BAROMETRICAI. TABLES. 



XXIU 



in treating of this humidity factor in connection witTi hypsometric tables 

 took the following position : 



' ' To introduce a separate correction for the expansion of aqueous vapor 

 is, in the writer's view, a doubtful improvement. The laws of the distri- 

 bution and transmission of moisture through the atmosphere are too little 

 known, and its amount, especially in mountain regions, is too variable, and 

 depends too much upon local winds and local condensation, to allow a 

 reasonable hope of obtaining the mean humidity of the layer of air between 

 the two stations by means of hygrometrical observations made at each of 

 them. These doubts are confirmed by the experience of the author and 

 of many other observers, which shows that, on an average, Laplace's 

 method works not only as well as the other, but more uniformly well. At 

 any rate the gain, if there be any, is not clear enough to compensate for 

 the undesirable complication of the formula. ' ' 



Since this position was taken by Dr. Guyot forty years ago, there has 

 been no such advance in our knowledge as to impair the practical conclusion 

 in conformity with which he constructed his hypsometric table. Accord- 

 ingly in treating this portion of the formula in the construction of the 

 present tables for the reduction of the barometer to sea level, it has been 

 deemed advantageous to retain the method adopted by Guyot, and to 

 incorporate the humidity factor in the temperature term, thereby assum- 

 ing the air to contain the average degree of humidity corresponding to the 

 actually prevailing condition of temperature. 



In evaluating the humidity factor as a function of the air temperature, 

 the tables given by Prof. Ferrel {^Meteorological researches. Part in. — Baro- 

 metric hypsometry and redtictioji of the barometer to sea level. Report, U. S. 

 Coast Survey, 1881. Appendix 10.) These tables by interpolation, and 

 by extrapolation below 0° F. , give the following values for /3 : 



For Fahrenheit temperatures. 



