ZENITH TUBE, an instrument designed for very accurate determinations of latitude by observing stars which transit the meridian close to the observer's zenith, or (when the latitude is known), for determining the dec linations of such stars. It consists of a rigid, immovable telescope tube so mounted as to point always to the Zenith; the plane of the telescope lens is thus horizontaL Within the title, at a distance below the lens of slightly less than one-half of the focal length, there is a basin of mercury; the rays from a zenith star after passing through the objective are re flected from the mercury surface and finally come to a focus and form the image a short distance above the lens, a hole being drilled through the centre of this to allow of their passage. The micrometer wires are adjusted above the lens in this focal plane, both they and the image being reflected horizontally from a prism placed at this point into the eye pieces, of which there are two on the sides of the main tube, one to the east and one to the nest of it.
The great advantage possessed by the in strument consists in the fact that the observer's horizon (and, therefore, his zenith) is deter mined wholly by the surface of the mercury; the more usual level tubes, which always pre sent irregular and annoying anomalies when employed for work of the highest accuracy.
are wholly dispensed with Only stars which enter the fixed field of the telescope, and hence pass within a few minutes of the zenith, can, however, he observed with it A very large zenith tube (aperture eight inches), was installed in the Flower Observa tory of the University of Pennsylvania at Phila delphia in the year 1904, for the purpose of find ing whether certain sudden changes of the local latitude, amounting in some cases to seseral feet, had a real existence. or whether they arose from undetected changes in the rcnith telescope (q.v 1, with which the latitudes had previously helm determined. The new instru ment agreed with the other in indicating oc casional sudden changes in latitude, rout their cause has not yet been discovered.