possible expenditure of energy. Is her purpose here, then, something
quite independent of man’s comfort and happiness? Of the whole solar
heat, we have just seen that less than 1/2,000,000,--less, that is,
than the one twenty-thousandth part of one per cent,--is made useful
to us. “But may there not be other planets on which intelligent life
exists, and where this heat, which passes us by, serves other beings
than ourselves?” There _may_ be; but if we could suppose all the other
planets of the solar system to be inhabited, it would help the matter
very little; for the whole together intercept so little of the great
sum, that all of it which Nature bestows on man is still as nothing to
what she bestows on some end--if end there be--which is to us as yet
inscrutable.
How is this heat maintained? Not by the miracle of a perpetual
self-sustained flame, we may be sure. But, then, by what fuel is such a
fire fed? There can be no question of simple burning, like that of coal
in the grate, for there is no source of supply adequate to the demand.
The State of Pennsylvania, for instance, is underlaid by one of the
richest coal-fields of the world, capable of supplying the consumption
of the whole country at its present rate for more than a thousand
years to come. If the source of the solar heat (whatever that is) were
withdrawn, and we were enabled to carry this coal there, and shoot it
into the solar furnace fast enough to keep up the known heat-supply,
so that the solar radiation would go on at just its actual rate, the
time which this coal would last is easily calculable. It would not last
days or hours, but the whole of these coal-beds would demonstrably be
used up in rather less than one one-thousandth of a second! We find by
a similar calculation that if the sun were itself one solid block of
coal, it would have burned out to the last cinder in less time than man
has certainly been on the earth. But during historic times there has as
surely been no noticeable diminution of the sun’s heat, for the olive
and the vine grow just as they did three thousand years ago, and the
hypothesis of an actual burning becomes untenable. It has been supposed
by some that meteors striking the solar surface might generate heat by
their impact, just as a cannon-ball fired against an armor-plate causes
a flash of light, and a heat so sudden and intense as to partly melt
the ball at the instant of concussion. This is probably a real source
of heat-supply so far as it goes, but it cannot go very far; and,
indeed, if our whole world should fall upon the solar surface like an
immense projectile, gathering speed as it fell, and finally striking
(as it would) with the force due to a rate of over three hundred miles
a second, the heat developed would supply the sun for but little more
than sixty years.[4]
[4] These estimates differ somewhat from those of Helmholtz and
Tyndall, as they rest on later measures.
It is not necessary, however, that a body should be moving rapidly to
develop heat, for arrested motion always generates it, whether the
motion be fast or slow, though in the latter case the mass arrested
must be larger to produce the same result. It is in the slow settlement
of the sun’s own substance toward its centre, as it contracts in
cooling, that we find a sufficient cause for the heat developed.
This explanation is often unsatisfactory to those who have not studied
the subject, because the fact that heat is so generated is not made
familiar to most of us by observation.
Perhaps the following illustration will make the matter plainer. When
we are carried up in a lift, or elevator, we know well enough that heat
has been expended under the boiler of some engine to drag us up against
the power of gravity. When the elevator is at the top of its course, it
is ready to give out in descending just the same amount of power needed
to raise it, as we see by its drawing up a nearly equal counterpoise
in the descent. It can and must give out in coming down the power that
was spent in raising it up; and though there is no practical occasion
to do so, a large part of this power could, if we wished, be actually
recovered in the form of heat again. In the case of a larger body,
such as the pyramid of Ghizeh, which weighs between six and seven
million tons, all the furnaces in the world, burning coal under all its
engines, would have to supply their heat for a measurable time to lift
it a mile high; and then, if it were allowed to come down, whether it
tell at once or were made to descend with imperceptible slowness, by
the time it touched the earth the same heat would be given out again.
