346 



The Realm of Nature 



was shielded from light by the mercury be in the tube. The portable 

 Aneroid barometer consists essentially of a metal box with an elastic 

 top and exhausted of air. When the atmospheric pressure increases 

 the top is forced in, when it diminishes the top curves out, and this 

 movement is transmitted by suitable mechanism to a hand moving 

 round a dial, or to a lever carrying a pen which records the fluctua- 

 tions of pressure in a curve drawn in ink on a rotating 

 cylinder. " Inches" and fractions are marked round 

 the dial by comparison of the aneroid with a mercurial 

 barometer, and a scale of heights is usually added, 

 for aneroids are of most value in hill-climbing. 



440. Thermometers are instruments for measuring 

 temperature by means of the difference of expansion of 

 a gas or liquid and the glass containing vessel. Mer- 

 cury is usually employed as the liquid, because it has 

 a low specific heat, great conducting power, expands 

 considerably when heated, has a low melting-point and 

 a high boiling-point. A mercurial thermometer con- 

 sists of a globular or cylindrical bulb (Fig. 65), and 

 a long tube of extremely small bore, which has been 

 sealed while filled with boiling mercury, so that, after 

 cooling, the bulb and part of the tube contain mercury 

 and the remainder is a vacuum. The freezing and 

 boiling points of any liquid depend only on the pres- 

 sure, and if the pressure remains unchanged the liquid 

 always freezes at one definite temperature, and always 

 boils at one definite temperature. Thermometers are 

 graduated by plunging them bodily into melting ice 

 and after the mercury has contracted to the full, mark- 

 ing its position by a scratch on the glass ; then by 

 hanging them in the vapour of boiling water at ordinary 

 atmospheric pressure, and when the mercury has ex- 

 panded to the full, marking its new position by a 

 scratch. Between the two fixed points any kind of 

 FIG. 65. Mer- subdivision might be made, but only three ways of 

 curial Ther- dividing the space into ' ' degrees " or steps are in use. 

 On the centigrade scale (often erroneously named after 

 Celsius) the freezing-point is marked o, the boiling-point 100, and the 

 space between is divided into 100 equal degrees, which are con- 

 tinued above 100 and below o as far as may be necessary (C, Fig. 65). 

 On Fahrenheit's scale, used popularly in English-speaking countries, 



F 



230- 

 220 



212 



200- 

 190- 



130- 

 120- 

 110- 

 100- 



32, 



r110 



-100 

 -90 

 ^80 

 70 

 -60 



50 

 h4C 

 L 30 

 ^20 



ho 



ho 



r20 



30 



