, and others show a faint haze looking like a microscopic
nebula.
All these comets are of considerable extent--some millions of miles
thick usually, and yet stars are clearly visible through them. Hence
they must be matter of very small density; their tails can be nothing
more dense than a filmy mist, but their nucleus must be something more
solid and substantial.
[Illustration: FIG. 100.--Various appearances of Halley's comet when
last seen.]
I have said that comets arrive from the depths of space, rush towards
and round the sun, whizzing past the earth with a speed of twenty-six
miles a second, on round the sun with a far greater velocity than that,
and then rush off again. Now, all the time they are away from the sun
they are invisible. It is only as they get near him that they begin to
expand and throw off tails and other appendages. The sun's heat is
evidently evaporating them, and driving away a cloud of mist and
volatile matter. This is when they can be seen. The comet is most
gorgeous when it is near the sun, and as soon as it gets a reasonable
distance away from him it is perfectly invisible.
The matter evaporated from the comet by the sun's heat does not
return--it is lost to the comet; and hence, after a few such journeys,
its volatile matter gets appreciably diminished, and so old-established
periodic comets have no tails to speak of. But the new visitants, coming
from the depths of space for the first time--these have great supplies
of volatile matter, and these are they which show the most magnificent
tails.
[Illustration: FIG. 101.--Head of Donati's comet of 1858.]
The tail of a comet is always directed away from the sun as if it were
repelled. To this rule there is no exception. It is suggested, and held
as most probable, that the tail and sun are similarly electrified, and
that the repulsion of the tail is electrical repulsion. Some great force
is obviously at work to account for the enormous distance to which the
tail is shot in a few hours. The pressure of the sun's light can do
something, and is a force that must not be ignored when small particles
are being dealt with. (Cf. _Modern Views of Electricity_, 2nd edition,
p. 363.)
Now just think what analogies there are between comets and meteors. Both
are bodies travelling in orbits round the sun, and both are mostly
invisible, but both become visible to us under certain circumstances.
Meteors become visible when they plunge into the extreme limits of our
atmosphere. Comets become visible when they approach the sun. Is it
possible that comets are large meteors which dip into the solar
atmosphere, and are thus rendered conspicuously luminous? Certainly they
do not dip into the actual main atmosphere of the sun, else they would
be utterly destroyed; but it is possible that the sun has a faint trace
of atmosphere extending far beyond this, and into this perhaps these
meteors dip, and glow with the friction. The particles thrown off might
be, also by friction, electrified; and the vaporous tail might be thus
accounted for.
[Illustration: FIG. 102.--Halley's Comet.]
Let us make this hypothesis provisionally--that comets are large
meteors, or a compact swarm of meteors, which, coming near the sun, find
a highly rarefied sort of atmosphere, in which they get heated and
partly vaporized, just as ordinary meteorites do when they dip into the
atmosphere of the earth. And let us see whether any facts bear out the
analogy and justify the hypothesis.
I must tell you now the history of three bodies, and you will see that
some intimate connection between comets and meteors is proved. The
three bodies are known as, first, Encke's comet; second, Biela's comet;
third, the November swarm of meteors.
Encke's comet (one of those discovered by Miss Herschel) is an
insignificant-looking telescopic comet of small period, the orbit of
which was well known, and which was carefully observed at each
reappearance after Encke had calculated its orbit. It was the quickest
of the comets, returning every 3-1/2 years.
[Illustration: FIG. 103.--Encke's comet.]
It was found, however, that its period was not quite constant; it kept
on getting slightly shorter. The comet, in fact, returned to the sun
slightly before its time. Now this effect is exactly what friction
against a solar atmosphere would bring about. Every time it passed near
the sun a little velocity would be rubbed out of it. But the velocity is
that which carries it away, hence it would not go quite so far, and
therefore would return a little sooner. Any revolving body subject to
friction must revolve quicker and quicker, and get nearer and nearer
its central body, until, if the process goes on long enough, it must
drop upon its surface. This seems the kind of thing happening to Encke's
comet. The effect is very small, and not thoroughly proved; but, so far
as it goes, the evidence points to a greatly extended rare solar
atmosphere, which rubs some energy out of it at every perihelion
passage.
[Illustration: FIG. 104.--Biela's comet as last seen, in two portions.]
Next, Biela's comet. This also was a well known and carefully observed
telescopic comet, with a period of six years. In one of its distant
excursions, it was calculated that it must pass very near Jupiter, and
much curiosity was excited as to what would happen to it in consequence
of the perturbation it must experience. As I have said, comets are only
visible as they approach the sun, and a watch was kept for it about its
appointed time. It was late, but it did ultimately arrive.
