from terrestrial chemistry, and the nebular hypothesis, the presence of
oxygen in the sun might have been strongly suspected; for this element
is currently stated to form eight-ninths of the water of the globe,
one-third of the crust of the earth, and one-fifth of the air, and
should therefore probably be a large constituent of every member of the
solar system. On the other hand, the discovery of oxygen, and probably
other non-metals, in the sun gives increased strength to the nebular
hypothesis, because to many persons the absence of this important group
has presented a considerable difficulty. I have already remarked on the
circumstance that we cannot, according to the known laws of gaseous
diffusion, accept the reasoning of those who have endeavoured to
explain the small density of the outer planets by the supposition that
the lighter gases were left behind by the great contracting nebulous
mass, out of which, on the nebular hypothesis, the solar system is
supposed to have been formed. It is important to notice, now, that if
on the one hand we find in the community of structure between the sun
and our earth, as confirmed by the discovery of oxygen and nitrogen
in the sun, evidence favouring the theory according to which all the
members of that system were formed out of what was originally a single
mass, we do not find evidence against the theory (as those who have
advanced the explanation above referred to may be disposed to imagine)
in the recognition in the sun’s mass of enormous quantities of one
of these elements which, according to their view, ought to be found
chiefly in the outer members of the solar system. If those who believe
in the nebular hypothesis (generally, that is, for many of the details
of the hypothesis as advanced by Laplace are entirely untenable in
the present position of physical science) had accepted the attempted
explanation of the supposed absence of the non-metallic elements in the
sun, they would now find themselves in a somewhat awkward position.
They would, in fact, be almost bound logically to reject the nebular
hypothesis, seeing that one of the asserted results of the formation of
our system, according to that hypothesis, would have been disproved.
But so far as I know no supporter of the nebular hypothesis possessing
sufficient knowledge of astronomical facts and physical laws to render
his opinion of any weight, has ever given in his adhesion to the
unsatisfactory explanation referred to.
The view which I have long entertained respecting the growth of the
solar system—viz., that it had its origin, not in contraction only or
chiefly, but in combined processes of contraction and accretion—seems
to me to be very strongly confirmed by Dr. Draper’s discovery. This
would not be the place for a full discussion of the reasons on which
this opinion is based. But I may remark that I believe no one who
applies the laws of physics, _as at present known_, to the theory of
the simple contraction of a great nebulous mass formerly extending
far beyond the orbit of Neptune, till, when planet after planet had
been thrown off, the sun was left in his present form and condition
in the centre, will fail to perceive enormous difficulties in the
hypothesis, or to recognize in Dr. Draper’s discovery a difficulty
added to those affecting the hypothesis _so presented_. Has it ever
occurred, I often wonder, to those who glibly quote the nebular theory
as originally propounded, to inquire how far some of the processes
suggested by Laplace are in accordance with the now known laws of
physics? To begin with, the original nebulous mass extending to a
distance exceeding the earth’s distance from the sun more than thirty
times (this being only the distance of Neptune), if we assign to it a
degree of compression making its axial diameter half its equatorial
diameter, would have had a volume exceeding the sun’s (roughly) about
120,000,000,000 times, and in this degree its mean density would have
been less than the sun’s. This would correspond to a density equal
(roughly) to about one-400,000th part of the density of hydrogen gas at
atmospheric pressure. To suppose that a great mass of matter, having
this exceedingly small mean density, and extending to a distance of
three or four thousand millions of miles from its centre, could under
any circumstances rotate as a whole, or behave in other respects after
the fashion attributed to the gaseous embryon of the solar system
in ordinary descriptions of the nebular hypothesis, is altogether
inconsistent with correct ideas of physical and dynamical laws. It is
absolutely a necessity of any nebular hypothesis of the solar system,
that from the very beginning a central nucleus and subordinate nuclei
should form in it, and grow according to the results of the motions (at
first to all intents and purposes independent) of its various parts.
