92 PHYSIOGRAPHY. [chap. 



Various forms have been given to the barometer, but 

 with the exception of the aneroid'^ — an entirely distinct 

 instrument — they all depend on the same principle, namely, 

 that of causing a column of liquid to be balanced against 

 the weight of the atmosphere. Almost any liquid may be 

 employed, but as a matter of convenience, mercury is the 

 only substance in common use.^ 



As the pressure of the atmosphere in any given locality 

 varies from day to day, and even from hour to hour, the 

 height of the mercurial column is subject to corresponding 

 fluctuation. The great use of the barometer, in fact, is to 

 indicate these changes of atmospheric pressure — changes 

 which are of vast importance to the meteorologist, since they 

 are related to general changes in the weather. Not that the 

 barometer forms a " weather-glass " as popularly understood : 

 it does not indicate absolutely the character of the forth- 

 coming weather, and the indications given on the dial of 

 common instruments are scarcely of any scientific value. 

 But still the changes in atmospheric pressure point to 

 changes in the winds ; and these are the prime movers in 

 effecting changes in our weather. Hence, the readings of 

 the barometer form the chief element in the weather-charts 

 and reports issued of late by most of the London daily 

 papers, and as these are forced every morning upon the 



^ Aneroid, from the privative c?, and vr\ph%, mros, moist ; an instrument 

 in wliich the pressure of the atmosphere acts upon a thin elastic metal 

 case, the movements of which are transmitted to a dial. 



* Water-barometers have occasionally been constructed, but their 

 great length renders them unwieldy, and they are also open to other 

 objections. One of these instruments may be seen in the Museum of 

 Practical Geology in Jerm)'n Street. Glycerine has also been used by 

 Mr. J. B. Jordan, as may be seen in an instrument erected by him at 

 South Kensington, But for all ordinary purposes mercury is invariably 

 used. 



