280 JIcAdie — JVeiv Units in Aero-phi/sics. 



These need further definition, and it nia_y be well to emphasize 

 somewhat how both fiducial jioints wander with change in 

 pressure. We find, for example, on the summit of Mount 

 Whitney* that the boiling point on an average summer day is 

 86° C. ('186-4° F.). Likewise the zero, that of melting ice 

 varies. It is raised by the decrease of pressure. Indeed, 

 throughout tlio western half of our country and especially in 

 mountain sections the thermometer scale is shortened at both 

 ends. A degree of temperature then is not a constant quan- 

 tity if the fiducial points are not definitely known. Further- 

 more even at sea level there are differences of pressure due to 

 passing atmospheric disturbances, and the change is sometimes 

 sufficiently great to cause a variation of several degrees in the 

 boiling and tiie melting points. But a new thermometer seaile 

 is much needed for another reason, and that is to avoid minus 

 signs. Over a large portion of the earth during several 

 mouths of the year temperatures are expressed by means of a 

 negative sign. In the upper air, temperatures of from —50° 0. 

 to —70° C. are not infrequent. To avoid this use of a negative 

 sign it is proposed to record air temperatures hereafter on a 

 scale, the zero of wliich shall be the absolute or natural zero, 

 273° below the zero of the Centigrade scale. The change is 

 easily made, and any temperature on the Centigrade scale is 

 readily given its new value by adding 273 degrees. In practice 

 the first figure, 2, can be dropped as the degrees, 200 A. to 

 299 A., cover temperatures from -73° C. to 26° C. 



Thus we have practically obtained the Kelvin or Absolute 

 scale and the temperature is read as it would be given by 

 a constant volume eras thermometer. 



to'^ 



T equals t + 273-10° C. 

 T " t + 459-58° F. 



It is a scale that is largely independent of the properties of 

 the material used. Dr. Shaw saysf that "the use of the 

 absolute as distinguished from the Centigrade scale is becom- 

 ing increasingly common in scientific publications, not only in 

 regard to subjects connected with very low temperature, such 

 as the liquefaction of gases ; but in other work also, on 

 account of its direct application in formulae connected with 

 thermal radiation, thermodynamics and the gaseous laws, with 

 all of which the investigation of the upper air is closely 

 concerned." 



* A small observatory has just been built by the Smithsonian Institution 

 at this point, which is the highest point in the United States, excluding 

 Alaska, 14,503 feet. See paper "Mountain Sites for Observatories," Pub. 

 Astronomical Society of Pacific, vol. xxi. No. 121, Feb. 1909, in which the 

 writer gives data for Mt. Shasta and Mt. Rainier. 



f British Meteorological Office, Pub. 202, pages 5 and 6. 



