66 BURTON EDWARD LIVINGSTON 



Whether a solid surface has altered with use is to be determined 

 by standardization at frequent intervals, an operation, with 

 which we shall have to deal in the detailed discussion of the 

 porous cup, to be presented below. 



Evaporation at Low Temperatures 



It is occasionally remarked that the porous cup atmometeris 

 inadequate for many purposes because it cannot be operated when 

 the air temperature falls below the freezing-point of water. 

 Short periods of such low temperatures, a few hours in length, 

 produce an ice or frost coating over the cup ; longer periods destroy 

 the instrument with the bursting of the cup through freezing of 

 the contained water. For measurements of evaporation under 

 such conditions of cold the only instrument so far employed at 

 all is the open pan of water, which thus exposes a free surface 

 of solid water, or ice. Measurements are usually made by 

 weighing. 



In connection with this matter it is to be observed that the 

 freezing of a free water surface is another modification of the 

 instrument, somewhat similar to dust accumulation, wave for- 

 mation, etc. Evaporation does not progress at the same rate 

 from an ice surface as from an equal surface of liquid water, 

 at the same temperature, 15 so that when freezing occurs it is as 

 though the instrument itself were profoundly altered. Thus 

 there is no logical direct comparison to be instituted between 

 readings made with the liquid surface and those made with ice. 

 Difficult as the problem thus raised undoubtedly is, the im- 

 portance of winter evaporation is so great that some rational 

 method for its measurement should be devised. We cannot 

 go further at the present time than to state that there is no 

 method yet described, suitable for measuring evaporation during 



15 Juhlin, J., Bestamning af Vattenangans Maximi-spanstighet ofver is mel- 

 lan 0° och - 50°C. samt ofver flytande Vatten mellan + 20° och - 13°C. Bihang 

 K. Svensk. Vet.-Akad. Handlingar 17 1 : 1-72, 1891. 



Marvin, F., Report of vapor pressure measurements and normal barometer 

 construction. Part I. Maximum pressures of aqueous vapor at low tempera- 

 tures. Ann. Rept. Chief Signal Office, Washington, 1891 : 351-3S3, 1S92. 



