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interference with the primary objects of the institution. From first to last Greenwich has been held to be an institution for the pursuit of mensurative astronomy with utilitarian ends, and its instruments, and, to some extent, its personnel, have been provided and organised accordingly. The succession of Royal Astronomers—Royal Observators, as they are officially styled Flamsteed, Halley, Bradley, Maskelyne, Pond, and the present occupant of the office, have, with one exception, laboured with a consistency truly remarkable, like one long-lived man, to carry out, with the best resources of their times, the strictly fundamental works committed to them: with what results we shall presently see.
Positions of stars for the formation of catalogues of starplaces, and positions of the sun, moon, and planets for the ultimate formation of tables of their motions, are the staple observations at Greenwich, and they consist almost entirely of meridian observations, namely, times of transit over the meridian, for determination of objects' Right Ascensions, and measures of the angular distances of the same objects from the celestial pole, for the determination of their North Polar distance; these last being also made on the meridian. The instruments for these observations are the “ Transit” and some form of - Circle,” both instruments moving in the plane of the meridian only, and the first having it accurately defined by a vertical wire, in modern times a cobweb thread. Flamsteed at first had a make-shift sextant, with which he could only measure one star from another in a straight line, and he had to leave these measures to be ultimately referred to fixed points for determination of Right Ascensions and Polar Distances. But in 1689, thirteen years after his appointment, he procured
—from his own resources, by the way, for from first to last he never had a penny for instruments from the Government-a large graduated arc, which was fixed upon a wall in the meridian plane, and upon the centre of which was pivoted a telescope with a vertical wire at its focus; and he took clocktimes of transit across this wire for Right Ascension, and read the position of the telescope upon the arc for Polar Distance. Halley had a transit instrument, with a 14-inch object-glass, generally similar in plan to those of the present day, specially for R.A. observations, and a meridianal quadrant of 8 feet radius specially for those in Polar Distance. Bradley at first had Halley's instruments, afterwards a better transit of 24-inch aperture, and a second quadrant, so that he could command the whole meridian from the North horizon to the South. Maskelyne used Bradley's instruments from first to last, but the defects of a quadrant, in its liabilities to distortion and errors of centering, were so obvious to hiin, that just before his death he ordered a complete circle, of 6 feet diameter, to be made, with 'a telescape, of 4-inch aperture, fixed upon it, graduated around its periphery. This circle turned upon a pivot carried through a wall to which were fixed six micrometers for reading the circle divisions. Maskelyne did not live to use this “ Mural circle,” but Pond had the advantage of it; and he subsequently had another one made like it, using the two together. Pond also had a new Transit made by Troughton, of 5-inch aperture, which was a masterpiece; and with these means he pushed observing accuracy to a point that had not before been aimed at, and has even now scarcely been surpassed.
The essential parts of all these instruments (except Flamsteed's) are shown in the accompanying sketch, upon a scale of
ANCIENT GREENWICH INSTRUMENTS.* 1. Halley's Transit (1721-1749). 4. Mural Quadrant (1725-1812). 2. Bradley's Transit (1750-1816). 5. Troughton's Transit (1816-1850.) 3. Bradley's Zenith Sector.
6. Mural Circle (1812-1850).
about 4 inch to the foot: the cut also shows the form of Bradley's famous Zenith Sector, with which he consummated his immortal aberration discovery.
The present Astronomer Royal, Sir George Airy, used Pond's instruments as he found them until about the year 1850, when finding them too small in their object-glasses for existing and future wants—especially for the observations of the newly-dis
* The woodcuts illustrating this article have been kindly lent by Messrs. Bell and Daldy, and Messrs. Bradbury and Evans.
covered minor planets—he made a revolutionary change of double character, first, by designing an instrument which, while carrying a large object-glass, should in itself combine the functions of both a Transit and a Mural Circle, and second, by causing such an instrument to be constructed of a solidity hitherto undreamt of. The engineer became instrument-maker in place of the optician, and cast and wrought iron-work of the former supplanted the tender and perishable brass of the latter. Not that the optician was ignored, but his work was confined to the delicate parts, the glasses, micrometers, and divided circles; and he and the engineer worked upon this instrument, as in all later additions to Greenwich, in judicious concert under one head.
