Ix INTRODUCTION. 



duced trom the temperature of boiling water observed at each station. It is 

 only necessary to find the barometric pressures which correspond to those 

 temperatures, and from these to compute the difference of height by the 

 tables given herein for computing heights from barometric observations. 



From the above, it may be seen that the heights determined by means 

 of the temperature of boiling water are less reliable than those deduced 

 from barometric observations. Both derive the difference of altitude from 

 the difference of atmospheric pressure. But the temperature of boiling 

 water is a less accurate measurement of the atmospheric pressure than is 

 the height of the barometer. In the present state of thermometry it would 

 hardly be safe, indeed, to rely, in the most favorable circumstances, upon 

 quantities so small as hundredths of a degree, even when the thermometer 

 has been constructed with the utmost care; moreover, the quality of the 

 glass of the instrument, the form and substance of the vessel containing the 

 water, the purity of the water itself, the position at which the bulb of the 

 thermometer is placed, whether in the current of the steam or in the water, 

 — all these circumstances cause no inconsiderable variations to take place 

 in the indications of thermometers observed under the same atmospheric 

 pressure. Owing to these various causes, an observation of the boiling point, 

 differing by one-tenth of a degree from the true temperature, ought to be 

 still admitted as a good one. Now, as the tables show, an error of one-tenth 

 of a degree Centigrade in the temperature of boiling water would cause 

 an error of 2 millimeters in the barometric pressure, or of from 70 to 80 feet 

 in the final result, while with a good barometer the error of pressure will 

 hardly ever exceed one-tenth of a millimeter, making a difference of 3 feet 

 in altitude. 



Notwithstanding these imperfections, the hypsometric thermometer is 

 of the greatest utility to travellers and explorers in rough countries, on 

 account of its being more conveniently transported and much less liable 

 to accidents than the mercurial barometer. A suitable form for it, designed 

 by Regnault {Annales de Chimie et de Physique, Tome xiv, p. 202), consists 

 of an accurate thermometer with long degrees, subdivided into tenths. 

 For observation the bulb is placed about 2 or 3 centimeters above the 

 surface of the water, in the steam arising from distilled water in a cylin- 

 drical vessel, the water being made to boil by a spirit-lamp. 



TABLES 72. 73. 



Barometric pressures at standard gravity corresponding to the tcuiperatiire 

 of boiling zvater. 



Table 72. Englisli Measures. 

 Table 73. Metric Measures. 



Table 72 is copied directly from Table 75. The argument is the tem- 

 perature of boiling water for every tenth of a degree from i85?o to 2i4°9 

 Fahrenheit. The tabular values are given to the nearest o.ooi inch. 



