The supply of material to the Meteorological Office is not the only use of the Greenwich meteorological observations. Two elements of meteorology, the temperature and the pressure of the atmosphere, have the very directest bearing upon astronomical work. And this in two ways. An instrument is sensible to heat and cold, and undergoes changes of form, size, or scale, which, however absolutely minute, yet become, with the increased delicacy of modern work, not merely appreciable, but important. So too with the density of the atmosphere: the light from a distant star, entering our atmosphere, suffers refraction; and being thus bent out of its path, the star appears higher in the heavens than it really is. The amount of this bending varies with the density of the layers of air through which the light has to pass. The two great meteorological instruments, the thermometer and barometer, are therefore astronomical instruments as well. In the arrangements at Greenwich the Magnetic Department is closely connected with the Meteorological, and it is because the two departments have been associated together that the building devoted to both is constructed of wood, not brick, since ordinary bricks are made of clay which is apt to be more or less ferruginous. Copper nails have alone been employed in the construction of the buildings. The fire-grates, coal-scuttles, and fire-irons are all of the same metal. The growth of the Observatory has, however, made it necessary to set up some of the new telescopes, into the mounting of which much iron enters, very close to the magnetic building. The present Astronomer-Royal has therefore erected a Magnetic Pavilion right out in the park at an ample distance from these disturbing causes. The double department is, therefore, the most widely scattered in the whole Observatory. It is located for computing purposes in the west wing of the New Observatory; many of its magnetic instruments are in the old Magnet House, others are across the park in the new Magnetic Pavilion; the anemometers are on the roof of the Octagon Room, Flamsteed's original observatory, and the self-registering thermometers are in the south ground between the old Magnet House and the New Observatory. [Illustration: MAGNETIC PAVILION--EXTERIOR. (_From a photograph by Mr. Lacey._)] The object of the Magnetic Observatory is to study the movements of the magnetic needle. The quaintest answer that I ever received in an examination was in reply to the question, 'What is meant by magnetic inclination and declination?' The examinee replied: 'To make a magnet, you take a needle, and rub it on a lodestone. If it refuses or _declines_ to become a magnet, that is magnetic declination; if it is easily made a magnet, or is _inclined_ to become one, that is magnetic inclination.'