Figure 71 gives curves of the thickness of year-old ice as a function of the number of degree- 

 days computed according to Stefan's formula, if we take the heat of crystallization as 80 g-cal and 

 ice density as 0.9. The curves were constructed for heat conductivities of ice equal to 0.002, 

 0.003, 0.004, and 0.005 g-cal sec"l degrees -1 cm"l. The same diagram shows both Weyprecht 's 

 and my empirical curves. It can be seen from the figure that the empirical formulas agree sur- 

 prisingly well with Stefan's theoretical curve, if we assume in the latter that the heat conduc- 

 tivity is 0.003, or more correctly, if we assume that Stefan's formula is of the form 



;? =0.14/2. 



(7) 



Formula (6) was used for further deductions . When dealing with considerable ice incre- 

 ments , from (6) we get 



(/„ + Mf + 50 (io + A/) = 8(/? + ^R), 



(8) 



whence 



A/2 + (50 + 2/o) A/ = 8A^, 



(9) 



M = -(25 + /„) + V{25 + iof+8KR, 



(10) 



or, since i - in + Ai 



/ = — 25 + v^(25 + /o)2-l-8A/?, 



(11) 



where Ai is the increment of ice which was i^ thick at the initial moment, AR is the number of 

 degree-days which had accumulated from the moment the ice became i^ thick. 



TABLE 69. ICE INCREMENT IN CM PER 24 HOUR WITH A GIVEN NEGATIVE AIR 

 TEMPERATURE (MEAN DAILY) AND A GIVEN INITIAL ICE THICK- 

 NESS IN CM 



208 



