an encircling mountainous wall consisting of volcano-like craters in
various stages of subsidence and dislocation. The sun-light coming from
the west casts strong shadows from all the elevations eastward, and is
just rising on the _terminator_, where the rugged structure of the
Moon's surface is best seen. The lighter portions are the more elevated
mountainous tracts and crater summits. The detailed description of this
Plate given in the body of the Manual is repeated here for convenience
of reference: The Mare Humorum, or sea of moisture, as it is called, is
one of the smaller gray lunar plains. Its diameter, which is very nearly
the same in all directions, is about 270 miles, the total area of this
plain being about 50,000 square miles. It is one of the most distinct
plains of the Moon, and is easily seen with the naked eye on the
left-hand side of the disk. The floor of the plain is, like that of the
other gray plains, traversed by several systems of very extended but low
hills and ridges, while small craters are disseminated upon its surface.
The color of this formation is of a dusky greenish gray along the
border, while in the interior it is of a lighter shade, and is of
brownish olivaceous tint. This plain, which is surrounded by high clefts
and rifts, well illustrates the phenomena of dislocation and subsidence.
The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000
feet in height, is seen in the upper left-hand corner of the gray plain.
Close to Vitello at the east is the large broken ring-plain Lee, and
farther east, and a little below, is a similarly broken crater called
Doppelmayer. Both of these open craters have mountainous masses and
peaks on their door, which is on a level with that of the Mare Humorum.
A little below, and to the left of these objects, is dimly seen a deeply
imbedded oval crater, whose walls barely rise above the level of the
plain. On the right-hand side of the great plain is a long _fault_, with
a system of fracture running along its border. On this right-hand side
may be seen a part of the line of the terminator, which separates the
light from the darkness. Towards the lower right-hand corner is the
great ring-plain Gassendi, 55 miles in diameter, with its system of
fractures and its central mountains, which rise from 3,000 to 4,000 feet
above its floor. This crater slopes towards the plain, showing the
subsidence to which it has been submitted. While the northern portion of
the wall of this crater rises to 10,000 feet, that on the plain is only
500 feet high, and is even wholly demolished at one place where the
floor of the crater is in direct communication with the plain. In the
lower part of the sea, and a little to the west of the middle line, is
found the crater Agatharchides, which shows below its north wall the
marks of rills impressed by a flood of lava, which once issued from the
side of the crater. On the left-hand side of the plain is seen the
half-demolished crater Hippalus resembling a large bay, which has its
interior strewn with peaks and mountains. On this same side can be seen
one of the most important systems of clefts and fractures visible on the
Moon, these clefts varying in length from 150 to 200 miles.
_Approximate scale, 15 miles--1 inch._
PLATE VII.--PARTIAL ECLIPSE OF THE MOON.
_Observed October_ 24, 1874.
A view of the Moon partially obscured by the Earth's shadow, whose
outline gives ocular proof of the earth's rotundity of form. The
shadowed part of the Moon's surface is rendered visible by the diffused
sun-light refracted upon it from the earth's atmosphere. Its reddish
brown color is due to the absorption, by vapors present near the earth's
surface, of a considerable part of this dim light. On both the obscured
and illuminated tracts the configurations of the Moon's surface are seen
as with the naked eye. The craters appear as distinct patches of lighter
color, and the noticeably darker areas are the depressed plains or
_Maria_. The large crater _Tycho_, at the lower part of the disk, is the
most prominent of these objects, with its extensive system of _radiating
streaks_. The largest crater above is _Copernicus_, at the left of which
is _Kepler_ and still above are _Aristarchus_ and _Herodotus_ appearing
as if blended in one. Above and at the left of the great crater _Tycho_,
the first dark tract is the _Mare Humorum_ of Plate VI., seen in its
natural position, with the crater _Gassendi_ at its northern (upper)
extremity and _Vitello_ on its southern (lower) border. The advancing
border of the shadow appears, as always, noticeably darker than the
remainder, an effect probably of contrast. The illuminated segment of
the Moon's disk has its usual appearance, the lighter portions being the
more elevated mountainous surfaces, and the dark spaces the floors of
extensive plains.
