4 SMITHSONIAN METEOROLOGICAL TABLES 



of mercury 760 mm. high, having a density of 13.5951 gm. cm.' 3 and subject to 

 a gravitational attraction of 980.665 cm. sec." 2 



A distinction is therefore necessary between the standard inch or millimeter 

 of mercury based on 980.665 cm. sec. -2 and the "45°" inch or millimeter of 

 mercury based on 980.616 cm. sec." 2 (see Table 1). Owing to the action of 

 the International Committee on Weights and Measures, in adopting the con- 

 ventional standard value 980.665 cm. sec.," 2 it is advisable to employ this datum 

 for all barometric readings. Accordingly, with a view to maintaining con- 

 sistency with the basis on which the International Temperature Scale of 1948 

 is established, the tables presented herein for conversions of inches or milli- 

 meters of mercury to millibars, and vice versa, are based on the conventional 

 standard acceleration of gravity (980.665 cm. sec." 2 ). 



If it is desired to convert "45°" inches or millimeters of mercury to milli- 

 bars, the tabular values in Tables 9 and 1 1 must be decreased by 0.005 percent. 

 Conversely, if millibars are to be converted to "45°" inches or millimeters 

 of mercury, the tabular values in Tables 10 and 12 must be increased by 

 0.005 percent. 



CALORIE 



15° gram-calorie. — The small- or gram-calorie 12 was originally defined as 

 the quantity of heat necessary to raise the temperature of 1 gram of water 

 1 degree centigrade. It was found necessary to specify the exact range of 

 temperature over which the water was heated and it became possible to define 

 many such calories, depending on the range. The 15° gram-calorie (cal.i 5 ), 

 the quantity of heat necessary to raise the temperature of 1 gram of water 

 from 14.5° C. to 15.5° C, has until recently been one of the most common 

 units of heat used in scientific work. 



International Steam Tables calorie. — Modern laboratory procedures for 

 the determination of amounts of heat usually involve electrical apparatus and 

 it has been found advantageous to define the calorie in terms of electrical 

 equivalents. Therefore in 1929 the First International Conference on Steam 

 Tables 18 defined the International Steam Tables calorie (ITcal.) as being 

 equivalent to 1/860 xlO 3 mean international kilowatt-hours, where 1 mean 

 international kilowatt-hour = 1.00019 absolute kilowatt-hours. 14 This defi- 

 nition has also been adopted by the International Meteorological Organiza- 



12 The large- or kilogram-calorie (Kcal.) has not been used in this volume. 



13 Mech. Eng., vol. 52, p. 120, 1930. 



14 In the United States, the National Bureau of Standards uses the relation 1 inter- 

 national joule = 1.000170 absolute joules, where the "international joule" is the inter- 

 national joule as maintained in this country, and not the mean international joule. This 

 definition yields the relation: 



1 ITcal. = 4.18674 absolute joules = 4.18605 international joules. 



