NEW ASTRONOMICAL APPLIANCES.
A CELESTIAL INDICATOR—ASTRONOMICAL OR COSMOGRAPHICAL CLOCK—A SIMPLE
GLOBE—A SOLAR CHRONOMETER.
Having said something concerning astronomy, we will give a few
instructions respecting the instruments not already described, and
make some observations, supplementing our directions in the previous
chapter, for many people will be glad to learn how to read the evening
skies.
Here we have an apparatus which will prove useful to amateurs; it is
a sort of celestial indicator by Mauperin, and will facilitate the
finding of every star or constellation, when the apparatus has been
made ready by pointing the rod, T, in the direction of the object it
is desired to view. This rod is mounted upon a rod, S, and is movable
upwards or downwards or sideways, and in the last-named movement it
will carry with it an indicator, I, which slides over the chart or
diagram of the heavens. The two arms of this indicator are always
parallel to the plane of the rod, T, no matter in what position they
may be on the chart or the inclination of the rod. The extremities of
this rod are terminated by an eye-slit, and by a crescent respectively.
When the apparatus is set, all one has to do is to look through the eye
aperture, O, and view the star which we have chosen in the centre of
the crescent, C. This star will be found named in the space between the
arms of the sliding indicator, I.
It is easy to perform the operation inversely—that is to say, to find
in the sky, by means of the sighting-rod, T, the stars we have chosen
on the chart between the prongs of the indicator. The chart represents
exactly the heavens as we see them, and this new mode is opposed to the
manner generally adopted with celestial charts, and is very important,
for it obviates the necessity of holding the map above one’s head, its
face downwards.
As we have already showed, it is not difficult to find the Polar
Star and the Great Bear. The latter is readily recognised by its
seven stars, and the Lesser Bear glides around the pole as shown in
the diagram on the preceding page (fig. 629). Now let us see how the
celestial indicator will work.
Let us take the apparatus into the open air, and place it upon its
tripod stand. The upper portion will be found movable by loosening the
screw, V. By another simple arrangement the table can be slanted, and
by turning a screw we can entirely slope the side of the chart where
midnight (_minuit_) is written. Being placed opposite the polar star we
take the upper part of the chart by the button, G, and bring it before
us by a horizontal rotative movement.
[Illustration: Fig. 632.—Celestial Indicator.]
We now place the sliding indicator upon midday (_midi_), and keep it in
that position while working the apparatus—that is, until we have caught
sight of the polar star in the centre of the crescent, C, by means of
the eye aperture, O. We have now obtained the meridian, and care should
be taken to tighten the screw, V. Then the table is raised to its fixed
place upon the support; it is regulated according to the latitude of
the place, and the apparatus is then “oriented.”
The upper disc is an elliptical opening, or aperture, which contains
for every moment the stars visible upon the horizon, and the
circumference is furnished with a graduated scale of hours divided into
five-minute divisions, and this is fixed upon the apparatus. The dotted
line between the midday and midnight points gives the meridian.
The disc placed underneath is the celestial chart, on the circumference
of which we shall find the days of the months. It can be moved around
the rod, S, which represents the axis of the earth around which the
heavens are supposed to revolve. When the stars have to be observed,
the day of the month has to be brought to the time at which the
observation is about to be made. We can easily read off the chart by
looking through the eye-piece as already explained. Every five minutes
it is necessary to move the chart one division, which indicates that
five minutes have passed; (other stars are, of course, arriving). The
apparatus can be packed away when done with, or the bearings taken, and
then the trouble of getting it into position again need not be repeated.
A small lamp, L, throws its light upon the chart in such a way that
the eyes of the observer are not incommoded, while the table is fully
illuminated. It can be placed at L´ if necessary. The inclination
varies according to the latitude of the place when the observations
are made. There is an arrangement underneath which admits of this
inclination according to longitude.
