PNEUMATICS. 



171 



Why should 

 t'le presence of 

 c indensed va- 

 por of water in 

 t!i3 atmos- 

 phere affect its 

 pressure ? 



naturally attributed to the variation in the weight or pres- ] 

 sure of the incumbent atmosphere, arising from various me- 

 teorological causes. 



Thus, when the air is moist or filled with vapors, it is lighter 

 than usual, and the column of mercury stands low in the ^ 

 tube ; but when the air is dry and free from vapor, it is heavier, 

 and supports a longer column of mercury. 



So long as the vapor of water exists in the 

 atmosphere, as a constituent part of it, it con- 

 tributes to the atmospheric pressure, and tlius 

 a portion of the column of mercury in the ba- 

 rometer tube is sustamed by the weight of the 

 vapor ; but when the vapor is condensed, and 

 takes on a visible form, as clouds, etc., then it no longer 

 forms a constituent part of the atmospliere, any more than dust, 

 smoke, or a balloon floating in it does, and the atmospheric 

 pressure being diminished, tlie mercury in the tube falls. In 

 this way the barometer, by showing variations in the weight 

 of the air, indicates also tlie changes in the weather. 



380. The space above the mercury in the 

 barometer tube, A D, Fig. 159, is called the 

 Tmricellian vacuum, and is the nearest approach to a perfect 

 vacuum that can be procured by art; for upon pressing the 

 lower end deeper in the mercury, the 

 whole tube becomes completely filled ; the fluid again 

 falling upon elevating the tube, it is therefore a per- 

 fect vacuum, with the exception of a small portion 

 of mercurial vapor. 



381. Barometers are constructed in very different 

 forms — the principle remaining the same, of course, in 

 all. The first barometer constructed was simply a tube 

 closed at one end, fiUed with mercur3-, and' inverted 

 in a vessel containing mercury, as in Fig. 159. 



__ ^ . ,^ A very common form of barometer, 



What IS the ■' 



construction of called the "Wheel-Barometer, con- 



^^rome'twr'" ^^*^ °^ ^ ^^^^^ ^^^' ^^^^ ^^ ^^® ^°*' 

 torn, and filled with mercury. (See 



Fig. 160.) The column of mercury in the long arm 

 of the tube is sustained by the pressure of the atmos- 

 phere upon the surface of the mercury in the shorter 

 arm, tlie end of which is open. A small float of iron 

 or glass rests upon the mercury in the shorter arm of 

 the tube, and is suspended by a slender thread, which 

 is passed round a wheel carrying an index, or pointer. 

 As the level of the mercury is altered by a variation 

 of the pressure of the atmosphere, the float resting 



What is the 

 most perfect 

 v.icuuni with 

 which we are 

 acquainted ? 



Fig. IGO. 



