The project was thought over in all its bearings, and in 1887 a great conference of astronomers at Paris resolved upon an international scheme for photographing the entire heavens. The work was to be divided between eighteen Observatories of different nationalities. It was to result in a photographic chart extending to the 14th magnitude, and probably embracing some forty million stars, and a catalogue made from measures of the photographs down to the 11th magnitude, which would probably include between two and three million stars. [Illustration: THE CONTROL PENDULUM AND THE BASE OF THE THOMPSON TELESCOPE.] The eighteen Observatories all undertook to use instruments of the same capacity. This was to be a photographic refractor, with an object-glass of 13 inches aperture and 11 feet focus. At Greenwich this telescope is mounted equatorially--that is, so as to follow the stars in their courses--and is mounted on the top of the pier that once supported Halley's quadrant. The telescope is driven by a most efficient clock, whose motive power is a heavy weight. The rate of the weight in falling is regulated by an ingenious governor, which brings its speed very nearly indeed to that of the star, and any little irregularities in its motion are corrected by the following device. A seconds pendulum is mounted in a glass case on the wall of the Observatory, and a needle at the lower end of the pendulum passes at each swing through a globule of mercury. On one of the wheels of the clock are arranged a number of little brass points, at such intervals apart that the wheel, when going at the proper rate, takes exactly one second to move through the distance between any pair. A little spring is arranged above the wheel, so that these points touch it as they pass. If this occurs exactly as the pendulum point passes through the mercury nothing happens, but if the clock is ever so little late or early, the electric current from the pendulum brings into action a second wheel, which accelerates or retards the driving of the clock, as the case may be. The total motion, therefore, is most beautifully even. [Illustration: THE ASTROGRAPHIC TELESCOPE. (_Reproduced from 'Engineering' by permission._)] But even this is not quite sufficient, especially as the plates for the great chart have to be exposed for at least forty minutes. Rigidly united with the 13-inch refractor, so that the two look like the two barrels of a huge double-barrelled gun, is a second telescope for the use of the observer. In its eyepiece are fixed two pairs of cross spider lines, commonly called wires, and a bright star, as near as possible to the centre of the field to be photographed, is brought to the junction of two wires. Should the star appear to move away from the wire, the observer has but to press one of two buttons on a little plate which he carries in his hand, and which is connected by an electric wire with the driving clock, to bring it back to its position.