terrestrial magnetic disturbances, we shall find altogether more
satisfactory testimony. This evidence is of all degrees of strength,
from probability up to what may be called certainty, and it is always
obtained, not by _a priori_ reasoning, but by the comparison of
independent observations of something which has happened on the sun and
on the earth. We will first take an instance of what we consider the
weakest degree of evidence (weak, that is, when any such single case
is considered), and we do so by simply quoting textually three records
which were made at nearly the same time in different parts of the world
in 1882.
A certain spot had been visible on the sun at intervals for some weeks;
but when on the 16th of November a glimpse was caught of it after
previous days of cloudy weather, the observer, it will be seen, is
struck by the great activity going on in it, and, though familiar with
such sights, describes this one as “magnificent.”
1. From the daily record at the Allegheny Observatory, November 16,
1882:--
“Very large spot on the sun; ... great variety of forms; inrush
from S. E. to S. W.; tendency to cyclonic action at several
points. The spot is apparently near its period of greatest
activity. A magnificent sight.”
At the same time a sketch was commenced which was interrupted by the
cloudy weather of this and following days. The outline of the main spot
only is here given (Fig. 49). Its area, as measured at Allegheny, was
2,200,000,000 square miles; at Greenwich its area, inclusive of some
outlying portions, was estimated on the same day to be 2,600,000,000
square miles. The earth is shown of its relative size upon it, to give
a proper idea of the scale.
2. From the “New York Tribune” of November 18th (describing what took
place in the night preceding the 17th):--
AN ELECTRIC STORM.
TELEGRAPH WIRES GREATLY AFFECTED.
THE DISTURBANCE WIDE-SPREAD.
... At the Mutual Union office the manager said, “Our wires are
all running, but very slowly. There is often an intermission of
from one to five minutes between the words of a sentence. The
electric storm is general as far as our wires are concerned.”...
The cable messages were also delayed, in some cases as much as an
hour.
The telephone service was practically useless during the day.
WASHINGTON, _Nov. 17_.--A magnetic storm of more than usual
intensity began here at an early hour this morning, and has
continued with occasional interruptions during the day,
seriously interfering with telegraphic communication.... As
an experiment one of the wires of the Western Union Telegraph
Company was worked between Washington and Baltimore this
afternoon with the terrestrial current alone, the batteries
having been entirely detached.
CHICAGO, _Nov. 17_.--An electric storm of the greatest violence
raged in all the territory to points beyond Omaha.... The
switch-board here has been on fire a dozen times during the
forenoon. At noon only a single wire out of fifteen between this
city and New York was in operation.
And so on through a column.
3. In Fig. 50 we give a portion of the automatic trace of the magnetic
needles at Greenwich.[3] These needles are mounted on massive piers in
the cellars of the observatory, far removed from every visible source
of disturbance, and each carries a small mirror, whence a spot of light
is reflected upon a strip of photographic paper, kept continually
rolling before it by clock-work. If the needle is still, the moving
strip of paper will have a straight line on it, traced by the point of
light, which is in this case motionless. If the needle swings to the
right or left, the light-spot vibrates with it, and the line it traces
becomes sinuous, or more and more sharply zigzagged as the needle
shivers under the unknown forces which control it.
[3] It appears here through the kindness of the Astronomer
Royal. We regret to say that American observers are
dependent on the courtesy of foreign ones in such matters,
the United States having no observatory where such records
of sun-spots and magnetic variation are systematically kept.
The upper part of Fig. 50 gives a little portion of this automatic
trace on November 16th before the disturbance began, to show the
ordinary daily record, which should be compared with the violent
perturbation occurring simultaneously with the telegraphic disturbance
in the United States. We may, for the reader’s convenience, remark
that as the astronomical day begins twelve hours later than the civil
day, the approximate Washington mean times, corresponding to the
Greenwich hours after twelve, are found by adding one to the days and
subtracting seventeen from the hours. Thus “November 16th, twenty-two
hours” corresponds in the eastern United States nearly to five o’clock
in the morning of November 17th.