The singular thing about it, however, was that it was now double. It had
apparently separated into two. This was in 1846. It was looked for again
in 1852, and this time the components were further separated. Sometimes
one was brighter, sometimes the other. Next time it ought to have come
round no one could find either portion. The comet seemed to have wholly
disappeared. It has never been seen since. It was then recorded and
advertised as the missing comet.
But now comes the interesting part of the story. The orbit of this Biela
comet was well known, and it was found that on a certain night in 1872
the earth would cross the orbit, and had some chance of encountering the
comet. Not a very likely chance, because it need not be in that part of
its orbit at the time; but it was suspected not to be far off--if still
existent. Well, the night arrived, the earth did cross the orbit, and
there was seen, not the comet, but a number of shooting-stars. Not one
body, nor yet two, but a multitude of bodies--in fact, a swarm of
meteors. Not a very great swarm, such as sometimes occurs, but still a
quite noticeable one; and this shower of meteors is definitely
recognized as flying along the track of Biela's comet. They are known as
the Andromedes.
This observation has been generalized. Every cometary orbit is marked by
a ring of meteoric stones travelling round it, and whenever a number of
shooting-stars are seen quickly one after the other, it is an evidence
that we are crossing the track of some comet. But suppose instead of
only crossing the track of a comet we were to pass close to the comet
itself, we should then expect to see an extraordinary swarm--a multitude
of shooting-stars. Such phenomena have occurred. The most famous are
those known as the November meteors, or Leonids.
This is the third of those bodies whose history I had to tell you.
Professor H.A. Newton, of America, by examining ancient records arrived
at the conclusion that the earth passed through a certain definite
meteor shoal every thirty-three years. He found, in fact, that every
thirty-three years an unusual flight of shooting-stars was witnessed in
November, the earliest record being 599 A.D. Their last appearance had
been in 1833, and he therefore predicted their return in 1866 or 1867.
Sure enough, in November, 1866, they appeared; and many must remember
seeing that glorious display. Although their hail was almost continuous,
it is estimated that their average distance apart was thirty-five miles!
Their radiant point was and always is in the constellation Leo, and
hence their name Leonids.
[Illustration: FIG. 105.--Radiant point perspective. The arrows
represent a number of approximately parallel meteor-streaks
foreshortened from a common vanishing-point.]
A parallel stream fixed in space necessarily exhibits a definite
aspect with reference to the fixed stars. Its aspect with respect
to the earth will be very changeable, because of the rotation and
revolution of that body, but its position with respect to
constellations will be steady. Hence each meteor swarm, being a
steady parallel stream of rushing masses, always strikes us from
the same point in stellar space, and by this point (or radiant) it
is identified and named.
The paths do not appear to us to be parallel, because of
perspective: they seem to radiate and spread in all directions from
a fixed centre like spokes, but all these diverging streaks are
really parallel lines optically foreshortened by different amounts
so as to produce the radiant impression.
The annexed diagram (Fig. 105) clearly illustrates the fact that
the "radiant" is the vanishing point of a number of parallel lines.
[Illustration: FIG. 106.--Orbit of November meteors.]
This swarm is specially interesting to us from the fact that we cross
its orbit every year. Its orbit and the earth's intersect. Every
November we go through it, and hence every November we see a few
stragglers of this immense swarm. The swarm itself takes thirty-three
years on its revolution round the sun, and hence we only encounter it
every thirty-three years.
The swarm is of immense size. In breadth it is such that the earth,
flying nineteen miles a second, takes four or five hours to cross it,
and this is therefore the time the display lasts. But in length it is
far more enormous. The speed with which it travels is twenty-five miles
a second, (for its orbit extends as far as Uranus, although by no means
parabolic), and yet it takes more than a year to pass. Imagine a
procession 200,000 miles broad, every individual rushing along at the
rate of twenty-five miles every second, and the whole procession so long
that it takes more than a year to pass. It is like a gigantic shoal of
herrings swimming round and round the sun every thirty-three years, and
travelling past the earth with that tremendous velocity of twenty-five
miles a second. The earth dashes through the swarm and sweeps up
myriads. Think of the countless numbers swept up by the whole earth in
crossing such a shoal as that! But heaps more remain, and probably the
millions which are destroyed every thirty-three years have not yet made
any very important difference to the numbers still remaining.
The earth never misses this swarm. Every thirty-three years it is bound
to pass through some part of them, for the shoal is so long that if the
head is just missed one November the tail will be encountered next
November. This is a plain and obvious result of its enormous length. It
may be likened to a two-foot length of sewing silk swimming round and
round an oval sixty feet in circumference. But, you will say, although
the numbers are so great that destroying a few millions or so every
thirty-three years makes but little difference to them, yet, if this
process has been going on from all eternity, they ought to be all swept
up. Granted; and no doubt the most ancient swarms have already all or
nearly all been swept up.
