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.
Fig. 633.—Cosmographical clock.
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.