Chemical and Physical Notes 



57 



that the lower the tension of the steam, the more efficient 

 is the thermometer for indicating it. But, for hypsometric 

 purposes, it must be remembered that the higher we climb 

 on a mountain the greater is the height required to produce 

 a given fall of the barometer. 



TABLE IV. Giving ttie Temperature (Celsius) of Saturated 

 Steam T, at ttie Pressure P, and the Difference of Pressure 

 D corresponding to i C. Difference of Temperature of 

 Saturation. 



That these two effects very nearly compensate each other 

 is shown in Tables V and VI 1 , which give the temperature of 

 boiling water in Celsius degrees in Table V, and in Fahrenheit 

 degrees in Table VI, and the barometric pressure, in inches 

 and millimetres, which is equal to the tension of saturated 

 steam at the particular temperature, with the height above 

 the sea, in metres and feet, at which this barometric pressure 

 would be found in a still, dry atmosphere at the temperature 

 of melting ice. It will be seen that throughout a range of 

 4500 metres, or 15,000 feet, a depression of the boiling-point 

 by i C. corresponds very closely to an ascent of 300 metres, 

 or looo feet. If the column of differences of the barometer 

 be inspected, it will be seen that the diminution of pressure 

 for a given increase in altitude has fallen at 4500 metres to 

 nearly one-half of what it is at the sea-level. Hence the 

 efficiency of the thermometer as a hypsometric instrument, 

 compared with that of the barometer, increases steadily with 



1 These tables are compiled from Tables 20, 25, 33 and 34 of the Smithsonian 

 Meteorological Tables, 1893. No. 844 of the Smithsonian Miscellaneous Col- 

 lections. 



