208 M. A. Hitter's Contributions to the Study 



Thomson's nomenclature for the point J of fig. 10, we might 

 also call it the triple edge). 



We may further call the steam-edge L D the cloud-edge, 

 since the beginning of condensation is marked by the forma- 

 tion of a cloud ; and the water-edge J W may be called the 

 rain-edge or the dew-edge, since the product of complete con- 

 densation exhibits itself as rain or dew. The edge F J may 

 be conceived as the line on which water begins to freeze, and 

 may therefore appropriately be called the frost-edge. The 

 edge S K can be conceived as the line in which ice begins to 

 melt, and may therefore be called the melting-edge. The edge 

 R K may be called the rime- or snow-edge, as the product of 

 the direct transformation of aqueous vapour into the solid state 

 appears as rime or snow. 



To conclude, exact proportions could not be given in the 

 above figures, from the nature of the case ; for if, for ex- 

 ample, the segment J K, so as to be perfectly discernible, were 

 drawn even only one millimetre long, the segment K L would 

 have a length of more than two kilometres on a diagram drawn 

 exactly to scale. 



§ 5. Angles formed at the Principal Edge. 



By Clapeyron and Clausius's law the relation between the 

 pressure and temperature of saturated steam can be expressed 

 by the differential equation 



dT AuT' ^ ; 



where r denotes the latent heat of steam, u the increment of 

 volume that occurs on vaporization, A=^^ the heat-equiva- 

 lent of a kilogrammetre. To the value T=273 (or £ = 0) cor- 

 respond the values ?*= 606*5 and ^ = 210*66*. In the iso- 

 thermal for 0° C, therefore, the above differential coefficient 

 takes the value 



dp _ 424x606-5 _ 1 . 1715 ( ^ 



dT 210-66x273 ^* iXO W 



If the principal edge (represented in fig. 10 by the point J) 

 lay exactly in the isothermal for 0° C, then would the above 

 value be the tangent of the angle marked </> in that figure. 

 As the point J really lies on the isothermal for o, 00744 C. 

 (as will appear later), the above coefficient, in order to repre- 

 sent tan <j> exactly, requires a slight correction, which we can 

 easily make by determining from tables by interpolation the 

 values of r and u that correspond to T= 273*00744 (or 



* Zeuner's Grundziige der mechanischen Warmetlieorie. 