[Illustration: FIG. 107.--Orbit of November meteors; showing their
probable parabolic orbit previous to 126 A.D., and its sudden conversion
into an elliptic orbit by the violent perturbation caused by Uranus,
which at that date occupied the position shown.]
The August meteors, or Perseids, are an example. Every August we cross
their path, and we have a small meteoric display radiating from the
sword-hand of Perseus, but never specially more in one August than
another. It would seem as if the main shoal has disappeared, and nothing
is now left but the stragglers; or perhaps it is that the shoal has
gradually become uniformly distributed all along the path. Anyhow, these
August meteors are reckoned much more ancient members of the solar
system than are the November meteors. The November meteors are believed
to have entered the solar system in the year 126 A.D.
This may seem an extraordinary statement. It is not final, but it is
based on the calculations of Leverrier--confirmed recently by Mr. Adams.
A few moments will suffice to make the grounds of it clear. Leverrier
calculated the orbit of the November meteors, and found them to be an
oval extending beyond Uranus. It was perturbed by the outer planets near
which it went, so that in past times it must have moved in a slightly
different orbit. Calculating back to their past positions, it was found
that in a certain year it must have gone very near to Uranus, and that
by the perturbation of this planet its path had been completely changed.
Originally it had in all probability been a comet, flying in a parabolic
orbit towards the sun like many others. This one, encountering Uranus,
was pulled to pieces as it were, and its orbit made elliptical as shown
in Fig. 107. It was no longer free to escape and go away into the depths
of space: it was enchained and made a member of the solar system. It
also ceased to be a comet; it was degraded into a shoal of meteors.
This is believed to be the past history of this splendid swarm. Since
its introduction to the solar system it has made 52 revolutions: its
next return is due in November, 1899, and I hope that it may occur in
the English dusk, and (see Fig. 97) in a cloudless after-midnight sky,
as it did in 1866.
NOTES FOR LECTURE XVII
The tide-generating force of one body on another is directly as the mass
of the one body and inversely as the cube of the distance between them.
Hence the moon is more effective in producing terrestrial tides than the
sun.
The tidal wave directly produced by the moon in the open ocean is about
5 feet high, that produced by the sun is about 2 feet. Hence the average
spring tide is to the average neap as about 7 to 3. The lunar tide
varies between apogee and perigee from 4.3 to 5.9.
The solar tide varies between aphelion and perihelion from 1.9 to 2.1.
Hence the highest spring tide is to the lowest neap as 5.9 + 2.1 is to
4.3 -2.1, or as 8 to 2.2.
The semi-synchronous oscillation of the Southern Ocean raises the
magnitude of oceanic tides somewhat above these directly generated
values.
Oceanic tides are true waves, not currents. Coast tides are currents.
The momentum of the water, when the tidal wave breaks upon a continent
and rushes up channels, raises coast tides to a much greater height--in
some places up to 50 or 60 feet, or even more.
Early observed connections between moon and tides would be these:--
1st. Spring tides at new and full moon.
2nd. Average interval between tide and tide is half a lunar, not a
solar, day--a lunar day being the interval between two successive
returns of the moon to the meridian: 24 hours and 50 minutes.
3rd. The tides of a given place at new and full moon occur always
at the same time of day whatever the season of the year.
LECTURE XVII
THE TIDES
Persons accustomed to make use of the Mersey landing-stages can hardly
fail to have been struck with two obvious phenomena. One is that the
gangways thereto are sometimes almost level, and at other times very
steep; another is that the water often rushes past the stage rather
violently, sometimes south towards Garston, sometimes north towards the
sea. They observe, in fact, that the water has two periodic motions--one
up and down, the other to and fro--a vertical and a horizontal motion.
They may further observe, if they take the trouble, that a complete
swing of the water, up and down, or to and fro, takes place about every
twelve and a half hours; moreover, that soon after high and low water
there is no current--the water is stationary, whereas about half-way
between high and low it is rushing with maximum speed either up or down
the river.
To both these motions of the water the name _tide_ is given, and both
are extremely important. Sailors usually pay most attention to the
horizontal motion, and on charts you find the tide-races marked; and the
places where there is but a small horizontal rush of the water are
labelled "very little tide here." Landsmen, or, at any rate, such of the
more philosophic sort as pay any attention to the matter at all, think
most of the vertical motion of the water--its amount of rise and fall.
Dwellers in some low-lying districts in London are compelled to pay
attention to the extra high tides of the Thames, because it is, or was,
very liable to overflow its banks and inundate their basements.
Sailors, however, on nearing a port are also greatly affected by the
time and amount of high water there, especially when they are in a big
ship; and we know well enough how frequently Atlantic liners, after
having accomplished their voyage with good speed, have to hang around
for hours waiting till there is enough water to lift them over the
Bar--that standing obstruction, one feels inclined to say disgrace, to
the Liverpool harbour.
