Fluid Pressure, \ \ "'(] \.< \\\ 1Q), 



flight of stairs. On this account, and because of their 

 ready portability, they are much used in determining 

 the heights of mountains or of balloon ascents. When 

 used for this purpose, a scale of heights is frequently 

 found upon them, in addition to the ordinary scale of 

 pressures. A decrease in pressure of 1" of mercury is 

 approximately equivalent to an increase of altitude of 

 about 900 feet. 



The dials of aneroid and hall barometers are fre- 

 quently marked at each half-inch, beginning at 28 

 inches, with the words Stormy, Much rain, Rain, 

 Change, Fair, and Set fair. The weather predictions 

 thus afforded are at best only approximate. They are 

 the result of long series of observations upon the 

 behaviour of the barometer during spells of weather of 

 various kinds. Thus it has been noted that, in our 

 latitudes, the barometer usually falls when the wind is 

 in the S.W., the quarter from which most of the rain 

 which falls in this country may be expected. On the 

 contrary, a N.E. wind in general causes a rise of 

 the barometer, accompanied by fine weather. 



In a later chapter, the question of the quantity of 

 water-vapour present in the air will be further dealt 

 with. For the present, it may be mentioned that 

 aqueous vapour is less dense than air and a column 

 of air containing much vapour is lighter, and conse- 

 quently exerts less pressure, than a column of drier air 

 of equal height. Warm air is also less dense than cold 

 air, and it is commonly the case in this country that 

 damp air is warmer than dry. Hence the barometer 

 registers lower pressures when moisture is contained in 

 large proportion in the air, and this is an atmospheric 

 condition conducive to the production of rain. 



It must be remembered that the height of the place 