Granting this state of things, we arrive, by considering the combined
effects of accretion and contraction, at a process of development
according fully as well as that ordinarily described with the general
relations described by Laplace, and accounting also (in a general way)
for certain peculiarities which are in no degree explained by the
ordinary theory. Amongst these may specially be noted the arrangement
and distribution of the masses within the solar system, and the fact
that so far as spectroscopic evidence enables us to judge, a general
similarity of structure exists throughout the whole of the system.
Inquiring as to the significance of his discovery, Dr. Draper remarks
that it seems rather difficult “at first sight to believe that an
ignited[5] gas in the solar atmosphere should not be indicated by dark
lines in the solar spectrum, and should appear not to act under the
law, ‘a gas when ignited absorbs rays of the same refrangibility as
those it emits.’ But, in fact, the substances hitherto investigated in
the sun are really metallic vapours, hydrogen probably coming under
that rule. The non-metals obviously may behave differently. It is easy
to speculate on the causes of such behaviour; and it may be suggested
that the reason of the non-appearance of a dark line may be that
the intensity of the light from a great thickness of ignited oxygen
overpowers the effect of the photosphere, just as, if a person were to
look at a candle-flame through a yard thickness of sodium vapour, he
would only see bright sodium lines, and no dark absorption.”
The reasoning here is not altogether satisfactory (or else is not
quite correctly expressed). In the first place, the difficulty dealt
with has no real existence. The law that a gas when glowing absorbs
rays of the same refrangibility as it emits, does not imply that a
gas between a source of light and the observer will show its presence
by spectroscopic dark lines. A gas so placed _does_ receive from the
source of light rays corresponding to those which it emits itself, if
it is cooler than the source of light; and it absorbs them, being in
fact heated by means of them, though the gain of temperature may be
dissipated as fast as received; but if the gas is hotter, it emits more
of those rays than it absorbs, and will make its presence known by its
bright lines. This is not a matter of speculation, but of experiment.
On the other hand, the experiment suggested by Dr. Draper would not
have the effect he supposes, if it were correctly made. Doubtless,
if the light from a considerable area of dully glowing sodium vapour
were received by the spectroscope at the same time as the light of a
candle-flame seen through the sodium vapour, the light of the sodium
vapour overcoming that of the candle-flame would indicate its presence
by bright lines; but if light were received only from that portion of
the sodium vapour which lay between the eye and the candle-flame, then
I apprehend that the dark lines of sodium would not only be seen, but
would be conspicuous by their darkness.
It is in no cavilling spirit that I indicate what appears to me
erroneous in a portion of Dr. Draper’s reasoning on his great
discovery. The entire significance of the discovery depends on the
meaning attached to it, and therefore it is most desirable to ascertain
what this meaning really is. There can be no doubt, I think, that we
are to look for the true interpretation of the brightness of the oxygen
lines in the higher temperature of the oxygen, not in the great depth
of oxygen above the photospheric level. The oxygen which produces
these bright lines need not necessarily be above the photosphere at
all. (In fact, I may remark here that Dr. Draper, in a communication
addressed to myself, mentions that he has found no traces at present
of oxygen above the photosphere, though I had not this circumstance
in my thoughts in reasoning down to the conclusion that the part of
the oxygen effective in showing these bright lines lies probably below
the visible photosphere.) Of course, if the photosphere were really
composed of glowing solid and liquid matter, or of masses of gas so
compressed and so intensely heated as to give a continuous spectrum,
no gas existing below the photosphere could send its light through,
nor could its presence, therefore, be indicated in any spectroscopic
manner. But the investigations which have been made into the structure
of the photosphere as revealed by the telescope, and in particular the
observations made by Professor Langley, of the Alleghany Observatory,
show that we have not in the photosphere a definite bounding envelope
of the sun, but receive light from many different depths below that
spherical surface, 425,000 miles from the sun’s centre, which we call
the photospheric level. We receive more light from the centre of
the solar disc, I feel satisfied, not solely because the absorptive
layer through which we there see the sun is shallower, but partly,
and perhaps chiefly, because we there receive light from some of the
interior and more intensely heated parts of the sun.[6] Should this
prove to be the case, it may be found possible to do what heretofore
astronomers have supposed to be impossible—to ascertain in some degree
how far and in what way the constitution of the sun varies below the
photosphere, which, so far as ordinary telescopic observation is
concerned, seems to present a limit below which researches cannot be
pursued.
