useful and important purposes_ _which the same thing is made to serve_.
Thus the sun is at once the great regulator of the planetary motions,
and the fountain of light and heat. The moon both gives light by night
and raises the tides. Or, if we would follow out this principle where
its operations are more within our comprehension, we may instance the
_atmosphere_. When man constructs an instrument, he deems it sufficient
if it fulfils one single purpose as the watch, to tell the hour of the
day, or the telescope, to enable him to see distant objects; and had a
being like ourselves made the atmosphere, he would have thought it
enough to have created a medium so essential to animal life, that to
live is to breathe, and to cease to breathe is to die. But beside this,
the atmosphere has manifold uses, each entirely distinct from all the
others. It conveys to plants, as well as animals, their nourishment and
life; it tempers the heat of Summer with its breezes; it binds down all
fluids, and prevents their passing into the state of vapor; it supports
the clouds, distils the dew, and waters the earth with showers; it
multiplies the light of the sun, and diffuses it over earth and sky; it
feeds our fires, turns our machines, wafts our ships, and conveys to the
ear all the sentiments of language, and all the melodies of music.
In the third place, the wisdom of the Creator is strikingly manifested
in the provision he has made for the _stability of the universe_. The
perturbations occasioned by the motions of the planets, from their
action on each other, are very numerous, since every body in the system
exerts an attraction on every other, in conformity with the law of
universal gravitation. Venus and Mercury, approaching, as they do at
times, comparatively near to the earth, sensibly disturb its motions;
and the satellites of the remoter planets greatly disturb each other's
movements. Nor was it possible to endow this principle with the
properties it has, and make it operate as it does in regulating the
motions of the world, without involving such an incident. On this
subject, Professor Whewell, in his excellent work composing one of the
Bridgewater Treatises, remarks: "The derangement which the planets
produce in the motion of one of their number will be very small, in the
course of one revolution; but this gives us no security that the
derangement may not become very large, in the course of many
revolutions. The cause acts perpetually, and it has the whole extent of
time to work in. Is it not easily conceivable, then, that, in the lapse
of ages, the derangements of the motions of the planets may accumulate,
the orbits may change their form, and their mutual distances may be much
increased or diminished? Is it not possible that these changes may go on
without limit, and end in the complete subversion and ruin of the
system? If, for instance, the result of this mutual gravitation should
be to increase considerably the eccentricity of the earth's orbit, or to
make the moon approach continually nearer and nearer to the earth, at
every revolution, it is easy to see that, in the one case, our year
would change its character, producing a far greater irregularity in the
distribution of the solar heat; in the other, our satellite must fall to
the earth, occasioning a dreadful catastrophe. If the positions of the
planetary orbits, with respect to that of the earth, were to change
much, the planets might sometimes come very near us, and thus increase
the effect of their attraction beyond calculable limits. Under such
circumstances, 'we might have years of unequal length, and seasons of
capricious temperature; planets and moons, of portentous size and
aspect, glaring and disappearing at uncertain intervals; tides, like
deluges, sweeping over whole continents; and perhaps the collision of
two of the planets, and the consequent destruction of all organization
on both of them.' The fact really is, that changes are taking place in
the motions of the heavenly bodies, which have gone on progressively,
from the first dawn of science. The eccentricity of the earth's orbit
has been diminishing from the earliest observations to our times. The
moon has been moving quicker from the time of the first recorded
eclipses, and is now in advance, by about four times her own breadth,
of what her own place would have been, if it had not been affected by
this acceleration. The obliquity of the ecliptic, also, is in a state of
diminution, and is now about two fifths of a degree less than it was in
the time of Aristotle."
