Hv INTRODUCTION. 



because the same result is attained by simply passing the curve through a 

 point more accurately known and as near as may be to the critical point. 

 This is equivalent to defining t and 6 by an "equation of condition." 

 Another "equation of condition " fixes the pressure at th£ boiling point which 

 by definition must be 760 mm. From the considerations given on page xi 

 computations are greatly facilitated by taking all temperatures on the 

 approximate absolute scale represented by T = 273 + f. 



A careful preliminary analysis of the observational data in the vicin- 

 ity of the critical temperature resulted in assigning values to 6 and w as 

 follows : 



6 = 643 , log. t = 5.1959000 



It is emphasized here again that these data do not represent critical tem- 

 perature conditions, but simply a convenient point on the pressure curve 

 slightly below the critical temperature, the value of which is fixed with 

 considerable accuracy by the observational data. 



The value of the constant A was fixed by the equation of condition, 

 e = 760 mm. when T = 373 (X = — 8). The remaining constants (b, m, 

 n, s) are computed by the method of least squares. The results are as fol- 

 lows: 



A = 3.1473172 

 b = .00295944 

 m = .0004191398 

 n = .0000001829924 

 5 = .00000008243516 



The number of significant figures in the constants is obviously greater 

 than the accuracy of the data justifies; but is justified to facilitate compu- 

 tation and to secure accuracy in the interpolation of values which should 

 themselves be as accurate as the data. 



Thiesen 1 has shown that the observed values of vapor pressure over ice 

 can be reproduced by the equation 



Log e = log e Q + 9.632 (1 - 0.00035 1) j, 

 where e = 4-5?85, and T = 273 + t. 



For convenience in computing this equation, for metric units it may be 

 written 



, /9- 6 3 2 —0.0033712 A A . . 



Log e = 0.66072 + ( / * 273 + ? ) L (2) 



For English units the equation becomes 



nnn , /9-6qi 93 - o.ooi 87289 /, \ , . f . 



Log . = I.255888+ C' 4 59 , 4 + f , ) & " ^ W 



t = degrees Centigrade; t,= degrees Fahrenheit. 



1 Thiesen M. Die Dampfspannung iiber Eis. (Mitteilung aus der Physikalisch-Tech- 

 nischen Reichsanstalt.) Annalen der Physik, 1909; 29: 1057. 