I hope we shall soon obtain news from Dr. Huggins’s Observatory that
the oxygen lines have been photographed, and possibly the bright lines
of other elements recognized in the solar spectrum. Mr. Lockyer also,
we may hope, will exercise that observing skill which enabled him early
to recognize the presence of bright hydrogen lines in the spectrum
of portions of the sun’s surface, to examine that spectrum for other
bright lines.
I do not remember any time within the last twenty years when the
prospects of fresh solar discoveries seemed more hopeful than they do
at present. The interest which has of late years been drawn to the
subject has had the effect of enlisting fresh recruits in the work of
observation, and many of these may before long be heard of as among
those who have employed Dr. Draper’s method successfully.
But I would specially call attention to the interest which attaches
to Dr. Draper’s discovery and to the researches likely to follow from
it, in connection with a branch of research which is becoming more
and more closely connected year by year with solar investigations—I
mean stellar spectroscopy. We have seen the stars divided into orders
according to their constitution. We recognize evidence tending to show
that these various orders depend in part upon age—not absolute but
relative age. There are among the suns which people space some younger
by far than our sun, others far older, and some in a late stage of
stellar decrepitude. Whether as yet spectroscopists have perfectly
succeeded in classifying these stellar orders in such sort that the
connection between a star’s spectrum and the star’s age can be at
once determined, may be doubtful. But certainly there are reasons for
hoping that before long this will be done. Amongst the stars, and
(strange to say) among celestial objects which are not stars, there
are suns in every conceivable stage of development, from embryon
masses not as yet justly to be regarded as suns, to masses which have
ceased to fulfil the duties of suns. Among the more pressing duties of
spectroscopic analysis at the present time is the proper classification
of these various orders of stars. Whensoever that task shall have been
accomplished, strong light, I venture to predict, will be thrown on our
sun’s present condition, as well as on his past history, and on that
future fate upon which depends the future of our earth.
_SUN-SPOT, STORM, AND FAMINE._
During the last five or six years a section of the scientific world
has been exercised with the question how far the condition of the
sun’s surface with regard to spots affects our earth’s condition as to
weather, and therefore as to those circumstances which are more or less
dependent on weather. Unfortunately, the question thus raised has not
presented itself alone, but in company with another not so strictly
scientific, in fact, regarded by most men of science as closely related
to personal considerations—the question, namely, whether certain
indicated persons should or should not be commissioned to undertake the
inquiry into the scientific problem. But the scientific question itself
ought not to be less interesting to us because it has been associated,
correctly or not, with the wants and wishes of those who advocate
the endowment of science. I propose here to consider the subject in
its scientific aspect only, and apart from any bias suggested by the
appeals which have been addressed to the administrators of the public
funds.
It is hardly necessary to point out, in the first place, that all
the phenomena of weather are directly referable to the sun as their
governing cause. His rays poured upon our air cause the more important
atmospheric currents directly. Indirectly they cause modifications
of these currents, because where they fall on water or on moist
surfaces they raise aqueous vapour into the air, which, when it
returns to the liquid form as cloud, gives up to the surrounding air
the heat which had originally vaporized the water. In these ways,
directly or indirectly, various degrees of pressure and temperature
are brought about in the atmospheric envelope of the earth, and,
speaking generally, all air currents, from the gentlest zephyr to the
fiercest tornado, are the movements by which the equilibrium of the
air is restored. Like other movements tending to restore equilibrium,
the atmospheric motions are oscillatory. Precisely as when a spring
has been bent one way, it flies not back only, but beyond the mean
position, till it is almost equally bent the other way, so the current
of air which rushes in towards a place of unduly diminished pressure
does more than restore the mean pressure, so that presently a return
current carries off the excess of air thus carried in. We may say,
indeed, that the mean pressure at any place scarcely ever exists, and
when it exists for a time the resulting calm is of short duration.
Just as the usual condition of the sea surface is one of disturbance,
greater or less, so the usual condition of the air at every spot on
the earth’s surface is one of motion not of quiescence. Every movement
of the air, thus almost constantly perturbed, is due directly or
indirectly to the sun.
