Elements of inorganic chemistry : descriptive and qualitative . 2. The earth is everywhere surrounded by an ocean ofgaseous vapor, called the atmosphere, which varies fromfifty to one hundred miles in height. This variation atany one point is never ceasing, for just as in the oceans ofwater, so in this ocean of air, do huge waves continuallysurge to and fro, —-waves so vast that their altitudes aremeasured in miles. Eveiy object upon the surface of the earth is subjectedto the pressure exerted by the weight of air above. Thispressure varies constantly, and, owing to the great mobilityof the pa
Elements of inorganic chemistry : descriptive and qualitative . 2. The earth is everywhere surrounded by an ocean ofgaseous vapor, called the atmosphere, which varies fromfifty to one hundred miles in height. This variation atany one point is never ceasing, for just as in the oceans ofwater, so in this ocean of air, do huge waves continuallysurge to and fro, —-waves so vast that their altitudes aremeasured in miles. Eveiy object upon the surface of the earth is subjectedto the pressure exerted by the weight of air above. Thispressure varies constantly, and, owing to the great mobilityof the particles of air, it is exerted in alldirections, — downwards, upwards, and side-wise. This pressure is measured by an instru-ment called a barometer (Fig. 15). A is aglass tube about 800mm long, sealed at theupper end, open at the lower, and providedwith a scale. This tube is filled with mer-cury, and inverted in a cup of mercury,C. Now, since the tube itself sustainsthe pressure which the atmosphere wouldFig. 15. exert on this column of mercury within the. THE ATMOSPHERE. 83 tube, in every direction except upwards, it follows thatthe column will remain at a higher altitude than the levelof the mercury in the cup. The height of this columnof mercury will depend upon how hard the atmospherepresses it upward. At the level of the sea, in the latitude of Paris, and at0° C, the average height of this column is 760mm; hence760mm is taken as the standard pressure of the air. As you ascend from the sea-level the column falls(why?), and as you descend it rises (why?). As the density (Art. 88) of the mercury and the at-mosphere varies, owing to changes of temperature, theheight of the barometer varies; hence the necessity oftaking a standard temperature, which is 0° C. /Off: 83. Measurement of the Temperature of the Atmos-phere.— This is accomplished by means of instrumentscalled thermometers. There are three scales in use, —Centigrade, Fahrenheit, and Reau-mur (Fig. 16). Thermo
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