The author having long considered that a good series of observations with a water barometer would be of great value as throwing light upon the theory of atmospheric tides, of the horary and other periodic oscillations of the barometer, and of the tension of vapourat different temperatures, was desirous of learning whether any such series of observations had ever been made. But he could meet with none having any pretensions to accuracy ; for neither those of Otto Guericke, in whose hands the water barometer was merely a philosophical toy, nor the cursory notices of the experiments of Mariotte upon this subject contained in the History of the French Academy of Sciences, can be considered as having any such claim. The difficulties which opposed the construction of a perfect instrument of this kind long appeared to be insurmountable; but the author at length proposed a plan for this purpose, which, having been approved of by the late Meteorological Committee of the Royal Society, was ordered by the President and Council to be carried into execution. The author then enters fully into the details of the methods he employed for constructing the whole of the apparatus, and for placing in its present situation in the centre of the winding staircase conducting to the apartments of the Royal Society. The tube was very skilfully made by Messrs. Pellattand Co. at the Falcon Glass-house. It was 40 feet long, and one inch in diameter at its lower end; and so nearly cylindrical, throughout its whole extent, as to diminish only by two tenths of an inch at its upper end. A second tube of the same dimensions was also made as a provision in reserve against any accident happening to the first. These tubes were both securely lodged in a square case by means of proper supports. A small thermometer with a platina scale, was introduced into the upper end of the tube. An external collar of glass was united to that end by heating it. This was done with a view of giving it additional support, and of preventing it from slipping. This end of the tube was then drawn out into a fine tube ready for sealing with the blowpipe; and a small stopcock was fitted on to it. The cistern of the barometer was formed by a small copper steam boiler, 18 inches long, 11 wide, and 10 deep, capable of being closed by a cock, and having at the bottom a small receptacle for holding the lower end of the tube, so as to allow of the water in the cistern being withdrawn, without dis turbing that contained in the tube.
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