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.

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.

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.

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.

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.