But amid so many seeming causes of irregularity and ruin, it is worthy
of a grateful notice, that effectual provision is made for the
_stability of the solar system_. The full confirmation of this fact is
among the grand results of physical astronomy. "Newton did not undertake
to demonstrate either the stability or instability of the system. The
decision of this point required a great number of preparatory steps and
simplifications, and such progress in the invention and improvement of
mathematical methods, as occupied the best mathematicians of Europe for
the greater part of the last century. Towards the end of that time, it
was shown by La Grange and La Place, that the arrangements of the solar
system are stable; that, in the long run, the orbits and motions remain
unchanged; and that the changes in the orbits, which take place in
shorter periods, never transgress certain very moderate limits. Each
orbit undergoes deviations on this side and on that side of its average
state; but these deviations are never very great, and it finally
recovers from them, so that the average is preserved. The planets
produce perpetual perturbations in each other's motions; but these
perturbations are not indefinitely progressive, but periodical, reaching
a maximum value, and then diminishing. The periods which this
restoration requires are, for the most part, enormous,--not less than
thousands, and in some instances, millions, of years. Indeed, some of
these apparent derangements have been going on in the same direction
from the creation of the world. But the restoration is in the sequel as
complete as the derangement; and in the mean time the disturbance never
attains a sufficient amount seriously to affect the stability of the
system. 'I have succeeded in demonstrating,' says La Place, 'that,
whatever be the masses of the planets, in consequence of the fact that
they all move in the same direction, in orbits of small eccentricity,
and but slightly inclined to each other, their secular irregularities
are periodical, and included within narrow limits; so that the planetary
system will only oscillate about a mean state, and will never deviate
from it, except by a very small quantity. The ellipses of the planets
have been and always will be nearly circular. The ecliptic will never
coincide with the equator; and the entire extent of the variation, in
its inclination, cannot exceed three degrees.'"
To these observations of La Place, Professor Whewell adds the following,
on the importance, to the stability of the solar system, of the fact
that those planets which have _great masses_ have orbits of _small
eccentricity_. "The planets Mercury and Mars, which have much the
largest eccentricity among the old planets, are those of which the
masses are much the smallest. The mass of Jupiter is more than two
thousand times that of either of these planets. If the orbit of Jupiter
were as eccentric as that of Mercury, all the security for the stability
of the system, which analysis has yet pointed out, would disappear. The
earth and the smaller planets might, by the near approach of Jupiter at
his perihelion, change their nearly circular orbits into very long
ellipses, and thus might fall into the sun, or fly off into remoter
space. It is further remarkable, that in the newly-discovered planets,
of which the orbits are still more eccentric than that of Mercury, the
masses are still smaller, so that the same provision is established in
this case, also."
With this hasty glance at the unity, power, and wisdom, of the Creator,
as manifested in the greatest of His works, I close. I hope enough has
been said to vindicate the sentiment that called 'Devotion, daughter of
Astronomy!' I do not pretend that this, or any other science, is
adequate of itself to purify the heart, or to raise it to its Maker; but
I fully believe that, when the heart is already under the power of
religion, there is something in the frequent and habitual contemplation
of the heavenly bodies under all the lights of modern astronomy, very
favorable to devotional feelings, inspiring, as it does, humility, in
unison with an exalted sentiment of grateful adoration.
LETTER XXXII.
RECENT DISCOVERIES.
"All are but parts of one stupendous whole."--_Pope._
WITHIN a few years, astronomy has been enriched with a number of
valuable discoveries, of which I will endeavor to give you a summary
account in this letter. The heavens have been explored with far more
powerful telescopes than before; instrumental measurements have been
carried to an astonishing degree of accuracy; numerous additions have
been made to the list of small planets or asteroids; a comet has
appeared of extraordinary splendor, remarkable, above all others, for
its near approach to the sun; the distances of several of the fixed
stars, an element long sought for in vain, have been determined; a large
planet, composing in itself a magnificent world, has been added to the
solar system, at such a distance from the central luminary as nearly to
double the supposed dimensions of that system; various nebulæ, before
held to be irresolvable, have been resolved into stars; and a new
satellite has been added to Saturn.