The apparatus can also be made available to ascertain what the aspect
of the heavens will be upon any particular evening of the month. We
have only to place the chart at the day and hour, and we shall then see
upon it all the stars visible above the horizon. We can thus find out
at what time the stars rise and set, and those which do not set—to find
the hour at which they pass the meridian (the line drawn between midday
and midnight upon the chart), and the time of their appearance on the
horizon. When the sliding indicator, I, does not show a star that is
discoverable in the sky, the observer may conclude that he is viewing a
planet. This apparatus is well adapted for beginners in astronomy, as
no deep preparatory study is necessary, and the tyro can read the sky
as easily as he could read a book.
A COSMOGRAPHICAL CLOCK.
We have, in the foregoing chapters upon Astronomy, endeavoured to
give the reader some idea respecting the inclination of the earth
and its rotation, and writers have often endeavoured to devise an
apparatus which shall show the position of the globe in space, its
diurnal motion—even its inclination and the succession of seasons in
its revolution round the sun. But such reproductions of simultaneous
movements have hitherto been obtained only on a very large scale, which
find their place well enough in the museum or lecture-room, but which
it is quite impossible to utilize in our sitting-rooms, on our tables,
or chimney-pieces. Besides, the usual apparatus employed is a very
costly one, and only serves for occasional representation; it will not
keep the facts constantly before the observer in the manner of a clock
showing the time.
But for all who are interested in Astronomy, or in Cosmography, or even
for a young person who desires merely to understand the reality of the
earth’s motion and how our earth is placed in the universe—for any one
who deems it of use that he or she should be able to see the signs and
the seasons, and the days and years, and how the earth revolves, may
obtain an astronomical or cosmographical clock, which will tell him or
her how the “world wags”; a useful as well as an ornamental timepiece.
Now this is precisely the result which the talented inventor of the
astronomical clock has arrived at. M. Mouret devoted a great portion
of his life and all his available means to the realization of his
great idea, and, sad to say, he died miserably in an attic the very
day before his great and deserving effort brought him the reward for
which he had so painfully striven and devoted himself to by a life of
self-denial and labour.
[Illustration: Fig. 633.—Cosmographical clock.]
M. Mouret communicated to his globe the astronomical movement, which
our earth possesses, by the aid of clock-work, which conveys to
it, second by second, at each stroke of the pendulum, the double
movement of rotation and progression. The globe turns upon its axis
in twenty-four hours, and thus one can perceive, without any mental
effort, the rotation of our planet, and the portions of the globe
which come under the influence of the sun in rotation, just as they
do actually on the earth. Not the least interesting attribute of this
ingenious arrangement is the fact that during breakfast or dinner one
can see the displacement and revolution of the earth with reference
to the sun to all people in the world. Here, on the meridian, all
are at midday. There, on the left, near the circle which defines the
limit between day and night, the sun is rising and day is beginning;
opposite, on the right, the sun is setting and day is closing. Yonder
is the Pacific Ocean in full daylight, while almost every continent is
in darkness and the inhabitants wrapped in slumber. Now the Chinese are
opening their eyes, and the Asiatic and European continents will soon
be illuminated and awake. This is the movement of the world as it has
ever been since time came into its calculations.
The ingenious inventor, who wished to make a clock of his apparatus,
and not being able to change its place on the earth from day to day
as the time changed, very cleverly reproduced the sun’s movement of
declination by making it describe a double cone at the axis of the
globe. At the equinoxes the poles are in a plane, and equal day and
night are shown. At the winter solstice the north pole is inclined
backwards at an angle 23° 28´, and our hemisphere is in the winter
season. We have then only eight hours’ daylight and sixteen of
darkness; six months later the pole is inclined towards the sun, and
the southern pole is plunged in darkness. We have the long days and
the southerners the long nights. An upright dial shows the time of the
country in which the globe may happen to be, and one can ascertain at
any moment what time it is anywhere else. A horizontal dial indicates
the day of the month, and changes every day in a manner corresponding
with the movement of the earth around the sun, reproduced by means of
the arrangement with the double cone. The spectator is supposed to be
turning his back to the sun.
