INTRODUCTION. 



DESCRIPTION AND USE OF TABLES. 



THERMOMETRY. 



The present standard for exact thermometry is the normal centigrade 

 scale of the constant-volume hydrogen thermometer as defined by the In- 

 ternational Bureau of Weights and Measures. The constant volume is one 

 liter and the pressure at the freezing point is one meter of mercury reduced 

 to freezing and standard gravity. The scale is completely defined by desig- 

 nating the temperature of melting ice, o°, and of condensing steam, ioo°, 

 both under standard atmospheric pressure. All other thermometric 

 scales that depend upon the physical properties of substances may by defi- 

 nition be made to coincide at the ice point and the boiling point with the 

 normal scale as above defined, but they will diverge more or less from it 

 and from each other at all other points. However, by international consent 

 it is customary in most cases to refer other working scales to the hydrogen 

 scale. 



The absolute or thermodynamic scale. To obviate the difficulty which 

 arises because thermometers of different type and substance inherently 

 disagree except at the fixed points. Lord Kelvin proposed that tempera- 

 tures be defined by reference to certain thermodynamic laws. This course 

 furnishes a scale independent of the nature or properties of any particular 

 substance. The resulting scale has been variously named the absolute, the 

 thermodynamic, and, more recently, in honor of its author, the Kelvin 

 scale. The temperature of melting ice by this scale on the centigrade basis 

 is not as yet accurately known, but it is very nearly 273.13, and that of the 

 boiling point, 373°i3. 



Many problems in physics and meteorology call for the use of the abso- 

 lute scale; but it is not convenient, and in many cases not necessary, to 

 adhere strictly to the true thermodynamic scale. In fact, the general require- 

 ments of science will very largely be met by the use of an approximate 

 absolute scale which for the centigrade system is defined by the equation 



T = (273° + t° C.) 

 The observed quantity, t°, may be referred to the normal hydrogen centi- 

 grade scale or be determined by any acceptable thermometric method. 



This scale differs from the true Kelvin scale, first, because 273° is not the 

 exact value of the ice point on the Kelvin scale, second, because each ob- 

 served value of t° other than 0° or 100° requires a particular correction to 



XV 