IMPROVEMENTS IN THE TELESCOPE.--Herschel's forty-feet telescope, of
which I gave an account in my fourth letter (see page 36), remained for
half a century unequalled in magnitude and power; but in 1842, Lord
Rosse, an Irish nobleman, commenced a telescope on a scale still more
gigantic. Like Herschel's, it was a _reflector_, the image being formed
by a concave mirror. This was six feet in diameter, and weighed three
tons; and the tube was fifty feet in length. The entire cost of the
instrument was sixty thousand dollars. Its reflecting surface is nearly
twice as great as the great Herschelian, and consequently it greatly
exceeds all instruments hitherto constructed in the _amount of light_
which it collects and transmits to the eye; and this adapts it
peculiarly to viewing those objects, as nebulæ, whose light is
exceedingly faint. Accordingly, it has revealed to us new wonders in
this curious department of astronomy. Some idea of the great dimensions
of the _Leviathan_ telescope (as this instrument has been called) may be
formed when it is said that the Dean of Ely, a full-sized man, walked
through the tube from one end to the other, with an umbrella over his
head.
But still greater advances have been made in refracting than in
reflecting telescopes. Such was the difficulty of obtaining large pieces
of glass which are free from impurities, and such the liability of large
lenses to form obscure and colored images, that it was formerly supposed
impossible to make a refracting telescope larger in diameter than five
or six inches; but their size has been increased from one step to
another, until they are now made more than fifteen inches in diameter;
and so completely have all the difficulties arising from the
imperfections of glass, and from optical defects inherent in lenses,
been surmounted, that the great telescopes of Pulkova, at St.
Petersburgh, and of Harvard University (the two finest refractors in the
world) are considered among the most perfect productions of the arts. A
lens of only 15 inches in diameter seems, indeed, diminutive when
compared with a concave reflector of six feet; but for most purposes of
the astronomer, the Pulkova and Cambridge instruments are more useful
than such great reflectors as those of Herschel and Rosse. If there is
any particular in which these are more effective, it is in observations
on the faintest nebulæ, where it is necessary to collect and convey to
the eye the greatest possible beam of light.
INSTRUMENTAL MEASUREMENTS.--When astronomical instruments were first
employed to measure the angular distance between two points on the
celestial sphere, it was not attempted to measure spaces smaller than
ten minutes--a space equal to the third part of the breadth of the full
moon. Tycho Brahe, however, carried his measures to sixty times that
degree of minuteness, having devised means of determining angles no
larger than ten seconds, or the one hundred and eightieth part of the
breadth of the lunar disk. For many years past, astronomers have carried
these measures to single seconds, or have determined spaces no greater
than the eighteen hundredth part of the diameter of the moon. This is
considered the smallest arc which can be accurately measured directly on
the limb of an instrument; but _differences_ between spaces may be
estimated to a far greater degree of accuracy than this, even to the
hundredth part of a second--a space less than that intercepted by a
spider's web held before the eye.
DISCOVERY OF NEW PLANETS.--In my twenty-third letter (see page 286), I
gave an account of the small planets called asteroids, which lie between
the orbits of Mars and Jupiter. When that letter was written, no longer
ago than 1840, only four of those bodies had been discovered, namely,
Ceres, Pallas, Juno, and Vesta. Within a few years past, nineteen more
have been added, making the number of the asteroids known at present
twenty-three, and every year adds one or more to the list.[17] The idea
first suggested by Olbers, one of the earliest discoverers of asteroids,
that they are fragments of a large single planet once revolving between
Mars and Jupiter, has gained credit since the discovery of so many
additional bodies of the same class, all, like the former, exceedingly
small and irregular in their motions, although there are still great
difficulties in tracing them to a common origin.
