He was not absolutely correct, however, for the great twenty-four-hour clock does not mark the exact meridian of Greenwich. To find the instrument which marks it out and defines it we must step inside the Observatory precincts, and just within the gate we see before us on the left hand a door which leads through a little lobby straight into the most important room of the whole Observatory--the Transit Room. [Illustration: THE GREAT CLOCK AND PORTER'S LODGE. (_From a photograph by Mr. Lacey._)] This room is not well adapted for representation by artist or photographer. Four broad stone pillars occupy the greater part of the space, and leave little more than mere passage room beside. Two of these pillars are tall, as well as broad and massive, and stand east and west of the centre of the room, carrying between them the fundamental instrument of the Observatory, the transit circle. The optical axis of this telescope marks 'Longitude Nought,' which is further continued by a pair of telescopes, one to the north of it, the other to the south, mounted on the third and fourth of the pillars alluded to above. This room has not always marked the meridian of Greenwich, for it stands outside the original boundary of the Observatory. But it is only a few feet to the east of the first transit instrument which was set up by Halley, the second Astronomer Royal, in the extreme N.-W. corner of the Observatory domain, a distance equivalent to very much less than one-tenth of a second of time, an utterly insensible quantity with the instruments of two hundred years ago. It would be a long story to tell in detail how the Greenwich transit room has come to define one of the two fundamental lines that encircle the earth. The other, the equator, is fixed for us by the earth itself, and is independent of any political considerations, or of any effort or enterprise of man. But of all the infinite number of great circles which could be drawn at right angles to the equator, and passing through the north and south poles, it was not easy to select one with such an overwhelming amount of argument in its favour as to obtain a practically universal acceptance. The meridians of Jerusalem and of Rome have both been urged, upon what we may call religious or sentimental grounds; that of the Great Pyramid at Ghizeh has been pressed in accordance with the fantastic delusion that the Pyramid was erected under Divine inspiration and direction; that of Ferro, in the Canaries, as being an oceanic station, well to the west of the Old World, and as giving a base line without preference or distinction for one nation rather than another.