OBSERVATORY, an institution supplied with instruments for the regular observation of natural phenomena. whether astronomical, meteorological, or magnetical. In sonic observatories all three classes of observation are carried on, but in most cases special attention is paid to astronomy alone, and only such meteorological observations are taken as are required for the calculation of the effect of atmospheric refraction on the position of a heavenly body; there are, however, a few observatories which are devoted solely to meteorological or magnetical observations. Oenfining our attention to astronomical observatories, it will be convenient to divide them into two classes—public and private observatories—the former being devoted to those observations which from their nature require to be continued on the same system for long periods of time, whilst the latter are usually founded for some special object, which may be attained with a comparatively small expenditure of time and labor.
The most important work which is carried out in public observatories is the deter mination of the movements of the sun, moon, and planets among the stars; and as a corollary to this, the relative positions of the stars to which the other heavenly bodies are referred. In early times the Greek astronomers fixed these positions by means of armil hay spheres and astrolabes, having concentric graduated circles, on which the latitudes and longitudes could be read off, when a pair of sights was pointed to the heavenly body. Ptolemy 19 use. of a quadrant, .with which he measured: zenith distances on the meridian; Mt 'Many centuries after, Tycho Brahe converted thi.sform of instrument into an altazimuth by mounting it on a vertical axis iu connection with a horizontal or azimuth circle. With this instrument Tycho Brahe made a long series of observations of the altitudes and azimuths of the heavenly bodies at the observatory which the king of Denmark erected for him, and he also measured with great assiduity their angular dis tances from each other by means of a sextant, a method of observation which Flamsteed after ward employed with a much improved form of the instrument, and which is now exten sively used with the reflecting sextant, for finding the longitude at sea. It was not till the middle of the last c., that the improvement of the clock by Graham enabled astronomers to rely on it for the determination of right ascensions by the times of passage across the meridian, instead of by measuring them with a graduated circle. The quadrant was then fixed in the meridian, and being attached to a massive wall, its dimensions were increased, and greater accuracy thereby secured in the determination of meridian zenith distances. Two such instruments pointing respectively n. and s. were erected at the royal observ atory, Greenwich, and used by Bradley and his successors from MO till they were dis placed by the mural circle (see CIRCLE, MURAL), an instrument vastly superior in prin ciple, since the troublesome errors of centering of the quadrant were got rid of by com bining the readings of opposite parts of a graduated circle, whilst the effect of division errors was much reduced by taking the mean of the readings at 6 or 8 equidistant points of the circle. At the same time, the accuracy of the readings was greatly increased by the invention of the micrometer-microscope, which made it possible to measure spaces to ,661600- of an inch. Neither the quadrant nor the mural circle, however, could be relied upon for accurate motion in the plane of the meridian, but Romer remedied this defect by inventing a separate instrument, the transit (q.v), which enabled astronomers to observe the times of meridian passage or transit with great accuracy, and thus to deter mine the differences of right ascension of the heavenly bodies by means of the apparent diurnal movement. With the transit and quadrant Bradley commenced that series of observations of the positions of the sun, moon, and planets, and of stars for reference, which have been continued ever since at Greenwich, and on which, in combination with less extensive series at Paris and KOnigsberg, all our tables of the motions of the heavenly bodies are founded. In modern observatories, the transit and mural circle are combined
into one instrument, the transit-circle, a change which has been rendered possible chiefly by the improvement in graduated circles since the invention of Troughton's dividing engine, the unwieldy size of the old quadrants and•mural circles necessitating an attach ment to a massive wall. Although Reichenbach made transit-circles at the beginning of this c. for several foreign observatories, including that of Dorpat, the lightness of their structure and their want of stability prevented their being introduced generally, and the mural circle held its place in the principal observatories till sir George Airy designed the Greenwich transit-circle in 1831, an instrument of a most massive character, which has served as model for nearly all that have been constructed in recent years. The main fea tures of the modern transit-circle are: (1) that it is not reversible, its collimation error being determined by means of two collimators, or reversed telescopes pointing at each other and at the transit telescope, n. and s. respectively; (2) that a spirit-level is not used, the level error being found by means of the reflection of the wires from the horizontal surface of mercury. These two negative characteristics, while admitting of great mas siveness in construction (the Greenwich instrument weighs more than a ton), have removed three troublesome sources of error—inequality in the pivots, lateral flexure of the telescope in the process of reversion, and the effect of currents of heated air on a spirit-level. An important auxilliary to the transit-circle is the chronograph, an Ameri can invention, which, in various forms, is now found in all well-equipped observatories, the principle in all cases being the same—viz., the registration on a revolving cylinder of paper of the times of transit across the system of spider-lines of the transit-circle, as well as of the seconds of the sidereal clock, by means of electric currents, which pass through electromagnets, when the circuit is closed either by the observer or the clock, thus causing a momentary attraction of a piece of soft iron, and producing it correspond ing mark on the paper either with a pen or a steel point. This system, while improving somewhat the accuracy of the individual observations, admits of a large number being made at intervals of two or three seconds, and leaves the observer free to make several observations of zenith distance during the passage of a star across the field of view. Allusion has been made to the importance of the sidereal clock in modern astronomy. Considerable improvements have been made in its construction since Graham's time, the original gridiron pendulum having been replaced successively by the mercurial and the zinc and steel, and the dead-beat escapement by Dennison's gravity and Airy's detached escapement. Recently an apparatus depending on the attraction of a movable magnet connected with a float in a siphon barometer has been applied by sir George Airy to the sidereal clock at Greenwich, to correct for the effect of variations in the atmospheric pressure on the motion of the pendulum. This clock is placed in a basement which is kept at a nearly uniform temperature, an important condition, which has contributed to make its peforma nee very far superior to that of any other clock hitherto constructed, and fully equal to the requirements of the methods of observation now in use. With instru ments such as have just been described, regular observations of the sun, moon, and planets, and of fundamental• stars, at. Greenwich, Paris, Washington and Oxford, supplemented at the first-named observatory by extra-meridian observations of the moon with a massive altazimuth, which can be employed when the moon is too near new moon to be seen on the meridian in full daylight, and which is in fact used to secure an observation on every night when the moon is visible.