In one way or another this work of the determination of geographical longitudes has been an important part of the extra-routine work at Greenwich, part of the work which has built up and sustained its claim to define 'longitude nought'; and many distinguished astronomers, especially from the leading observatories of the Continent, have come here from time to time to obtain more accurately the longitude of their own cities. The traces of their visits may be seen here and there about the Observatory grounds in flat stones which lie level with the surface, and bear a name and date like the gravestones in some old country churchyard. These are not, as one might suppose, to mark the burial-places of deceased astronomers, but record the sites where, on their visits for longitude purposes, different foreign astronomers have set up their transit instruments. Now, however, a permanent pier has been erected in the courtyard, and a neat house--the Transit Pavilion--built over it, so that in all probability no fresh additions will be made to these sepulchral-looking little monuments. It might be asked, What reason is there for a foreign observer to come over to England for such a purpose? Would it not be sufficient for the clock signals to be exchanged? But a curious little fact has come out with the increase of accuracy of transit observation, and that is, that each observer has his own particular habit or method of observation. A hundred years ago, Maskelyne, the fifth Astronomer Royal, was greatly disturbed to find that his assistant, David Kinnebrook, constantly and regularly observed a star-transit a little later than he did himself. The offender was scolded, warned, exhorted, and finally, when all proved useless to bring his observations into exact agreement with the Astronomer Royal's, dismissed as an incompetent observer. As a matter of fact, poor Kinnebrook has a right to be regarded as one of the martyrs of science, and Maskelyne, by this most natural but mistaken judgment, missed the chance of making an important discovery, which was not made until some thirty years later. Astronomers now would be more cautious of concluding that observations were bad simply because they differed from what had been expected. They have learnt by experience that these unexpected differences are the most likely hunting-ground in which to look for new discoveries.