Perhaps the fact that the sun is gaseous rather than solid makes it
less easy to realize the enormous weight which is consistent with this
vaporous constitution. A cubic mile of hydrogen gas (the lightest
substance known) would weigh much more at the sun’s surface than the
Great Pyramid does here, and the number of these cubic miles in a
stratum one mile deep below its surface is over 2,000,000,000,000! This
alone is enough to show that as they settle downward as the solar globe
shrinks, here is a _possible_ source of supply for all the heat the sun
sends out. More exact calculation shows that it _is_ sufficient, and
that a contraction of three hundred feet a year (which in ten thousand
years would make a shrinkage hardly visible in the most powerful
telescope) would give all the immense outflow of heat which we see.
There is an ultimate limit, however, to the sun’s shrinking, and
there must have been some bounds to the heat he can already have thus
acquired; for--though the greater the original diameter of his sphere,
the greater the gain of heat by shrinking to its present size--if the
original diameter be supposed as great as possible, there is still a
finite limit to the heat gained.
Suppose, in other words, the sun itself and all the planets ground to
powder, and distributed on the surface of a sphere whose radius is
infinite, and that this matter (the same in amount as that constituting
the present solar system) is allowed to fall together at the centre.
The actual shrinkage cannot possibly be greater than in this extreme
case; but even in this practically impossible instance, it is easy
to calculate that the heat given out would not support the _present_
radiation over eighteen million years, and thus we are enabled to look
back over past time, and fix an approximate limit to the age of the sun
and earth.
We say “present” rate of radiation, because, so long as the sun is
purely gaseous, its temperature rises as it contracts, and the heat
is spent faster; so that in early ages before this temperature was as
high as it is now, the heat was spent more slowly, and what could have
lasted “only” eighteen million years at the present rate might have
actually spread over an indefinitely greater time in the past; possibly
covering more than all the æons geologists ask for.
If we would look into the future, also, we find that at the present
rate we may say that the sun’s heat-supply is enough to last for some
such term as four or five million years before it sensibly fails. It
is certainly remarkable that by the aid of our science man can look
out from this “bank and shoal of time,” where his fleeting existence
is spent, not only back on the almost infinite lapse of ages past, but
that he can forecast with some sort of assurance what is to happen
in an almost infinitely distant future, long after the human race
itself will have disappeared from its present home. But so it is, and
we may say--with something like awe at the meaning to which science
points--that the whole future radiation cannot last so long as ten
million years. Its probable life in its present condition is covered
by about thirty million years. No reasonable allowance for the fall
of meteors or for all other known causes of supply could possibly at
the present rate of radiation raise the whole term of its existence to
sixty million years.
This is substantially Professor Young’s view, and he adds:--
“At the same time it is, of course, impossible to assert that
there has been no catastrophe in the past, no collision with
some wandering star ... producing a shock which might in a few
hours, or moments even, restore the wasted energy of ages.
Neither is it wholly safe to assume that there may not be ways,
of which we as yet have no conception, by which the energy
apparently lost in space may be returned. But the whole course
and tendency of Nature, so far as science now makes out, points
backward to a beginning and forward to an end. The present order
of things seems to be bounded both in the past and in the future
by terminal catastrophes which are veiled in clouds as yet
inscrutable.”
There is another matter of interest to us as dwellers on this planet,
connected not with the amount of the sun’s heat so much as with the
degree of its temperature; for it is almost certain that a very
little fall in the temperature will cause an immense and wholly
disproportionate diminution of the heat-supply. The same principle may
be observed in more familiar things. We can, for instance, warm quite
a large house by a very small furnace, if we urge this (by a wasteful
use of coal) to a dazzling white heat. If we now let the furnace cool
to half this white-heat temperature, we shall be sure to find that
the heat radiated has not diminished in proportion, but out of all
proportion,--has sunk, for instance, not only to one-half what it was
(as we might think it would do), but to perhaps a twentieth or even
less, so that the furnace which heated the house can no longer warm a
single room.
