SECT, xv.] BAROMETKICAL MEASUREMENTS. 133 



Smyth, R.N., and Sir John Herschel measured the height 

 of Etna by the barometer without any communication and 

 in different years ; Captain Smyth made it 10,874 feet, and 

 Sir John Herschel 10,873, the difference being only one 

 foot. In consequence of the diminished pressure of the 

 atmosphere, water boils at a lower temperature on the moun- 

 tain tops than in the valleys, which induced Fahrenheit to 

 propose this mode of observation as a method of ascertaining 

 their heights. It is very simple, as Professor Forbes has 

 ascertained that the temperature of the boiling point varies 

 in an arithmetical proportion with the height, or 549'5 feet 

 for every degree of Fahrenheit, so that the calculation of 

 height becomes one of arithmetic only without the use of 

 any table. 



The atmosphere when in equilibrio is an ellipsoid flattened 

 at the poles from its rotation with the earth. In that state 

 its strata are of uniform density at equal heights above the 

 level of the sea, and it is sensible of finite extent when it 

 consists of particles infinitely divisible or not. On the latter 

 hypothesis it must really be finite, and, even if its particles 

 be infinitely divisible, it is known by experience to be of 

 extreme tenuity at very small heights. The barometer rises 

 in proportion to the super-incumbent pressure. At the level 

 of the sea in the latitude of 45 and at the temperature of 

 melting ice, the mean height of the barometer being 29'922 

 inches, the density of the air is to the density of a similar vo- 

 lume of mercury as 1 to 10477'9. Consequently the height of 

 the atmosphere supposed to be of uniform density would be 

 about 4'95 miles. But, as the density decreases upwards in 

 geometrical progression, it is considerably higher, probably 

 about fifty miles ; at that height it must be of extreme tenuity, 

 for the decrease in density is so rapid that three-fourths of 

 all the air contained in the atmosphere is within four miles 

 of the earth ; and, as its superficial extent is 200 millions of 

 square miles, its relative thickness is less than that of a heet 

 of paper when compared with its breadth. The air even on 