We may add that these movements are all self-acting, and there is no
need to interfere with the clock, which is wound up like ordinary
timepieces. By an ingenious forethought the inventor provided that the
sphere should be independent of the other movements, and it can be used
for demonstration in the hands of the lecturer, and be explained with
all its motions without in any way disarranging the clock-work. The
globe must, of course, when replaced, be put exactly at the correct day
and hour.
A SIMPLE TERRESTRIAL GLOBE.
A terrestrial globe without any mechanism, so long as its axis is
parallel to that of the earth, _exposed to the direct rays of the sun_,
represents our planet with its recurrence of day and night.
The figure (634) shows us a globe without any support. The axis is
north and south, and makes, with the horizon, an angle equal to the
latitude of Paris, if the support, A B, be horizontal. To make the axis
of the globe parallel to the axis of our earth, the line, N S, must
correspond to the meridian of the place; this can be done with the
compass, for instance.
The solar rays always illuminate one-half of a sphere, no matter what
its dimensions. If we look at the illustration we shall see that the
line of separation between the light and dark portions of the globe
corresponds with that in our earth. This globe, then, tells us the
passage of light and darkness for the day, and even for the moment
of the day, when it is turned as the earth moves. The place examined
should be placed in the meridian of that place (Paris, for instance),
and occupy the most elevated spot on the globe. The earth is then in
just the same position, and daylight and darkness are shown exactly as
they exist on the earth at the time.
If this globe be then observed for a few minutes, the sun will be seen
rising and setting, as it were, in various places (we must remember we
have concentrated the _sun’s_ rays, not a lamp, upon the globe). The
places on the right, if the observer be placed facing the sun, will
come out of the shade, and those on the left will enter it. The former
are then really enjoying the sunrise, and the others are actually
witnessing his setting.
The globe represented, making the double revolution of our planet in
the year, will reproduce all the actual phenomena of day and night as
taking place on the earth itself if we stand at a little distance so as
to observe it all at once.
[Illustration: Fig. 634.—A simple terrestrial globe.]
Of course, the employment of this simple apparatus should not exclude
more complicated ones, for the former can only be used on a fine bright
day. But the advantage claimed for it is that in it we can imitate
nature exactly. Illuminated, as it is, by the real sun, the portions of
light and shade are indicated by the rays and not by a metallic circle.
In order that the line of demarcation may be exactly defined, it will
be necessary that the sun’s rays be concentrated upon the globe, and
that no lateral or vertical light be admitted. The curtains should
therefore be so arranged, and the blinds pulled down to a certain
point, and if the stand or support be painted black it will be found an
advantage. If the globe be a small one, it will be sufficient to place
the stand upon an ordinary table, without verifying the horizontal
plane. With a large globe the arrangement must be very exact.
A SOLAR CHRONOMETER.
M. Flechet’s chronometer, of which we give an illustration, is a kind
of equitorial reduced to its most simple form. It is possible to
ascertain the exact time by it very easily. It consists of a disc, AB,
divided into twenty-four hours and fractions of hours. This disc turns
upon itself around an arc, CD, which has a direction parallel to the
axis of the world, and can be moved on a joint, E, according to the
latitude of the place; F is a lens which can be moved and presented
to the sun at any time, forming the centre of a concave and exactly
spherical plate represented at GH.
[Illustration: Fig. 635.—Solar chronometer.]
When the instrument is fixed so that the axis, CD, is parallel to the
axis of the globe, the disc, AB, is turned so that the centre of the
image of the sun, produced by the lens, shall fall at _m_. The real
time is found by an examination of the position of the index, A, upon
the hour graduations of the disc. A French writer, Ch. Delounay, has
mentioned this instrument, and considers it easy of arrangement, exact
in time, and very useful.