Entering the transit room, the first thing that strikes the visitor is the extreme solidity with which the great telescope is mounted. It turns but in one plane, that of 'longitude nought,' and its pivots are supported by the pair of great stone pillars which we have already spoken of as occupying the principal part of the transit-room area, and the foundations of which go deep down under the surface of the hill. On the west side of the telescope, and rigidly connected with it, is a large wheel some six feet in diameter, and with a number of wooden handles attached to it, resembling the steering-wheel of a large steamer. This wheel carries the setting circle, which is engraved upon a band of silver let into its face near its circumference, a similar circle being at the back of the wheel nearer the pillar. Eleven microscopes, of which only seven are ordinarily used, penetrate through the pier, and are directed on to this second circle. The present transit is the fourth which the Observatory has possessed, and its three predecessors, known as Halley's, Bradley's, and Troughton's, respectively, are still preserved and hang on the walls of the transit room, affording by their comparison an interesting object-lesson in the evolution of a modern astronomical instrument. The watcher who wishes to observe the passing of a star must note two things: he must know in what direction to point his telescope, and at what time to look for the star. Then, about two minutes before the appointed time, he takes his place at the eyepiece. As he looks in he sees a number of vertical lines across his field of view. These are spider-threads placed in the focus of the eye-piece. Presently, as he looks, a bright point of silver light, often surrounded by little flashing, vibrating rays of colour, comes moving quickly, steadily onward--'swims into his ken,' as the poet has it. The watcher's hand seeks the side of the telescope till his finger finds a little button, over which it poises itself to strike. On comes the star, 'without haste, without rest,' till it reaches one of the gleaming threads. Tap! The watcher's finger falls sharply on the button. Some three or four seconds later and the star has reached another 'wire,' as the spider-threads are commonly called. Tap! Again the button is struck. Another brief interval and the third wire is reached, and so on, until ten wires have been passed, and the transit is over. The intervals are not, however, all the same, the ten wires being grouped into three sets, two of three apiece, and the third of four. [Illustration: THE CHRONOGRAPH.]