120 BAROMETRICAL MEASUREMENTS. SECT. XV. 



suited, the determination becomes more complicated. Mr. Ivory's 

 method of computing heights from barometrical measurements 

 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. 

 Admiral Smyth, E.N., and Sir John Herschel measured the 

 height of Etna by the barometer, without any communication 

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

 Sir John Herchel 10,873, the difference being only one foot. In 

 consequence of the diminished pressure of the atmosphere water 

 boils at a lower temperature on mountain 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 ascertained that the temperature of 

 the boiling point varies in arithmetical proportion with the 

 height, or 5495 feet for every degree of Fahrenheit, so that the 

 calculation of height becomes one of arithmetic only, without the 

 use of any table. 



The mean pressure of the atmosphere is not the same all over 

 the globe. It is less by P 24 of an inch at the equator than at 

 the tropics or in the higher latitudes, in consequence of the 

 ascent of heated air and vapour from the surface of the ocean. It 

 is less also on the shores of the Baltic Sea than it is in France, 

 and it was observed by Sir James C. Eoss that throughout the 

 whole of the Antarctic Ocean, from 68 to 74 S. latitude, and 

 from 8 to 7 W. longitude, there is a depression of the barometer 

 amounting to an inch and upwards, which is equivalent to an 

 elevation above the sea level of 800 feet. A similar depression 

 was observed by M. Erman in the sea of Ochotzk, and in the 

 adjacent continent of eastern Siberia. Sir John Herschel assigns 

 as the cause of these singular anomalies the great system of 

 circulation of the trade and antetrade winds, in both hemi- 

 spheres, reacting upon the general mass of the continents as 

 obstacles in their path, which is modified by the configuration of 

 the land. 



There are various periodic oscillations in the atmosphere, which, 

 rising and falling like waves in the sea, occasion corresponding 

 changes in the height of the barometer, but they differ as much 

 from the trade-winds, monsoons, and other currents, as the tides 

 of the sea do from the Gulf-stream and other oceanic rivers. 



