•■ -w^m 



ANNALS 



OP i> 



PHILOSOPHY. 



^ ^ .^^v AUGUST, 1326^^^^^^^^'^ ;-.t«**>i .^ 



Article I. * 



Explanation of the Theory of the Baromettical Measurement of 

 Heights, By Mr. Nixon. 



o {Continued from p. 52.) * 



Of f^e K«^z^o of Demi ty of drif Air%o Mercui'y\ '' , 



In possession of foiniulse enabling us to calculate the -density 

 of dry air as varying from pressure and temperature, we have to 

 ascertain in the next place the ratio of its specific gravity at any 

 given temperature compared to that of the liquid of the baro- 

 meter indicating the pressure. From the numerous experiments 

 of MM. Arago and Biot made (at Paris, at an elevation of about 

 200 feet above the level of the sea)' on moist air of various tem- 

 peratures, it is inferred that 12j000 cubic feet of perfectly dry 

 air, of the temperature of 32° F. supporting at the level of the 

 sea, in latitude 45°, a pressure of 26-0988 inches^ would be equal 

 in weight to 1 cubic foot of the liquid of the barometer. Con- 

 sequently a column of the dry air^ if uniformly dense^, and of the 

 vertical height of 12,000 inches, or 1000 feet, would exactly 

 counterpoise a column of the liquid^ of the same base, one inch 

 in height.^' 



♦ Whatever the specific gravity of the mercury (or other liquid) of the barometer, 

 12,000 measures of the dry air would equal in weight one measure of the same liquid 

 as that contained in the barometer ; the liquid in the instrument and in the scales being 

 of the same temperature. 



Repeating the experiment with water substituted for mercury, and specifically lighte/ 

 in the ratio of IS to 1, the barometer would now exhibit a pressure of 13 times 26*0988 

 inches, or 339^2^44 inches, and one-thirteenth part of 12,000 cubic feet, or 923 + cubic 

 feet of the air, wowld counterpoise one of water. Yet as the densities of dry air are 

 directly as the pressures, we should infer that when the water barometer (carried to the 

 requisite altitude) stood at 2t3-098S inches, then would the density of the air there be 

 diminished one-thirteenth, and that 12,000 measures of it would be required to balance 

 one of water. 



, • The value of the inch or foot being lost might be easily regained by a'-certaining how 

 jmany measures of dry air equalled iu weight one measure of the mercury of the baro- 

 meter. Then dividing 31 3 1 85-6 inches (- 26-0988 x 12-000) by the number of 

 measures (or ratio of the sjiecific gravity of the mercuiy to that of the air) we sliould have 



j^ew Seriefi, vol. x. g 



