Excerpt from Reduction of Twenty Years' Photographic Records of the Barometer and Dry-Bulb and Wet-Bulb Thermometers, and Twenty-Seven Years' Observations of the Earth Thermometers: Made at the Royal Observatory, Greenwich The cylinder on which the record was originally made is of glass, 115 inches high in its cylindrical part, and 145 inches in circumference, having hemispherical end (similar to those used as shades or protectors to clocks, works Of art, The cylinder is covered internally with a black pigment, and stopped at the Open end by insertion in a metallic cap. The cap has no pivot, but there is a perforation through its center, which, when the cylinder is mounted, is fitted upon a vertical spindle projecting upward from the center of a horizontal circular brass plate, which sustains the weight Of the vertical cylinder and turns horizontally, being supported by three antifriction wheels (each in a vertical plane) carried by the fixed frame. The glass cylinder was covered when in use by another cylinder slightly larger and having similar hemispherical end; its open end was fixed, by friction, on the rim Of the metallic cap to which the inner cylinder was attached, a collar of tape being inserted between; a second collar was also placed between the cylinders near their hemispherical ends. In the month Of April 1868 an accurately turned cylinder Of ebonite, 115 inches high and inches in circumference, was substituted for the glass cylinder used until that time. TO the ends Of the cylinder are fixed circular brass plates; the lower plate has a diameter somewhat greater than that of the cylinder, and is perforated at its center, to allow the cylinder to be fitted upon the vertical spindle projecting Upward from the center of the horizontal circular brass plate resting on antifriction wheels before mentioned. The ebonite cylinder is covered when in use by Slipping over it a tube of glass, kept in position at its lower end by a narrow projecting collar of ebonite, and at its upper end by being fixed to a flat plate of brass having a central aperture, which is fitted to a small projection on the flat top Of the ebonite cylinder. Uniform rotatory motion is given to the horizontal brass plate resting on anti friction wheels and supporting the cylinder, as before described, by means of a strong chronometer movement. The center of the chronometer is vertically below the axis of the rotating cylinder. An arm revolves on the face Of the chronometer in the same manner as the ordinary hour hand, and a fork at the end of the arm takes hold of a winch fixed to the horizontal brass plate that sustains the cylinder. About the Publisher Forgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.com This book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully; any imperfections that remain are intentionally left to preserve the state of such historical works.