GREAT COMET OF 1843.--This is the most wonderful body that has appeared
in the heavens in modern times; first, on account of its appearing, when
first seen, in the broad light of noonday; and, secondly, on account of
its approaching so near the sun as almost to graze his surface. It was
first discovered, in New England, on the 28th of February, a little
eastward of the sun, shining like a white cloud illuminated by the solar
rays. It arrested the attention of many individuals from half past seven
in the morning until three o'clock in the afternoon, when the sky became
obscured by clouds. In Mexico, it was observed from nine in the morning
until sunset. At a single station in South America, it was said to have
been seen on the 27th of February, almost in contact with the sun. Early
in March, it had receded so far to the eastward of that body as to be
visible in the southwest after sunset, throwing upward a long train,
which increased in length from night to night until it covered a space
of 40 degrees. Its position may be seen on a celestial globe adjusted to
the latitude of New Haven (41° 18´) for the 20th of March, by tracing a
line, or, rather, a broad band proceeding from the place of the sun
towards the bright star Sirius, in the south, between the ears of the
Hare and the feet of Orion.
The comet passed its perihelion on the 27th of February, at which time
it almost came in contact with the sun. To prevent its falling into the
sun it was endued with a prodigious velocity; a velocity so great that,
had it continued at the same rate as at the instant of perihelion
passage, it would have whirled round the sun in two hours and a half. It
did, in fact, complete more than half its revolution around the sun in
that short period, and it made more than three quarters of its circuit
around the sun in one day. Its velocity, when nearest the sun, exceeded
a million of miles per hour, and its tail, at its greatest elongation,
was one hundred and eight millions of miles; a length more than
sufficient to have reached from the sun to the earth. Its heat was
estimated to be 47,000 times greater than that received by the earth
from a vertical sun, and consequently it was more intense than that
produced by the most powerful blowpipes, and sufficient to melt like wax
the most infusible bodies. No doubt, when in the vicinity of the sun,
the solid matter of the comet was first melted and then converted into
vapor, which itself became red hot, or, more properly speaking, _white
hot_. Much discussion has arisen among astronomers respecting the
periodic time of this comet. Its most probable period is about 175
years.
DISTANCES OF THE STARS.--I have already mentioned (page 389) that the
distance of at least one of the fixed stars has at length been
determined, although at so great a distance that its annual parallax is
only about one third of a second, implying a distance from the sun of
nearly sixty millions of millions of miles. Of a distance so immense the
mind can form no adequate conception. The most successful effort towards
it is made by gradual and successive approximations. Let us, therefore,
take the motion of a rail-way car as the most rapid with which we are
familiar, and apply it first to the planetary spaces, and then to the
vast interval that separates these nether worlds from the fixed stars. A
rail-way car, travelling constantly night and day at the rate of twenty
miles per hour, would make 480 miles per day. At this rate, to travel
around the earth on a great circle would require about 50 days, and 500
days to reach the moon. If we took our departure from the sun, and
journeyed night and day, we should reach Mercury in a little more than
200 years, Venus in nearly 400, and the Earth in 547 years; but to reach
Neptune, the outermost planet, would require 16,000 years. Great as
appear the dimensions of the solar system, when we imagine ourselves
thus borne along from world to world, yet this space is small compared
with that which separates us from the fixed stars; for to reach 61 Cygni
it would take 324,000,000 years. But this is believed, for certain
satisfactory reasons, to be one of the nearest of the stars. Several
other stars whose parallax has been determined are at a much greater
distance than 61 Cygni. The pole star is five times as far off; and the
greater part of the stars are at distances inconceivably more remote.
Such, especially, are those which compose the faintest nebulæ.
DISCOVERY OF THE PLANET NEPTUNE.--From the earliest ages down to the
year 1781, the solar system was supposed to terminate with the planet
Saturn, at the distance of nine hundred millions of miles from the sun;
but the discovery of Uranus added another world, and doubled the
dimensions of the solar system. It seemed improbable that any more
planets should exist at a distance still more remote, since such a body
could hardly receive any of the vivifying influences of the central
luminary. Still, certain irregularities to which the Uranus was subject,
led to the suspicion that there exists a planet beyond it, which, by its
attractions, caused these irregularities. Impressed with this belief,
two young astronomers of great genius, Le Verrier, of France, and Adams,
of England, applied themselves to the task of finding the hidden planet.