The Allegheny observer, it will be remembered, in his glimpse of the
spot on November 16th, was struck with the great activity of the
internal motions then going on in it. The Astronomer Royal states that
a portion of the spot became detached on November 17th or 18th, and
that several small spots which broke out in the immediate neighborhood
were seen for the first time on the photographs taken November 17th,
twenty-two hours.
“Are we to conclude from this,” it may be asked, “that what went on
in the sun was the cause of the trouble on the telegraph wires?” I
think we are not at all entitled to conclude so from this instance
_alone_; but though in one such case, taken by itself, there is nothing
conclusive, yet when such a degree of coincidence occurs again and
again, the habitual observer of solar phenomena learns to look with
some confidence for evidence of electrical disturbance here following
certain kinds of disturbance there, and the weight of this part of the
evidence is not to be sought so much in the strength of a single case,
as in the multitude of such coincidences.
We have, however, not only the means of comparing sun-spot _years_ with
years of terrestrial electric disturbance, but individual instances,
particular _minutes_ of sun-spot changes, with particular minutes of
terrestrial change; and both comparisons are of the most convincing
character.
First, let us observe that the compass needle, in its regular and
ordinary behavior, does not point constantly in any one direction
through the day, but moves a very little one way in the morning, and
back in the afternoon. This same movement, which can be noticed even
in a good surveyor’s compass, is called the “diurnal oscillation,”
and has long been known. It has been known, too, that its amount
altered from one year to another; but since Schwabe’s observations it
has been found that the changes in this variation and in the number
of the spots went on together. The coincidences which we failed to
note in the comparison of the spots with the prices of grain are here
made out with convincing clearness, as the reader will see by a simple
inspection of this chart (Fig. 51, taken from Professor Young’s work),
where the horizontal divisions still denote years, and the height of
the continuous curve the relative number of spots, while the height of
the dotted curve is the amount of the magnetic variation. Though we
have given but a part of the curve, the presumption from the agreement
in the forty years alone would be a strong one that the two effects,
apparently so widely remote in their nature, are really due to a common
cause.
[Illustration: FIG. 51.--SUN-SPOTS AND MAGNETIC VARIATIONS.]
Here we have compared years with years; let us next compare minutes
with minutes. Thus, to cite (from Mr. Proctor’s work) a well-known
instance: On Sept. 1, 1869, at eighteen minutes past eleven, Mr.
Carrington, an experienced solar observer, suddenly saw in the sun
something brighter than the sun,--two patches of light, breaking out so
instantly and so intensely that his first thought was that daylight
was entering through a hole in the darkening screen he used. It was
immediately, however, made certain that something unusual was occurring
in the sun itself, across which the brilliant spots were moving,
travelling thirty-five thousand miles in five minutes, at the end of
which time (at twenty-three minutes past eleven) they disappeared from
sight. By good fortune, another observer a few miles distant saw and
independently described the same phenomenon; and as the minute had been
noted, it was immediately afterward found that recording instruments
registered a magnetic disturbance at the same time,--“at the very
moment,” says Dr. Stewart, the director of the observatory at Kew.
“By degrees,” says Sir John Herschel, “accounts began to pour in of
... great electro-magnetic disturbances in every part of the world....
At Washington and Philadelphia, in America, the telegraphic signal men
received severe electric shocks. At Boston, in North America, a flame
of fire followed the pen of Bain’s electric telegraph.” (Such electric
disturbances, it may be mentioned, are called “electric storms,” though
when they occur the weather may be perfectly serene to the eye. They
are shown also by rapid vibrations of the magnetic needle, like those
we have illustrated.)
On Aug. 3, 1872, Professor Young, who was observing at Sherman in the
Rocky Mountains, saw three notable paroxysms in the sun’s chromosphere,
jets of luminous matter of intense brilliance being projected at 8h.
