436 



THERMOMETRICAL MEASUREMENT OF HEIGHTS. 



TABLES 



FOR DEDUCING DIFFERENCES OF LEVEL FROM THE TEMPERATURE OF THE 

 BOILING POINT OF WATER. 



When water is heated in the open air, the elastic force of the vapors produced 

 from it gradually increases, until it becomes equal to the incumbent weight of the 

 atmosphere. Then, the pressure of the atmosphere being overcome, the steam 

 escapes rapidly in large bubbles, and the water boils. The temperature at which, 

 in the open air, water boils, thus depends upon the weight of the atmospheric col- 

 umn above it, and under a less barometric pressure the water will boil at a lower 

 temperature than under a greater pressure. Now, as the weight of the atmosphere 

 decreases with the elevation, it is obvious that, in ascending a mountain, the higher 

 the station where an observation is taken, the lower the temperature at which water 

 boils at that station will be. 



The difference of elevation between two places, therefore, can be deduced from 

 the temperature of boiling water observed at each station. It is only necessary to 

 find the barometric pressures which correspond to those temperatures, and, the at- 

 mospheric pressures at both places being known, to compute the difference of level 

 by a formula, or by the tables given above for computing heights from barometrical 

 observations. 



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

 perature of boiling water are less reliable than those deduced from barometrical 

 observations. Both derive the difference of altitude from the difference of atmos- 

 pheric pressure. But the temperature of boiling water gives only indirectly the 

 atmospheric pressure, which is given directly by the barometer. This method is 

 thus liable to all the chances of error which may affect the measurements by means 

 of the barometer, besides adding to them new ones peculiar to itself, the principal 

 of which, not to speak of the differences exhibited in the various tables of the force 

 of vapor, is the difficulty of ascertaining with the necessary accuracy the true 

 temperature of boiling water. In the present state of thermometry it would hardly 

 be safe, indeed, 1o answer, in the most favorable circumstances, for quantities so 

 small as hundredths of degrees, even when the thermometer has been constructed 

 with the utmost care ; moreover, the quality of the glass of the instrument, the form 

 and the substance of the vessel containing the water, the nature of the water itself, 

 the place at which the bulb of the thermometer is placed, whether in the current of 

 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 



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