Page:The American Cyclopædia (1879) Volume II.djvu/346

 326 BAROMETER feet or more in length with mercury, and clos- ing the open end with his finger, he introduced this by inverting the tube under the surface of mercury in a basin. On removing the finger, the mercury in the tube sank down, and after oscillating stood at about 28 inches above the surface of that in the vessel, leaving in the upper end a vacant space. (See fig. 1.) Tor- ricelli continued his experiments, X'ig.l- and discovered the fluctuations in the height of the column of mer- cury caused by the changes of the weather, and in 1645 an account of his observations was published ; but he soon after died, before his great discovery was fully completed. The subject was taken up with great zeal by Pascal at Rouen in France. It occurred to him that if it were the atmospheric pressure which sup- ported the column of mercury or water, the height of the column should be lessened as the pressure is reduced by ascending to greater elevations above the surface. He communicated his views to his brother-in-law P6rier, who lived at Clermont in Auvergne, near the high conical mountain of Puy-de-D6me, with the request that he should test the theory upon this elevation. This was not accomplish- ed, however, till Sept. 19, 1648. Perier at this time, provided with mercury and tubes, ob- served in the garden of a monastery in the lowest part of Clermont the height at which the mercury stood in two tubes, which was 26 French inches and 3 lines. Leaving one of the barometers to be noticed in his absence, he took the other up the mountain, and at the summit found the height of the column was only 23 inches and 2 lines. At lower points, as he descended, the mercury rose in the tube, and at the base it occupied the same space in the tube as at first. This was the first observa- tion ever made upon the different pressures of the atmosphere at different elevations. Perier repeated the experiment upon the highest tower of Clermont; and Pascal, on learning the result, made similar observations upon the top of a high house and the belfry of a church in Paris. Satisfied with the results, he soon proposed this process for determining dif- ferences of elevation. Attention began now to be directed to the variations in the height of the mercurial column caused by the atmo- spheric changes. Otto Guericke, an ingenious and wealthy burgomaster of Magdeburg, con- trived a gigantic barometer for indicating the state of the weather. It was a glass tube near- ly filled with water, 30 feet in length, placed within the wall of his house and rising above the roof, the lower end terminating in a cistern of water. In the upper part, which was of larger dimensions than the rest, was placed the figure of a man, large enough to be visible from the street. In fine weather this figure, floating upon the surface of the water, appeared in full size above the roof; but as the fluid sub- sided with the change of weather, the manikin withdrew into the building. From the origi- nal invention of the barometer to the present time, the ingenuity of the most distinguished men of science has been exercised in improving its construction. Numerous modifications of its form have been contrived, and yet those now most approved are but slightly varied from the straight inverted tube of Torricelli, and the siphon tube also proposed by him. The liquid selected by him is still preferred to all others by reason of the required weight of it occupying so little space. It is also not liable to be volatilized by slight elevations of temperature, and thus fill with its vapor the vacant space in the top of the tube. The simplest form of the instrument is that called the cistern barometer. The straight tube of Torricelli terminates at its foot in a cistern of mer- cury. By the rising and fall- ing of the liquid in the tube, the level of that in the cistern must change. The absolute J height of the mercury, there- fore, is found by rendering the scale movable, and bringing its zero point always to the sur- face of the mercury in the cis- tern; or by making the scale fixed, and bringing the mercury to its zero point by means of a screw, which is made to press against a flexible bag that forms the lower part of the cylinder, as represented in fig. 2, where the details of the upper, middle, and lower part of the barome- ter are shown separately. The latter method is the most gen- erally adopted in the best in- struments. By means of a slid- ing vernier, the scale may be read to the - of an inch. Though various contrivances have been suggested for taking the place of these minute divisions and vernier readings, no sub- stitute has yet been found to give such good results. By a skilful observer they can be read with great minuteness, and much within the limits of accuracy of the instrument in other re- spects. The barometer adopted by the Smith- sonian institution is that of Mr. James Greene of New York. A full description of this, with the drawings that are required to render it in- telligible, is published in the 10th annual re- port of the institution. In the same article are also directions for the use of the instru- ment, and for making barometrical observa- tions. The instrument is designed for service as a mountain barometer as well as for sta- tionary uses. In fig. 3 is represented the tri- pod serving for its support during observations when used as a mountain or travelling barom-