ZENITH SECTOR

An astronomical instrument, contrived for the purpose of measuring with great accuracy the zenith distances of stars which pa near the zenith. It was by means of a zenith sector that Bradley discovered the existence and magnitude of two most important astronomical elements, — the aberration of light, and the nutation of the earth's axis. The instrument has also been generally used (in this country at least) in trigonometrical surveys for determining the difference of latitude of two stations, a purpose for which it is very convenient; for the difference of the zenith distances of the same star, observed at its meridional pa ages at two places, gives the difference of the astronomical latitudes of the places without any regard to the star's declination. The general description of the zenith sector has been given by the present Astronomer Royal as follows {Ency. Metr., art. " Figure of the Earth "): — " In the annexed figure A B is a bar of iron with a cro piece C D, the whole in one piece. The top^A is formed in such a way that the instrument can be turned half round in azimuth when suspended at the top, and that the bottom can be moved freely in the directions D C or C D. The bracket or other support E, on which it rests, is attached to some firm part of the building. To the bar A B is firmly attached a telescope F G. At a point a, near A, is attached a plumb line a H; sometimes it is fastened at a point of attachment which is moveable, in order that by moving the point of suspension the plumb line may be made to pa o^ er a tine dot at a. The limb C D is graduated, sometimes on a circular arc of which a is the centre, and sometimes on a straight line. L D is a screw pa ing through a block strongly connected with the floor, and acting with its point against the end 1) of the piece C D; and M N O are a string and weight pre ing the sector against the point of the screw." The method of observing with the zenith sector is this: — A clock being regulated by transits or equal altitudes, the direction of the meridian is found, and guides fixed to compel the sector to move in the meridian. When a star is to be observed, the screw L D is turned till the plumb line falls exactly on some point K of the graduated arc, such that the telescope is very nearly directed to the point through which the star will pa when it comes to the meridian. When the star enters the field of view, the screw is again turned till the star appears to glide exactly along the wire fixed in the focus of the eye-gla ; and the motion of the screw being ascertained, and the value of the division known, the apparent zenith distance of the star is found. But it is evident that this is not the true zenith distance, unle the zero point of the division on the scale be quite accurate: that is to say, 1339 ZINC. so placed that when the plumb line falls on it the optical axis of the telescope is exactly vertical. It is impo ible to insure this accuracy; but it is easy to see that if the instrument be turned half round in azimuth, and the same observation made, the apparent zenith distance will be just as much greater than the true as in the former determination it was le . The mean of the two will therefore be the true zenith distance of the star. The advantages of the zenith sector are these: — The stars observed being very near the zenith, the tremor, and dancing which generally affect stars in other positions are seldom seen; there is no uncertainty about the effects of refraction; the telescope and the whole instrument are not subject to flexure; and the variation of temperature produces no sensible effect. (For a detailed account of Ramsden's zenith sector used in the measurement of the English arcs of the meridian, see the second volume of the Trigonometrical Survey of England and Wales, or Phil. Trans, for 1803. This superb instrument was unfortunately burnt in the great fire which took place in the Tower of London in October, 1841. Another sector, on a different plan from that above described (the zenith point being determined by levels instead of a plumb line) has recently been constructed for the use of the survey by Trough ton and Simms, under the directions of the Astronomer Royal. A description of the new instrument is given in the Monthly Notices of the Royal Astronomical Society for May, 1842.) ZEOLITE. {Gr.(;,iu, I boil, imAXido;, stone.) Aname given to a family of minerals, which, when heated before the blowpipe, melt with considerable ebullition. They mostly consist of silica, alumina, lime, and water. 'Natrolite. A species of zeolite containing soda: from natron, or soda. [s. 1352]