_Approximate scale, 140 miles--1 inch._
PLATE VIII.--THE PLANET MARS.
_Observed September_ 3, 1877, _at 11h. 55m. P. M._
A view of the southern hemisphere of Mars, when in the most favorable
position for observation, and when exceptionally free from the clouds,
which very frequently hide its surface configurations. Since, of all the
planets, Mars is most like the earth, Plate VIII. may give a fair idea
of the appearance of our globe to a supposed observer on Mars. The dark
gray and black markings, are regarded as tracts of water, or of some
liquid with similar powers of absorbing light; and, for the same reason,
the lighter portions, of a prevailing reddish tint, are supposed to be
bodies of land, while the bright white portions are variously due to
clouds, to polar snow or ice, and the bright rim of white along the
limb, to the depth of the atmosphere through which the limb is seen. The
chief permanent features of the planet's surface have been named in
honor of various astronomers.
The large dark tract on the left is _De La Rue Ocean_, the isolated oval
spot near the centre is _Terby Sea_, and on the right is the western end
of _Maraldi Sea_, with strongly indented border. Directly north of
(below) De La Rue Ocean, is _Maedler Continent_; above it stretches
_Jacob Land_; and surrounding Terby Sea is _Secchi Continent_. Extending
into the centre of De La Rue Ocean is a curious double peninsula,
called, in consequence of the dimness of former observations, _Hall
Island_. The sharply defined, white-crested northern borders of De La
Rue Ocean and Maraldi Sea may indicate the existence there of lofty
coast ranges, more or less constantly covered with opaque clouds
strongly reflecting light. The white spot in the centre of Maedler
Continent, of a temporary nature, has a similar explanation. The
intervals of olivaceous gray on Secchi Continent and elsewhere may
perhaps be ascribed to the flooding and drying up of marshes and
lowlands, as these markings have been observed to vary somewhat in
connection with the change of seasons on Mars. The greenish tints
observed along the planet's limb, alike on the darker and lighter
surfaces, are probably due to an optical effect, the green being
complementary to the prevailing red of the disk. The brilliant oval
white spot near the southern (upper) pole of the planet is a so-called
polar spot, in all probability consisting of a material similar to snow
or ice and here observed in the midst of a dark open sea.
_Approximate scale, 300 miles--1 inch._
PLATE IX.--THE PLANET JUPITER.
_Observed November_ 1, 1880, _at 9h. 30m. P. M._
This planet is perpetually wrapped in dense clouds which hide its inner
globe from view. The drawing shows Jupiter's outer clouded surface with
its usual series of alternate light and dark belts, the disk as a whole
appearing brighter in the centre than near the limb. The darker gray and
black markings indicate in general the lower cloud-levels; that is,
partial breaks or rifts in the cloudy envelope, whose prevailing depth
apparently exceeds four thousand miles. While the deepest depression in
the cloudy envelope is within the limits of the Great Red Spot, the
vision may not even here penetrate very deeply. Two of Jupiter's four
moons present bright disks near the planet's western limb, and cast
their shadows far eastward on the disk, that of the "second satellite"
falling upon the Red Spot. On the Red Spot are seen in addition two
small black spots, no explanation of which can yet be offered. The broad
white ring of clouds bordering the Red Spot appeared in constant motion.
The central, or equatorial belt, shows brilliant cloudy masses of both
the _cumulus_ and _stratus_ types, and the underlying gray and black
cloudy surfaces are pervaded with the pinkish color characteristic of
this belt. The dark circular spots on the wide white belt next north
showed in their mode of formation striking resemblances to sun-spots.
They afterward coalesced into a continuous pink belt. The diffusion of
pinkish color over the three northernmost dark bands, as here observed,
is unusual. About either pole is seen the uniform gray segment or polar
cap. The equatorial diameter is noticeably longer than the polar
diameter, a consequence of the planet's extraordinary swiftness of
rotation. To the same cause may also be due chiefly the distribution of
the cloudy belts parallel to the planet's equator, though the analogy of
the terrestrial trade-winds fails to explain all the observed phenomena.
