8cr. XV. BAROMETRICAL MEASUREMENTS. 113 



decreases as the height above the surface of the earth 

 increases. And it appears from recent investigations 

 that the mean temperature of space is 58 below the 

 zero point of Fahrenheit, which would probably be the 

 temperature of the surface of the earth also were it 

 not for the non-conducting power of the air, whence it 

 is enabled to retain the heat of the sun's rays, which 

 the earth imbibes and radiates in all directions. The 

 decrease in heat is very irregular ; each authority gives 

 a different estimate : probably because the decrease 

 varies with the latitude as well as the height, and some- 

 thing is due also to local circumstances. But from the 

 mean of five different statements, it seems to be about 

 one degree for every 334 feet, which is the cause of the 

 severe cold and eternal snows on the summits of the 

 Alpine chains. Of the various methods of computing 

 heights from barometrical measurements, that of Mr. 

 Ivory has the advantage of combining accuracy with the 

 greatest simplicity. Indeed the accuracy with which 

 the heights of mountains can be obtained by this method 

 is very remarkable. Captain Smyth, R.N., and Sir 

 John Herschel measured the height of Etna by the 

 barometer without any communication ^and hi 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 atmos- 

 phere, 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 as- 

 certaining then* heights. It is very simple, as Professor 

 Forbes has ascertained that the temperature of the boil- 

 ing 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 divisi- 

 ble or not. On the latter hypothesis it must really be 

 finite, and even if its particles be infinitely divisible it is 

 8 IL2 