45m., 10h. 30m., and 11h. 50m. of the local time. “At dinner,” he
says, “the photographer of the party, who was making our magnetic
observations, told me, before knowing anything about what I had been
observing, that he had been obliged to give up work, his magnet having
swung clear off the limb.” Similar phenomena were observed August 5th.
Professor Young wrote to England, and received from Greenwich and
Stonyhurst copies of the automatic record, which he gives, and which
we give in Fig. 52. After allowing for difference of longitude, the
reader who will take the pains to compare them may see for himself that
both show a jump of the needles in the cellars at Greenwich at the same
_minute_ in each of the four cases of outburst in the Rocky Mountains.
[Illustration: FIG. 52.--GREENWICH MAGNETIC OBSERVATIONS, AUG. 3 AND 5,
1872.]
While we admit that the evidence in any single case is rarely so
conclusive as in these; while we agree that the spot is not so much
the cause of the change as the index of some other solar action which
does cause it; and while we fully concede our present ignorance of
the nature of this cause,--we cannot refuse to accept the cumulative
evidence, of which a little has been submitted.
It is only in rare cases that we can feel quite sure; and yet, in
regard even to one of the more common and less conclusive ones, we
may at least feel warranted in saying that if the reader forfeited
a business engagement or missed an invitation to dinner through the
failure of the telegraph or telephone on such an occasion as that of
the 17th of November, 1882, the far-off sun-spot was not improbably
connected with the cause.
Probably we should all like to hear some at least equally positive
conclusion about the weather also, and to learn that there was a
likelihood of our being able to predict it for the next year, as the
Signal Service now does for the next day; but there is at present
no such likelihood. The study of the possible connection between
sun-spots and the weather is, nevertheless, one that will always have
great interest to many; for even if we set its scientific aim aside
and consider it in its purely utilitarian aspect, it is evident that
the knowledge how to predict whether coming harvests would be good or
bad, would enable us to do for the whole world what Joseph’s prophetic
vision of the seven good and seven barren years did for the land of
Egypt, and confer a greater power on its discoverer than any sovereign
now possesses. There is something to be said, then, for the cyclists;
for if their zeal does sometimes outrun knowledge, their object is a
worthy one, and their aims such as we can sympathize with, and of which
none of us can say that there is any inherent impossibility in them,
or that they may not conceivably yet lead to something. Let us not,
then, treat the inquirer who tries to connect panics on ‘Change with
sun-spots as a mere lunatic; for there is this amount of reason in his
theory, that the panics, together with the general state of business,
are connected in some obscure way with the good or bad harvests, and
these again in some still obscurer way with changes in our sun.
We may leave, then, this vision of forecasting the harvests and the
markets of the world from a study of the sun, as one of the fair dreams
for the future of our science. Perhaps the dream will one day be
realized. Who knows?
IV.
THE SUN’S ENERGY (_Continued_).
If we paused on the words with which our last chapter closed, the
reader might perhaps so far gather an impression that the whole
all-important subject of the solar energy was involved in mystery and
doubt. But if it be indeed a mystery when considered in its essence, so
are all things; while regarded separately in any one of its terrestrial
effects of magnetic or chemical action, or of light or heat, it may
seem less so. Since there is not room to consider all these aspects,
let us choose the last, and look at this energy in its familiar form of
the _heat_ by which we live.
We, the human race, are warming ourselves at this great fire which
called our bodies into being, and when it goes out we shall go too.
What is it? How long has it been? How long will it last? How shall we
use it?
To look across the space of over ninety million miles, and to try to
learn from that distance the nature of the solar heat, and how it is
kept up, seemed to the astronomers of the last century a hopeless task.
The difficulty was avoided rather than met by the doctrine that the sun
was pure fire, and shone because “it was its nature to.” In the Middle
Ages such an idea was universal; and along with it, and as a logical
sequence of it, the belief was long prevalent that it was possible
to make another such flame here, in the form of a lamp which should
burn forever and radiate light endlessly without exhaustion. With
the philosopher’s stone, which was to transmute lead into gold, this
perpetual lamp formed a prime object of research for the alchemist and
student of magic.