So also every drop of rain or snow, every particle of liquid or of
frozen water in mist or in cloud, owes its birth to the sun. The
questions addressed of old to Job, “Hath the rain a father? or who
hath begotten the drops of dew? out of whose womb came the ice? and
the hoary frost of heaven, who hath gendered it?” have been answered
by modern science, and to every question the answer is, The Sun. He
is parent of the snow and hail, as he is of the moist warm rains of
summer, of the ice which crowns the everlasting hills, and of the mist
which rises from the valleys beneath his morning rays.
Since, then, the snow that clothes the earth in winter as with a
garment, and the clouds that in due season drop fatness on the earth,
are alike gendered by the sun; since every movement in our air, from
the health-bringing breeze to the most destructive hurricane, owns him
as its parent; we must at the outset admit, that if there is any body
external to the earth whose varying aspect or condition can inform us
beforehand of changes which the weather is to undergo, the sun is that
body. That for countless ages the moon should have been regarded as
the great weather-breeder, shows only how prone men are to recognize
in apparent changes the true cause of real changes, and how slight
the evidence is on which they will base laws of association which
have no real foundation in fact. Every one can see when the moon is
full, or horned, or gibbous, or half-full; when her horns are directed
upwards, or downwards, or sideways. And as the weather is always
changing, even as the moon is always changing, it must needs happen
that from time to time changes of weather so closely follow changes
of the moon as to suggest that the two orders of change stand to each
other in the relation of cause and effect. Thus rough rules (such as
those which Aratus has handed down to us) came to be formed, and as
(to use Bacon’s expression) men mark when such rules hit, and never
mark when they miss, a system of weather lore gradually comes into
being, which, while in one sense based on facts, has not in reality a
particle of true evidence in its favour—every single fact noted for
each relation having been contradicted by several unnoted facts opposed
to the relation. There could be no more instructive illustration of
men’s habits in such matters than the system of lunar weather wisdom in
vogue to this day among seamen, though long since utterly disproved by
science. But let it be remarked in passing, that in leaving the moon,
which has no direct influence, and scarcely any indirect influence, on
the weather, for the sun, which is all-powerful, we have not got rid of
the mental habits which led men so far astray in former times. We shall
have to be specially careful lest it lead us astray yet once more,
perhaps all the more readily because of the confidence with which we
feel that, at the outset anyway, we are on the right road.
I suppose there must have been a time when men were not altogether
certain whether the varying apparent path of the sun, as he travels
from east to west every day, has any special effect on the weather.
It seems so natural to us to recognize in the sun’s greater mid-day
elevation and longer continuance above the horizon in summer, the cause
of the greater warmth which then commonly prevails, that we find it
difficult to believe that men could ever have been in doubt on this
subject. Yet it is probable that a long time passed after the position
of the sun as ruler of the day had been noticed, before his power as
ruler of the seasons was recognized. I cannot at this moment recall
any passage in the Bible, for example, in which direct reference is
made to the sun’s special influence in bringing about the seasons, or
any passage in very ancient writings referring definitely to the fact
that the weather changes with the changing position of the sun in the
skies (as distinguished from the star-sphere), and with the changing
length of the day. “While the earth remaineth,” we are told in Genesis,
“seed-time and harvest, and cold and heat, and summer and winter, and
day and night, shall not cease;” but there is no reference to the sun’s
aspect as determining summer and winter. We find no mention of any of
the celestial signs of the seasons anywhere in the Bible, I think, but
such signs as are mentioned in the parable of the fig tree—“When his
branch is yet tender, and putteth forth leaves, ye know that summer is
nigh.” Whether this indicates or not that the terrestrial, rather than
the celestial signs of the progress of the year were chiefly noted by
men in those times, it is tolerably certain that in the beginning a
long interval must have elapsed between the recognition of the seasons
themselves, and the recognition of their origin in the changes of the
sun’s apparent motions.
