ZEND LANGUAGE: see IRANIAN LANGUAGES and PERSIAN. ZENITH TELESCOPE, a form of telescope specially de vised for the accurate determination of the latitude of a station. It is used both in geodetic surveys and also at fixed stations for measuring the variation of latitude. The usual form of instrument consists of a telescope which can be clamped at any angle to a vertical axis and rotated about the vertical. Two stars of known declination and 52 are chosen which transit within a few minutes of one another, the one north and the other south of the zenith at nearly equal zenith distances. If c/) is the latitude the respective zenith distances are north and south, so that the small difference between them is Settings of a micrometer wire are made on each star in turn, the telescope being of course rotated from the north to the south direction between the two observations : the small distance that the wire has to be moved measures the quantity 24) and hence determines 0. This method, known as the Talcott method, de
pends on securing accurate rotation about the vertical; usually delicate spirit levels attached to the telescope are used to determine the correction for imperfect fulfilment of this condition. In a floating zenith telescope devised by B. Cookson and now employed for latitude variation at Greenwich observatory, rotation about the true vertical is obtained by floating the whole instrument in mer cury. The observations are made photographically ; the trails of the pair of stars are shown close together on the plate, and the distance between them (equivalent to is measured with a suitable micrometer.