We recall the use which Scott has made of the belief in this product
of “gramarye” in the “Lay of the Last Minstrel,” where it is sought to
open the grave of the great wizard in Melrose Abbey. It is midnight
when the stone which covers it is heaved away, and Michael’s undying
lamp, buried with him long ago, shines out from the open tomb and
illuminates the darkness of the chancel.
“I would you had been there to see
The light break forth so gloriously;
That lamp shall burn unquenchably
Until the eternal doom shall be,”
says the poet. Now we are at liberty to enjoy the fiction as a fiction;
but if we admit that the art which could make such a lamp would indeed
be a black art, which did not work under Nature’s laws, but against
them, then we ought to see that as the whole conception is derived from
the early notion of a miraculous constitution of the sun, the idea of
an eternal self-sustained sun is no more permitted to us than that of
an eternal self-sustained lamp. We must look for the cause of the sun’s
heat in Nature’s laws, and we know those laws chiefly by what we see
here.
Before examining the source of the sun’s heat, let us look a little
more into its amount. To find the exact amount of heat which it sends
out is a very difficult problem, especially if we are to use all the
refinements of the latest methods in determining it. The underlying
principle, however, is embodied in an old method, which gives, it is
true, rather crude results, but by so simple a treatment that the
reader can follow it readily, especially if unembarrassed with details,
in which most of the actual trouble lies. We must warn him in advance
that he is going to be confronted with a kind of enormous sum in
multiplication, for whose general accuracy he may, however, trust to us
if he pleases. We have not attempted exact accuracy, because it is more
convenient for him that we should deal with round numbers.
[Illustration: FIG. 53.--ONE CUBIC CENTIMETRE.]
[Illustration: FIG. 54.--POUILLET’S PYRHELIOMETER.]
The apparatus which we shall need for the attack of this great problem
is surprisingly simple, and moderate in size. Let us begin by finding
how much sun-heat falls in a small known area. To do this we take a
flat, shallow vessel, which is to be filled with water. The amount it
contains is usually a hundred cubic centimetres (a centimetre being
nearly four-tenths of an inch), so that if we imagine a tiny cubical
box about as large as a backgammon die, or, more exactly, having each
side just the size of this (Fig. 53), to be filled and emptied into the
vessel one hundred times, we shall have a precise idea of its limited
capacity. Into this vessel we dip a thermometer, so as to read the
temperature of the water, seal all up so that the water shall not run
out, and expose it so that the heat at noon falls perpendicularly on
it. The apparatus is shown in Fig. 54, attached to a tree. The stem
of the instrument holds the thermometer, which is upside down, its
bulb being in the water-vessel. Now, all the sun’s rays do not reach
this vessel, for some are absorbed by our atmosphere; and all the heat
which falls on it does not stay there, as the water loses part of it
by the contact of the air with the box outside, and in other ways.
When allowance is made for these losses, we find that the sun’s heat,
if all retained, would have raised the temperature of the few drops of
water which would fill a box the size of our little cube (according
to these latest observations) nearly three degrees of the centigrade
thermometer in one minute,--a most insignificant result, apparently,
as a measure of what we have been told is the almost infinite heat of
the sun! But if we think so, we are forgetting the power of numbers, of
which we are about to have an illustration as striking in its way as
that which Archimedes once gave with the grains of sand.
There is a treatise of his extant, in which he remarks (I cite from
memory) that as some people believe it possible for numbers to express
a quantity as great as that of the grains of sand upon the sea-shore,
while others deny this, he will show that they can express one even
larger. To prove this beyond dispute, he begins by taking a small
seed, beside which he ranges single grains of sand in a line, till he
can give the number of these latter which equal its length. Next he
ranges seeds beside each other till their number makes up the length
of a span; then he counts the spans in a stadium, and the stadia in
the whole world as known to the ancients, at each step expressing his
results in a number certainly _greater_ than the number of sand-grains
which the seed, or the span, or the stadium, or finally the whole
world, is thus successively shown to contain. He has then already got
a number before his reader’s eyes demonstrably larger than that of all
the grains of sand on the sea-shore; yet he does not stop, but steps
off the earth into space, to calculate and express a number _greater_
than that of all the grains of sand which would fill a sphere embracing
the earth and the sun!