[Illustration: FIG. 108.--The Mersey]
To us in Liverpool the tides are of supreme importance--upon them the
very existence of the city depends--for without them Liverpool would not
be a port. It may be familiar to many of you how this is, and yet it is
a matter that cannot be passed over in silence. I will therefore call
your attention to the Ordnance Survey of the estuaries of the Mersey and
the Dee. You see first that there is a great tendency for sand-banks to
accumulate all about this coast, from North Wales right away round to
Southport. You see next that the port of Chester has been practically
silted up by the deposits of sand in the wide-mouthed Dee, while the
port of Liverpool remains open owing to the scouring action of the tide
in its peculiarly shaped channel. Without the tides the Mersey would be
a wretched dribble not much bigger than it is at Warrington. With them,
this splendid basin is kept open, and a channel is cut of such depth
that the _Great Eastern_ easily rode in it in all states of the water.
The basin is filled with water every twelve hours through its narrow
neck. The amount of water stored up in this basin at high tide I
estimate as 600 million tons. All this quantity flows through the neck
in six hours, and flows out again in the next six, scouring and
cleansing and carrying mud and sand far out to sea. Just at present the
currents set strongest on the Birkenhead side of the river, and
accordingly a "Pluckington bank" unfortunately grows under the Liverpool
stage. Should this tendency to silt up the gates of our docks increase,
land can be reclaimed on the other side of the river between Tranmere
and Rock Ferry, and an embankment made so as to deflect the water over
Liverpool way, and give us a fairer proportion of the current. After
passing New Brighton the water spreads out again to the left; its
velocity forward diminishes; and after a few miles it has no power to
cut away that sandbank known as the Bar. Should it be thought desirable
to make it accomplish this, and sweep the Bar further out to sea into
deeper water, it is probable that a rude training wall (say of old
hulks, or other removable partial obstruction) on the west of Queen's
Channel, arranged so as to check the spreading out over all this useless
area, may be quite sufficient to retain the needed extra impetus in the
water, perhaps even without choking up the useful old Rock Channel,
through which smaller ships still find convenient exit.
Now, although the horizontal rush of the tide is necessary to our
existence as a port, it does not follow that the accompanying rise and
fall of the water is an unmixed blessing. To it is due the need for all
the expensive arrangements of docks and gates wherewith to store up the
high-level water. Quebec and New York are cities on such magnificent
rivers that the current required to keep open channel is supplied
without any tidal action, although Quebec is nearly 1,000 miles from the
open ocean; and accordingly, Atlantic liners do not hover in mid-river
and discharge passengers by tender, but they proceed straight to the
side of the quays lining the river, or, as at New York, they dive into
one of the pockets belonging to the company running the ship, and there
discharge passengers and cargo without further trouble, and with no need
for docks or gates. However, rivers like the St. Lawrence and the Hudson
are the natural property of a gigantic continent; and we in England may
be well contented with the possession of such tidal estuaries as the
Mersey, the Thames, and the Humber. That by pertinacious dredging the
citizens of Glasgow manage to get large ships right up their small
river, the Clyde, to the quays of the town, is a remarkable fact, and
redounds very highly to their credit.
We will now proceed to consider the connection existing between the
horizontal rush of water and its vertical elevation, and ask, Which is
cause and which is effect? Does the elevation of the ocean cause the
tidal flow, or does the tidal flow cause the elevation? The answer is
twofold: both statements are in some sense true. The prime cause of the
tide is undoubtedly a vertical elevation of the ocean, a tidal wave or
hump produced by the attraction of the moon. This hump as it passes the
various channels opening into the ocean raises their level, and causes
water to flow up them. But this simple oceanic tide, although the cause
of all tide, is itself but a small affair. It seldom rises above six or
seven feet, and tides on islands in mid-ocean have about this value or
less. But the tides on our coasts are far greater than this--they rise
twenty or thirty feet, or even fifty feet occasionally, at some places,
as at Bristol. Why is this? The horizontal motion of the water gives it
such an impetus or momentum that its motion far transcends that of the
original impulse given to it, just as a push given to a pendulum may
cause it to swing over a much greater arc than that through which the
force acts. The inrushing water flowing up the English Channel or the
Bristol Channel or St. George's Channel has such an impetus that it
propels itself some twenty or thirty feet high before it has exhausted
its momentum and begins to descend. In the Bristol Channel the gradual
narrowing of the opening so much assists this action that the tides
often rise forty feet, occasionally fifty feet, and rush still further
up the Severn in a precipitous and extraordinary hill of water called
"the bore."
Some places are subject to considerable rise and fall of water with very
little horizontal flow; others possess strong tidal races, but very
little elevation and depression. The effect observed at any given place
entirely depends on whether the place has the general character of a
terminus, or whether it lies _en route_ to some great basin.