The direction in which the disturbed body was moved afforded some clue
to the part of the heavens where the disturbing body lay concealed; the
kind of action it excited at different times indicated that it was
beyond Uranus, and not this side of that planet; and the magnitude of
the forces it exerted gave some intimation of its size and mass. The law
of distances from the sun which the superior planets observe (Saturn
being nearly twice the distance of Jupiter, and Uranus twice that of
Saturn), led both these astronomers to assume that the body sought was
nearly double the distance of Uranus from the sun. With these few and
imperfect data, as so many leading-strings proceeding from the planet
Uranus, they felt their way into the abysses of space by the aid of two
sure guides--the law of gravitation and the higher geometry. Both
astronomers arrived at nearly the same results, although they wrought
independently of each other, and each, indeed, without the knowledge of
the other. Le Verrier was the first to make public his conclusions,
which he communicated to the French Academy at their sitting, August 31,
1846. They saw that there existed, at nearly double the distance of
Uranus from the sun, a planet larger than that body; that it lay near a
certain star seen at that season in the southwest, in the evening sky;
that, on account of its immense distance, it was invisible to the naked
eye, and could be distinctly seen with a perceptible disk only by the
most powerful telescopes; being no brighter than a star of the ninth
magnitude, and subtending an angle of only three seconds. Le Verrier
communicated these results to Dr. Galle, of Berlin, with the request
that he would search for the stranger with his powerful telescope,
pointing out the exact spot in the heavens where it would be found. On
the same evening, Dr. Galle directed his instrument to that part of the
heavens, and immediately the planet presented itself to view, within one
degree of the very spot assigned to it by Le Verrier. Subsequent
investigations have shown that its apparent size is within half a second
of that which the same sagacious mind foresaw, and that its diameter is
nearly equal to that of Uranus, being 31,000, while Uranus is 35,000
miles.[18] The distance from the sun is less than was predicted, being
only about 3000, instead of 3600 millions of miles; and its periodic
time is 164-1/2, instead of 217 years, as was supposed by Le Verrier.
One satellite only has yet been discovered, and this was first seen by
Professor Bond with the great telescope of Harvard University.
RECENT TELESCOPIC DISCOVERIES.--The great reflecting telescope of Lord
Rosse, and the powerful refracting telescopes of Pulkova and Cambridge,
have opened new fields of discovery to the delighted astronomer. A new
satellite has been added to Saturn, first revealed to the Cambridge
instrument, making the entire number of moons that adorn the nocturnal
sky of that remarkable planet no less than eight. Still more wonderful
things have been disclosed among the remotest _Nebulæ_. A number of
these objects before placed among the irresolvable nebulæ, and supposed
to consist not of stars, but of mere nebulous matter, have been resolved
into stars; others, of which we before saw only a part, have revealed
themselves under new and strange forms, one resembling an animal with
huge branching arms, and hence called the _crab_ nebula; another
imitating a scroll or vortex, and called the _whirlpool_ nebula; and
other figures, which to ordinary telescopes appear only as dim specks on
the confines of creation, are presented to these wonderful instruments
as glorious firmaments of stars.
In the year 1833, Sir John Herschel left England for the Cape of Good
Hope, furnished with powerful instruments for observing the stars and
nebulæ of the southern hemisphere, which had never been examined in a
manner suited to disclose their full glories. This great astronomer and
benefactor to science devoted five years of the most assiduous toil in
observing and delineating the astronomical objects of that portion of
the heavens. He had before extended the catalogue of nebulæ begun by his
illustrious father, Sir William Herschel, to the number of 2307; and
beginning at that point, he swelled the number, by his labors at the
Cape of Good Hope, to 4015. He extended also the list of double stars
from 3346 to 5449, and showed that the luminous spots near the South
Pole, known to sailors by the name of the "Magellan Clouds," consist of
an assemblage of several hundred brilliant nebulæ.