The human race, as we have said, is warming itself at the great solar
furnace, which we have just seen contains an internal source for
generating heat enough for millions of years to come; but we have
also learned that if the sun’s internal circulation were stopped,
the surface would cool and shut up the heat inside, where it would
do us no good. The _temperature_ of the surface, then, on which the
rate of heat-emission depends, concerns us very much; and if we had
a thermometer so long that we could dip its bulb into the sun and
read the degrees on the stem here, we should find out what observers
would very much like to know, and at present are disposed to quarrel
about. The difficulty is not in measuring the heat,--for that we have
just seen how to do,--but in telling what temperature corresponds to
it, since there is no known rule by which to find one from the other.
One certain thing is this--that we cannot by any contrivance raise
the temperature in the focus of any lens or mirror beyond that of
its source (practically we cannot do even so much); we cannot, for
instance, by any burning-lens make the image of a candle as hot as the
original flame. Whatever a thermometer may read when the candle-heat is
concentrated on its bulb by a lens, it would read yet more if the bulb
were dipped in the candle-flame itself; and one obvious application of
this fact is that though we cannot dip our thermometer in the sun, we
know that if we could do so, the temperature would at least be greater
than any we get by the largest burning-glass. We need have no fear of
making the burning-glass too big; the temperature at its solar focus is
_always_ and necessarily lower than that of the sun itself.
For some reason no very great burning-lens or mirror has been
constructed for a long time, and we have to go back to the eighteenth
century to see what can be done in this way. The annexed figure (Fig.
55) is from a wood-cut of the last century, describing the largest
burning-lens then or since constructed in France, whose size and
mode of use the drawing clearly shows. All the heat falling on the
great lens was concentrated on a smaller one, and the smaller one
concentrated it in turn, till at the very focus we are assured that
iron, gold, and other metals ran like melted butter. In England, the
largest burning-lens on record was made about the same time by an
optician named Parker for the English Government, who designed it as
a present to be taken by Lord Macartney’s embassy to the Emperor of
China. Parker’s lens was three feet in diameter and very massive, being
seven inches thick at the centre. In its focus the most refractory
substances were fused, and even the diamond was reduced to vapor, so
that the temperature of the sun’s surface is at any rate higher than
_this_.
[Illustration: FIG. 55.--BERNIÈRES’S GREAT BURNING-GLASS. (AFTER AN OLD
FRENCH PRINT.)]
(What became of the French lens shown, it would be interesting to know.
If it is still above ground, its fate has been better than that of the
English one. It is said that the Emperor of China, when he got his
lens, was much alarmed by it, as being possibly sent him by the English
with some covert design for his injury. By way of a test, a smith was
ordered to strike it with his hammer; but the hammer rebounded from
the solid glass, and this was taken to be conclusive evidence of magic
in the thing, which was immediately buried, and probably is still
reposing under the soil of the Celestial Flowery Kingdom.)
We can confirm the evidence of such burning-lenses as to the sun’s
high temperature by another class of experiment, which rests on an
analogous principle. We can make the comparison between the heat from
some artificially heated object and that which would be given out
from an equal area of the sun’s face. Now, supposing like emissive
powers, if the latter be found the hotter, though we cannot tell what
its temperature absolutely is, we can at least say that it is greater
than that of the thing with which it is compared; so that we choose
for comparison the hottest thing we can find, on a scale large enough
for the experiment. One observation of my own in this direction I will
permit myself to cite in illustration.
Perhaps the highest temperature we can get on a large scale in the arts
is that of molten steel in the Bessemer converter. As many may be as
ignorant of what this is as I was before I tried the experiment, I will
try to describe it.
[Illustration: FIG. 56.--A “POUR” FROM THE BESSEMER CONVERTER.]