We are going to use our little unit of heat in the same way, for
(to calculate in round figures and in English measure) we find that
we can set over nine hundred of these small cubes side by side in a
square foot, and, as there are 28,000,000 feet in a square mile, that
the latter would contain 25,000,000,000 of the cubes, placed side by
side, touching each other, like a mosaic pavement. We find also, by
weighing our little cup, that we should need to fill and empty it
almost exactly a million times to exhaust a tank containing a ton of
water. The sun-heat falling on one square mile corresponds, then, to
over seven hundred and fifty tons of water raised _every minute_ from
the freezing-point to boiling, which already is becoming a respectable
amount!
But there are 49,000,000 square miles in the cross-section of the
earth exposed to the sun’s rays, which it would therefore need
1,225,000,000,000,000,000 of our little dies to cover one deep; and
therefore in each _minute_ the sun’s heat falling on the earth would
raise to boiling 37,000,000,000 tons of water.
We may express this in other ways, as by the quantity of ice it would
melt; and as the heat required to melt a given weight of ice is 79/100
of that required to bring as much water from the freezing to the
boiling point, and as the whole surface of the earth, including the
night side, is four times the cross-section exposed to the sun, we
find, by taking 526,000 minutes to a year, that the sun’s rays would
melt in the year a coating of ice over the whole earth more than one
hundred and sixty feet thick.
We have ascended already from our small starting-point to numbers which
express the heat that falls upon the whole planet, and enable us to
deal, if we wish, with questions relating to the glacial epochs and
other changes in its history. We have done this by referring at each
step to the little cube which we have carried along with us, and which
is the foundation of all the rest; and we now see why such exactness
in the first determination is needed, since any error is multiplied
by enormous numbers. But now we too are going to step off the earth
and to deal with numbers which we can still express in the same way
if we choose, but which grow so large thus stated that we will seek
some greater term of comparison for them. We have just seen the almost
incomprehensible amount of heat which the sun must send the earth in
order to warm its oceans and make green its continents; but how little
this is to what passes us by! The earth as it moves on in its annual
path continually comes into new regions, where it finds the same amount
of heat already pouring forth; and this same amount still continues to
fall into the empty space we have just quitted, where there is no one
left to note it, and where it goes on in what seems to us utter waste.
If, then, the whole annual orbit were set close with globes like ours,
and strung with worlds like beads upon a ring, each would receive the
same enormous amount the earth does now. But this is not all; for not
only along the orbit, but above and below it, the sun sends its heat in
seemingly incredible wastefulness, the final amount being expressible
in the number of _worlds_ like ours that it could warm like ours, which
is 2,200,000,000.
We have possibly given a surfeit of such numbers, but we cannot escape
or altogether avoid them when dealing with this stupendous outflow of
the solar heat. They are too great, perhaps, to convey a clear idea to
the mind, but let us before leaving them try to give an illustration of
their significance.
Let us suppose that we could sweep up from the earth all the ice and
snow on its surface, and, gathering in the accumulations which lie
on its Arctic and Antarctic poles, commence building with it a tower
greater than that of Babel, fifteen miles in diameter, and so high as
to exhaust our store. Imagine that it could be preserved untouched by
the sun’s rays, while we built on with the accumulations of successive
winters, until it stretched out 240,000 miles into space, and formed an
ice-bridge to the moon, and that then we concentrated on it the sun’s
whole radiation, neither more nor less than that which goes on every
moment. In _one_ second the whole would be gone, melted, boiled, and
dissipated in vapor. And this is the rate at which the solar heat is
being (to human apprehension) _wasted_!