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.
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.