The “converter” is an enormous iron pot, lined with fire-brick, and
capable of holding thirty or forty thousand pounds of melted metal;
and it is swung on trunnions, so that it can be raised by an engine
to a vertical position, or lowered by machinery so as to pour its
contents out into a caldron. First the empty converter is inclined,
and fifteen thousand pounds of fluid iron streams down into the mouth
from an adjacent furnace where it has been melted. Then the engine
lifts the converter into an erect position, while an air-blast from
a blowing-engine is forced in at the bottom and through the liquid
iron, which has combined with it nearly half a ton of silicon and
carbon,--materials which, with the oxygen of the blast, create a heat
which leaves that of the already molten iron far behind. After some
time the converter is tipped forward, and fifteen hundred pounds more
of melted iron is added to that already in it. What the temperature
of this last is, may be judged from the fact that though ordinary
melted iron is dazzlingly bright, the melted metal in the converter
is so much brighter still, that the entering stream is dark brown by
comparison, presenting a contrast like that of chocolate poured into
a white cup. The contents are now no longer iron, but liquid steel,
ready for pouring into the caldron; and, looking from the front down
into the inclined vessel, we see the almost blindingly bright interior
dripping with the drainage of the metal running down its side, so that
the circular mouth, which is twenty-four inches in diameter, presents
the effect of a disk of molten metal of that size (were it possible to
maintain such a disk in a vertical position). In addition, we have the
actual stream of falling metal, which continues nearly a minute, and
presents an area of some square feet. The shower of scintillations from
this cataract of what seems at first “sunlike” brilliancy, and the area
whence such intense heat and light are for a brief time radiated, make
the spectacle a most striking one. (See Fig. 56.)
The “pour” is preceded by a shower of sparks, consisting of little
particles of molten steel which are projected fully a hundred feet in
the direction of the open mouth of the converter. In the line of this
my apparatus was stationed in an open window, at a point where its view
could be directed down into the converter on one side, and up at the
sun on the other. This apparatus consisted of a long photometer-box
with a _porte-lumière_ at one end. The mirror of this reflected the
sun’s rays through the box and then on to the pouring metal, tracing
their way to it by a beam visible in the dusty air (Fig. 57). In the
path of this beam was placed the measuring apparatus, both for heat
and light. As the best point of observation was in the line of the
blast, a shower of sparks was driven over the instrument and observer
at every “pour;” and the rain of wet soot from chimneys without, the
bombardment from within, and the moving masses of red-hot iron around,
made the experiment an altogether peculiar one. The apparatus was
arranged in such a way that the effect (except for the absorption of
its beams on the way) was independent of the size or distance of the
sun, and depended on the absolute radiation there, and was equivalent,
in fact, to taking a sample piece of the sun’s face _of equal size_
with the fluid metal, bringing them face to face, and seeing which
was the hotter and brighter. The comparison, however, was unfair to
the sun, because its rays were in reality partly absorbed by the
atmosphere on the way, while those of the furnace were not. Under these
circumstances the heat from any single square foot of the sun’s surface
was found to be _at least_ eighty-seven times that from a square foot
of the melted metal, while the light from the sun was proved to be,
foot for foot, over five thousand times that from the molten steel,
though the latter, separately considered, seemed to be itself, as I
have said, of quite sunlike brilliancy.
[Illustration: FIG. 57.--PHOTOMETER-BOX.]
We must not conclude from this that the _temperature_ of the sun was
five thousand times that of the steel, but we may be certain that it
was at any rate a great deal the higher of the two. It is probable,
from all experiments made up to this date, that the solar effective
temperature is not less than 3,000 nor more than 30,000 degrees of
the centigrade thermometer. Sir William Siemens, whose opinion on any
question as to heat is entitled to great respect, thought the lower
value nearer the truth, but this is doubtful.
[Illustration: FIG. 58.--MOUCHOT’S SOLAR ENGINE. (FROM A FRENCH
PRINT.)]
* * * * *
We have, in all that has preceded, been speaking of the sun’s
constitution and appearance, and have hardly entered on the question
of its industrial relations to man. It must be evident, however, that
if we derive, as it is asserted we do, almost all our mechanical power
from this solar heat,--if our water-wheel is driven by rivers which
the sun feeds by the rain he sucks up for them into the clouds, if
the coal is stored sun-power, and if, as Stevenson said, it really is
the sun which drives our engines, though at second hand,--there is an
immense fund of possible mechanical power still coming to us from him
which might be economically utilized. Leaving out of sight all our
more important relations to him (for, as has been already said, he
is in a physical sense our creator, and he keeps us alive from hour
to hour), and considering him only as a possible servant to grind
our corn and spin our flax, we find that even in this light there
are startling possibilities of profit in the study of our subject.
