The outlines of physics: an elementary text-book . by means of whichthe height of the top of the mercury column, above thelevel of the mercury in the cistern, may be read in centi-meters and decimal parts of a centimeter (or in countrieswhere the British system of measurement is used, in inchesand fractions of an inch). In order to maintain the mercury in the cistern at a con-stant level, the latter is commonly providedwith a flexible bottom of leather which canbe raised and lowered by means of a screwfrom below (Fig. 121), and with a pointerthe apex of which is rigidly adjusted tothe zero poi
The outlines of physics: an elementary text-book . by means of whichthe height of the top of the mercury column, above thelevel of the mercury in the cistern, may be read in centi-meters and decimal parts of a centimeter (or in countrieswhere the British system of measurement is used, in inchesand fractions of an inch). In order to maintain the mercury in the cistern at a con-stant level, the latter is commonly providedwith a flexible bottom of leather which canbe raised and lowered by means of a screwfrom below (Fig. 121), and with a pointerthe apex of which is rigidly adjusted tothe zero point of the scale. Before mak-ing a reading the mercury is raised untilits surface makes contact with the pointer. Barometers thus arranged are known asFortin barometers. Sometimes barometers are given theform shown in Fig. 122, in which a benttube, with one open and one closed arm, isfilled with mercury. In this case the dif-ference in the height of the two mercurycolumns indicates the pressure, and thereis no correction for capillary Fig. 121. 128. Nature of the Atmosphere. — From the phenome-non presented by the barometric column certain importantconclusions may be drawn concerning the constitutionof the atmosphere. A layer of air everywhere of thesame density as the atmosphere at the level of the sea, 142 THE OUTLINES OF PHYSICS and capable of sustaining a mercury column 76 cm. highagainst gravity, would need to have a height If, such that, jff: h : : d„ : d^ or S=h- where h is the height of the mercury column,d„ is the density of mercury, and d„ is the den-sity of air at 76 cm. pressure. Now 1 ce. ofmercury weighs grams, and 1 cc. of airat the same temperature (0°) weighs 0-001293gram. The value of IT is, therefore, = 4-95 miles nearly. The air being a compressible fluid, however,does not lie in a layer of uniform density en-veloping the earth, but diminishes rapidly indensity from the surface upwards. There isabundant evidence of the existence o
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Keywords: ., bookcentury1800, bookdecade1890, bookpublishe, booksubjectphysics