The Transit Circle consists essentially of a telescope, 12 feet long with an object-glass of 8 inches diameter, turning between two massive stone piers. The telescope tube and the pivots are of cast iron strengthened by internal braces. In the focus is the series of vertical cobweb wires, the central one marking the meridian, for observations in Right Ascension. Upon the western side of the axis (right hand in the cut) is a circle 6 feet in diameter, divided on silver to five minutes of arc, and read by six long micrometers whose eye-ends are on the other side of the pier, which is pierced for the purpose. The micrometers read to .01 of a minute. The circle reads Zenith Distances, which are afterwards converted into North Polar Distances. One horizontal wire in the telescope, whose position is registered by a micrometer screw on the eye-piece, is used to bisect the star or object under observation, and the reading of its micrometer is combined with the circle reading. Upon the eastern axis (left hand in the cut) is another circle; this is for clamping only. A trough of mercury (for obtaining the horizontal point of the circle by observation of the same star directly and by reflexion) is carried on a circular tramway on the eastern pier by means of parallel bars which, with the counter-weights of the trough, appear in the picture : the trough does not appear. North and south of the instrument are two fixed inverted telescopes (the end of one appears in the cut); these are for giving a line of collimation : each has a wire at its focus which can be viewed by the other through an aperture in the central cube of the great telescope, or by raising the great telescope from its bearings, for which there is due provision ; the wires are adjusted to coincidence, and then by observing each with the great telescope the collimation error of the latter is obtained. The instrument is never reversed. The error of level of the axis is found by measuring the amount of non-coincidence of the meridional cobweb with its own image reflected from a trough of mercury placed perpendicularly under the telescope.
In front of the observer, as he looks south, is the Transit clock (not seen in the picture) for observing times of passage
the second and decimal of a second at which the object passed each wire. Since 1851 they have been registered by an electric chronograph. A sheet of paper on a cylinder rotating uniformly receives a puncture from a point on the armature of an electro-magnet at every beat of the Transit-clock: thus a time scale is formed by the clock. The observer taps a key mounted on the telescope eye-piece as the object under observation crosses each wire; his tap completes a galvanic circuit which works another electro-magnetic pricker; his punctures fall among the clock punctures, and their times can be read off thereby. The prickers travel down the cylinder as it rotates, and thus give a spiral line of registers. Uniform rotation of the cylinder being of the highest importance, it is secured by a driving-clock controlled by a rotating in place of a vibrating pendulum.
Since the Transit-clock is a measuring instrument of the most accurate kind, its excellence and the steadiness of its rate should be of the highest character attainable. The one in the Transit Circle room at Greenwich having grown old and exhibited some slight infirmities, a new Sidereal Motor has lately been mounted, in a basement where the temperature is nearly uniform, at a distance from the Transit instrument. It is, however, in connection with the chronograph, and nearly all Transit observations are made (galvanically) by it. It also gives controlling currents to several subsidiary clocks. Its rate is wonderfully regular; so uniform, indeed, that it faithfully exhibits the small changes which are produced by barometric fluctuations.
The Transit Circle being the chief instrument of the Observatory, it is in almost constant use whenever the sky is clear. A daily course of work with it includes Transits and North Polar Distances of the sun, moon (when visible on the meridian), all the large planets that pass before 3 o'clock in the morning, all the small planets passing between 10 P.M. and 1 A.M. during the first half of the lunation,* transits of a selection of the principal or fundamental stars for error and rate of the clock, an upper and lower culmination of a polar star for azimuthal error, an observation of the reflected meridional wire for level error, and of each collimating mark for collimation error of the Transit telescope; and observations of four stars, two north, two south of the zenith, each by reflexion and directly, for the horizontal-point error of the circle. These secured, as many extra or small stars as can be are observed for the ultimate formation
• By arrangement with the Paris Observatory the small planet observations are divided ; Greenwich observing from new to full moon, and Paris from full to new moon.
VOL. XI.-NO. XLIV.