From recent measures it appears that from every square yard of the
earth exposed perpendicularly to the sun’s rays, in the absence of an
absorbing atmosphere, there could be derived more than one horse-power,
if the heat were all converted into this use, and that even on such a
little area as the island of Manhattan, or that occupied by the city
of London, the noontide heat is enough, could it all be utilized, to
drive all the steam-engines in the world. It will not be surprising,
then, to hear that many practical men are turning their attention to
this as a source of power, and that, though it has hitherto cost more
to utilize the power than it is worth, there is reason to believe
that some of the greatest changes which civilization has to bring
may yet be due to such investigations. The visitor to the last Paris
Exposition may remember an extraordinary machine on the grounds of
the Trocadéro, looking like a gigantic inverted umbrella pointed
sunward. This was the sun-machine of M. Mouchot, consisting of a great
parabolic reflector, which concentrated the heat on a boiler in the
focus and drove a steam-engine with it, which was employed in turn to
work a printing-press, as our engraving shows (Fig. 58). Because these
constructions have been hitherto little more than playthings, we are
not to think of them as useless. If toys, they are the toys of the
childhood of a science which is destined to grow, and in its maturity
to apply this solar energy to the use of all mankind.
Even now they are beginning to pass into the region of practical
utility, and in the form of the latest achievement of Mr. Ericsson’s
ever-young genius are ready for actual work on an economical scale.
We present in Fig. 59 his new actually working solar engine, which
there is every reason to believe is more efficient than Mouchot’s,
and probably capable of being used with economical advantage in
pumping water in desert regions of our own country. It is pregnant
with suggestion of the future, if we consider the growing demand for
power in the world, and the fact that its stock of coal, though vast,
is strictly limited, in the sense that when it _is_ gone we can get
absolutely no more. The sun has been making a little every day for
millions of years,--so little and for so long, that it is as though
time had daily dropped a single penny into the bank to our credit for
untold ages, until an enormous fund had been thus slowly accumulated in
our favor. We are drawing on this fund like a prodigal who thinks his
means endless, but the day will come when our check will no longer be
honored, and what shall we do then?
[Illustration: FIG. 59.--ERICSSON’S NEW SOLAR ENGINE, NOW IN PRACTICAL
USE IN NEW YORK.]
The exhaustion of some of the coal-beds is an affair of the immediate
future, by comparison with the vast period of time we have been
speaking of. The English coal-beds, it is asserted, will, from present
indications, be quite used up in about three hundred years more.
Three hundred years ago, the sun, looking down on the England of our
forefathers, saw a fair land of green woods and quiet waters, a land
unvexed with noisier machinery than the spinning-wheel, or the needles
of the “free maids that weave their threads with bones.” Because of the
coal which has been dug from its soil, he sees it now soot-blackened,
furrowed with railway-cuttings, covered with noisy manufactories,
filled with grimy operatives, while the island shakes with the throb
of coal-driven engines, and its once quiet waters are churned by the
wheels of steamships. Many generations of the lives of men have passed
to make the England of Elizabeth into the England of Victoria; but what
a moment this time is, compared with the vast lapse of ages during
which the coal was being stored! What a moment in the life of the
“all-beholding sun,” who in a few hundred years--his gift exhausted
and the last furnace-fire out--may send his beams through rents in the
ivy-grown walls of deserted factories, upon silent engines brown with
rust, while the mill-hand has gone to other lands, the rivers are clean
again, the harbors show only white sails, and England’s “black country”
is green once more! To America, too, such a time may come, though at a
greatly longer distance.
Does this all seem but the idlest fancy? That something like it will
come to pass sooner or later, is a most certain fact--as certain as any
process of Nature--if we do not find a new source of power; for of the
coal which has supplied us, after a certain time we can get no more.