1825.] Barometrical Mediiirifhint of Heights, 167 



connected by means of a little apparatus furnished with a cock, 

 the whole of the mercury may be confined within the longer 

 branch. 2. The two branches being fixed within a leathern bag* 

 containing mercury, a stopper is inserted in the shorter branch,* 

 and the mercury forced to the summit of the interior of both 

 branches by means of a screw pressing against the bag. The 

 screw serves further to bring the shorter column of mercury; 

 under every variety of pressure, to the zero of the scale. 3. The 

 syphon being filled with mercury in the usual manner, the 

 shorter branch is hermetically closed, and a capillary opening 

 made a little above z to admit the atmospheric air, but too small 

 to permit the mercury to escape. To break the force of the 

 shocks to which the instrument may be exposed, the longer 

 tube is bent out of a straight line, or both branches are con- 

 tracted in bore somewhere near the summit. 



If we fill a number of strait glass tubes of different interior 

 diameters with mercury, and immerse them (inverted) in a basin 

 of the same fluid placed in a vacuum, it will be observed that 

 the mercury within the tubes will descend below the level of the 

 fluid in the basin ; the depression being most considerable within 

 the tube having the smallest bore. The atmospheric air being 

 admitted to press on the mercury in the basin, it will be found 

 that the heights of the mercurial columns (exhibiting the pres^ 

 sures) will fall short of the elevation of the column within an 

 extremely wide tube, and that the discrepancies will be equal to 

 the depressions below the surface observed before the admission 

 of the air. Hence it is clear that when the interior of the 

 syphon barometer is not of the same diameter in both branches 

 within the range of the summits of the mercurial columns, a 

 correction for capillarity (otherwise unnecessary) will be 

 required : the height of the mercury 

 within the narrower tube must be 

 augmented by the difference of the 

 corrections proper for the respective 

 tubes. 



It has been objected to the syphon 

 barometer that the adhesion of the 

 mercury taking place through the 

 length of two tubes as well as in the 

 part connecting them, the settling op-^^^m^:* 

 the fluid v;ill be more uncertain that! '^f^^ '''•. 

 in the single tube of the cistern bar6^ t^l^rt 

 meter; and that two measurements-' ■'' 

 are more liable to error in noting the - 

 pressure than when one only is re- 

 quired. ' ■' ^^ :'OV07- !f'V.M^ ^v-r tj if 



The cistern barometer consists in' 

 its simplest form of a strait tube A 



3 



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36 



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