However, in 1851, as already mentioned, Airy substituted for the two separate instruments--the transit and mural circle--the transit circle, which, unlike the mural circle, is equally supported on both sides. This, however, does not free it from the liability to some minute flexure in the direction of its length, from the weight of its two ends, and the mercury trough is used for the detection of such bending, should it exist. The present practice is to observe a star both by reflection and directly in the course of the same transit. The observer sets the telescope carefully before ever the star comes into the field of view, and reads his seven microscopes. Then he climbs up a narrow wooden staircase and watches the star transit nearly half across the field. Then comes a rush, the observer swings himself down the ladder, unclamps the telescope, turns it rapidly up to the star itself, clamps it again, flings himself on his back on a bench below the telescope, and does it so quickly that he is able to observe the star across the second half of the field. There is no time for dawdling, no room for making any mistakes; the stars never forgive; 'they haste not, they rest not;' and if the unfortunate observer is too slow, or makes some slip in his second setting, the star, cold and inexorable, takes no pity, and moves regardless on. It will be seen that a considerable amount of work is involved in taking a single observation of a star-place. But in making a star-catalogue it is always deemed necessary to obtain at least three observations of each star; and many are observed much more frequently. A modern star-catalogue contains, like Ptolemy's, four columns. It contains also several more. Of these the principal are devoted to the effect of precession. As precession is caused by a movement of the earth's axis making the pole of the sky seem to describe a circle in the heavens, it follows that the celestial poles, and the celestial equator with them are slowly, but continually, changing their place with respect to the stars, and therefore that the declinations of the stars are always undergoing change, and as the equator changes, the point where the sun crosses it in spring--the first point of Aries--changes also, and with it the stars' right ascensions.