_Approximate scale, 5,500 miles--1 inch._
PLATE X.--THE PLANET SATURN.
_Observed November_ 30, 1874, _at 5h. 50m. P. M._
Saturn is unique amongst the planets in that its globe is encircled by a
series of concentric rings, which lie in the plane of its equator, and
consist, according to present theories, of vast throngs of minute bodies
revolving about the planet, like so many satellites, in closely parallel
orbits. The globe of Saturn, like that of Jupiter, is surrounded by
cloudy belts parallel to its equator. The broad equatorial belt, of a
delicate pinkish tint, is both brighter and more mottled than the
narrower yellowish white belts, which alternate with darker belts of
ashy gray on both the north and south sides, but are seen here only on
the northern side. The disk has an oval shape, owing to the extreme
polar compression of the globe.
The outer, middle and inner rings, with their various subdivisions, are
clearly shown in Plate X., and are best seen on the so-called _ansæ_,
or handles, projecting on either side. The gray _outer ring_ is
separated by the dusky pencil line into two divisions, both of which
appear slightly mottled on the ansæ, as if with clouds. The _middle
ring_ has three sub-divisions which are clearly distinguishable,
although separated by no dark interval, viz., a brilliant white outer
zone, distinctly mottled, as seen on the extremities of the ansæ, and
two interior zones of gradually diminishing brightness. The _gauze_ or
_dusky ring_ is seen at its full width on the ansæ, but on the
background of the strongly illuminated globe only its outer and
presumably denser border is visible. The shadow of the globe on the
rings is seen on the lower portion of the eastern ansæ. The shadow on
the dusky ring is with difficulty perceptible; the shadow on the middle
ring is slightly concave toward the planet, which concavity is abruptly
increased on the outer zone of this ring; while the shadow on the outer
ring slants away from the globe. These appearances are fully accounted
for by supposing a general increase of level from the inner edge of the
dusky ring to the outer margin of the middle ring, and a uniform lower
level on the outer ring. Other observers have regarded the deflection of
the shadow as an effect of irradiation. The inner margin of the double
outer ring presents on both ansæ a number of slight indentations,
which, if not actual irregularities in the contour of this ring, may be
explained as shadows caused by elevations on the outer border of the
middle ring, or possibly by over-hanging clouds.
_Approximate scale, 6,500 miles--1 inch._
PLATE XI.--THE GREAT COMET OF 1881.
_Observed on the night of June_ 25-26, _at 1h. 30m. A. M._
A view of the comet 1881, III., drawn as if seen with the naked eye, the
minute details, however, being reproduced as seen with the telescope.
The star-like _nucleus_ is attended by four conical wings which cause it
to appear diamond-shaped. The _coma_ appears double, the brilliant inner
coma, immediately enveloping the nucleus, being surrounded by a fainter
exterior coma, which has a noticeable depression corresponding to that
of the inner edge of the principal coma. The tail is divided lengthwise
by a dark rift and is brightest on its convex or forward side. An inner
portion of the tail, brighter than the rest, is more strongly curved, as
if by solar repulsion. Stars are seen through the brighter parts of the
tail, as they may be seen even through the coma and nucleus, with little
diminution of their light.
PLATE XII.--THE NOVEMBER METEORS.
_As observed on the night of November_ 13-14, 1868.
A partially ideal view of the November Meteors, combining forms observed
at different times during the night of Nov. 13th, 1868. It is not,
however, a fanciful view, since a much larger number of meteors were
observed falling at once during the shower of November, 1833, and at
other times. The locality of the observation is shown by the Polar Star
seen near the centre of the Plate, and Cassiopeia's Chair at the left.
The general direction of the paths of the meteors is from the
north-east, the _radiant point_ of the shower having been in the
constellation Leo, beyond and above Ursa Major. While the orbits of the
meteors are, in general, curved regularly and slightly, several are
shown with very eccentric paths, among them one which changed its course
at a sharp angle. In the upper left-hand corner appear two vanishing
trails of the "ring-form," and several others still further transformed
into faint luminous patches of cloud. Red, yellow, green, blue and
purple tints were observed in the meteors and their trails, as
represented in the Plate.
