Airy's altazimuth was but a small instrument of three and three-quarter inches aperture, mounted in a high tower built on the site of Flamsteed's mural arc; and, after a life history of about half a century, has been succeeded by a far more powerful instrument. The 'New Altazimuth' has an aperture of eight inches, and is housed in a very solidly constructed building of striking appearance, the connection of the Observatory with navigation being suggested by a row of circular lights which strongly recall a ship's portholes. This building is at the southern end of the narrow passage, 'the wasp's waist,' which connects the older Observatory domain with the newer. It is the first building we come to in the south ground. The computations of the department are carried on in the south wing of the new Observatory. It will be seen from the photograph that the instrument is much larger, heavier, and less easy to move in azimuth than the old altazimuth. It is, therefore, not often moved in azimuth, but is set in some particular direction, not necessarily north and south, in which it is used practically as a transit circle. [Illustration: NEW ALTAZIMUTH BUILDING.] There is quite another way of determining the place of the moon, which is sometimes available, and which offers one of the prettiest of observations to the astronomer. As the moon travels across the sky, moving amongst the stars from west to east, it necessarily passes in front of some of them, and hides them from us for a time. Such a passage, or 'occultation,' offers two observations: the 'disappearance,' as the moon comes up to the star and covers it; the 'reappearance,' as it leaves it again, and so discloses it. [Illustration: THE NEW ALTAZIMUTH. (_From a photograph by Mr. Lacey._)]