48 VAPORIZATION. 



which air presses upon a man of ordinary size has been 

 estimated at fifty tons ; yet, from all the cavities of the animal 

 frame being filled with equally elastic air, we support this great 

 pressure without being sensible of it ; indeed, we should suffer 

 the greatest inconvenience from its sudden removal. Now the 

 pressure of the atmosphere is not always the same at the same 

 place, but is found by the barometer to vary within the limits of 

 one' tenth of the whole pressure. This difference affects the 

 boiling point to the extent of 4^ degrees. Thus, when the 

 height of the mercury in the barometer is expressed by the 

 numbers in the first column, water boils at the temperatures 

 placed against them in the second column. 



Barometer in inches of mercury water boils 



27.74 .... 2080 



28.29 .... 20.9 



28.84 .... 210 



29.41 ... v. 211 



29.8 .... 212 



30.6' .... 213 



It appears, from this table, that for every inch of variation in 

 the barometer, the boiling point of water varies 1 . J6 degree. 

 And consequently a rise or fall in the barometer of 0. 1 inch, 

 raises or lowers the boiling point 0. 176 degree. On this account 

 the pressure of the atmosphere must be attended to in fixing 

 the boiling point of water on thermometers. Water boils at 

 212, only when the pressure of the atmosphere is equivalent 

 to a column of 29. 8 inches mercury. 



The pressure of the atmosphere will be greatest at the level 

 of the sea, and will diminish as we ascend to any height above 

 it, for then we have less of the atmosphere above and pressing 

 upon us, part of it being below us. Hence, water boils on the 

 tops of mountains at a considerably lower temperature than at 

 their bases. On the top of Mount Blanc, which is the pinnacle 

 of Europe, water was observed by Saussure to boil at 184. 

 In deep pits, on the other hand, water requires a higher 

 temperature to boil it, than at the surface of the earth. An 

 instrument has been constructed for ascertaining the heights of 

 mountains on this principle. It consists essentially of a ther- 

 mometer, graduated with great care about the boiling point of 

 water, by means of which the temperature at which water boils 

 at different altitudes can be ascertained with minute accuracy. 