PLATE XIII.--PART OF THE MILKY WAY.
_From a study made during the years_ 1874, 1875 _and_ 1876.
The course of the portion of the Galaxy represented in Plate XIII. is as
follows: From Cassiopeia's chair, three bright stars of which appear at
the upper edge of the Plate, the Galaxy, forming two streams, descends
south, passing partly through Lacerta on the left, and Cepheus on the
right; at this last point it approaches nearest to the Polar Star, which
is itself outside of the field of view. Then it enters Cygnus, where it
becomes very complicated and bright, and where several large cloudy
masses are seen terminating its left branch, which passes to the right,
near the bright star _Deneb_, the leader of this constellation. Below
_Deneb_, the Galaxy is apparently disconnected and separated from the
northern part by a narrow, irregular dark gap. From this rupture, the
Milky-way divides into two great streams, separated by an irregular dark
rift. An immense branch extends to the right, which, after having formed
an important luminous mass between the stars _Gamma_ and _Beta_,
continues its southward progress through parts of Lyra, Vulpecula,
Hercules, Aquila and Ophiuchus, where it gradually terminates a few
degrees south of the equator. The main stream on the left, after having
formed a bright mass around _Epsilon Cygni_, passes through Vulpecula
and then Aquila, where it crosses the equinoctial just below the star
_Eta_, after having involved in its nebulosity the bright star _Altair_,
the leader of Aquila. In the southern hemisphere the Galaxy becomes very
complicated and forms a succession of very bright, irregular masses, the
upper one being in Scutum Sobieskii, while the others are respectively
situated in Sagittarius and in Scorpio; the last, just a little above
our horizon, being always considerably dimmed by vapors. From Scutum
Sobieskii, the Galaxy expands considerably on the right, and sends a
branch into Scorpio, in which the fiery red star _Antares_ is somewhat
involved. In the upper left-hand corner of the Plate, at some distance
from the Milky-way, is seen dimly the Nebula in Andromeda, which becomes
so magnificent an object to telescopic view.
PLATE XIV.--STAR-CLUSTER IN HERCULES.
_From a study made in June_, 1877.
In the constellation Hercules, a small nebulous mass is faintly visible
to the eye, a telescopic view of which is presented in Plate XIV. It is
one of the most beautiful of the easily resolvable globular clusters.
The brilliancy of the centre gives the cluster a distinctly globular
appearance, while the several wings curving in various directions, have
suggested to some observers an irregularly spiral structure. The large
stars of the cluster are arranged in several groups which correspond, in
a general way, with the faintly luminous wings.
PLATE XV.--THE GREAT NEBULA IN ORION.
_From a study made in the years_ 1875-76.
This nebula, which is one of the most brilliant and wonderful of
telescopic objects, readily visible to the naked eve as a patch of
nebulous light immediately surrounding the middle star of the three
which form the sword of Orion, and a little south of the three
well-known stars forming the belt. The small stars in this, as in other
Plates of the series, are somewhat exaggerated in size, as was
unavoidable with any mode of reproduction that could be employed. The
bright pentagonal centre of the nebula is traversed by less luminous
rifts, the several subdivisions thus outlined being irregularly mottled
as if by bright fleecy clouds. Toward the lower part of this bright
pentagonal centre is a comparatively dark space containing four bright
stars which form a trapezium and together constitute the quadruple star
_Theta Orionis_, which, to the naked eye, appears as the single star in
the centre of the sword. On three sides of the central mass extend long
bright wisps, whose curves fail, however, to reveal the spiral structure
often attributed to this nebula. On the east a broad wing, with
wave-shaped inner border, stretches southward. East of the trapezium are
two especially noticeable dark spaces. Close to the main nebula on the
north-east, a small faint nebula surrounds a bright star, and a branch
from another faint stream of nebulous matter forming a loop to the
southward, encloses the nebulous star (_Iota Orionis_) shown at the top
of the Plate.