When this discovery was effected, men made the most important and, I
think, the most satisfactory step towards the determination of cyclic
associations between solar and terrestrial phenomena. It is for that
reason that I refer specially to the point. In reality, it does not
appertain to my subject, for seasons and sun-spots are not associated.
But it admirably illustrates the value of cyclic relations. Men might
have gone on for centuries, we may conceive, noting the recurrence
of seed-time and harvest-time, summer and winter, recognizing the
periodical returns of heat and cold, and (in some regions) of dry
seasons and wet seasons, of calm and storm, and so forth, without
perceiving that the sun runs through his changes of diurnal motion in
the same cyclic period. We can imagine that some few who might notice
the connection between the two orders of celestial phenomena would
be anxious to spread their faith in the association among their less
observant brethren. They might maintain that observatories for watching
the motions of the sun would demonstrate either that their belief
was just or that it was not so, would in fact dispose finally of the
question. It is giving the most advantageous possible position to those
who now advocate the erection of solar observatories for determining
what connection, if any, may exist between sun-spots and terrestrial
phenomena, thus to compare them to observers who had noted a relation
which unquestionably exists. But it is worthy of notice that if those
whom I have imagined thus urging the erection of an observatory for
solving the question whether the sun rules the seasons, and to some
degree regulates the recurrence of dry or rainy, and of calm or
stormy weather, had promised results of material value from their
observations, they would have promised more than they could possibly
have performed. Even in this most favourable case, where the sun is,
beyond all question, the efficient ruling body, where the nature of
the cyclic change is most exactly determinable, and where even the way
in which the sun acts can be exactly ascertained, no direct benefit
accrues from the knowledge. The exact determination of the sun’s
apparent motions has its value, and this value is great, but it is most
certainly not derived from any power of predicting the recurrence of
those phenomena which nevertheless depend directly on the sun’s action.
The farmer who in any given year knows from the almanac the exact
duration of daylight, and the exact mid-day elevation of the sun for
every day in the year, is not one whit better able to protect his crops
or his herds against storm or flood than the tiller of the soil or the
tender of flocks a hundred thousand years or so ago, who knew only when
seed-time and summer and harvest-time and winter were at hand or in
progress.
The evidence thus afforded is by no means promising, then, so far as
the prediction of special storms, or floods, or droughts is concerned.
It would seem that if past experience can afford any evidence in such
matters, men may expect to recognize cycles of weather change long
before they recognize corresponding solar cycles (presuming always that
such cycles exist), and that they may expect to find the recognition
of such association utterly barren, so far as the possibility of
predicting definite weather changes is concerned. It would seem that
there is no likelihood of anything better than what Sir J. Herschel
said _might_ be hoped for hereafter. “A lucky hit may be made; nay,
some rude approach to the perception of a ‘cycle of seasons’ may
_possibly_ be obtainable. But no person in his senses would alter
his plans of conduct for six months in advance in the most trifling
particular on the faith of any special prediction of a warm or a cold,
a wet or a dry, a calm or a stormy, summer or winter”—far less of a
great storm or flood announced for any special day.
But let us see what the cycle association between solar spots and
terrestrial weather actually is, or rather of what nature it promises
to be, for as yet the true nature of the association has not been made
out.
It has been found that in a period of about eleven years the sun’s
surface is affected by what may be described as a wave of sun-spots.
There is a short time—a year or so—during which scarce any spots are
seen; they become more and more numerous during the next four or five
years, until they attain a maximum of frequency and size; after this
they wane in number and dimensions, until at length, about eleven years
from the time when he had before been freest from spots, he attains
again a similar condition. After this the spots begin to return,
gradually attain to a maximum, then gradually diminish, until after
eleven more years have elapsed few or none are seen. It must not be
supposed that the sun is always free from spots at the time of minimum
spot frequency, or that he always shows many and large spots at the
time of maximum spot frequency. Occasionally several very large spots,
and sometimes singularly large spots, have been seen in the very heart
of the minimum spot season, and again there have been occasions when
scarcely any spots have been seen for several days in the very heart
of the maximum spot season. But, taking the average of each year,
the progression of the spots in number and frequency from minimum
to maximum, and their decline from maximum to minimum, are quite
unmistakeable.