THE BAROMETER. 



53 



Fig: 50. Sy 

 phon barometer. 



of mercury; the mercury will rise to a certain height (30 inches), and no 



higher in vacuo. When the pressure of the air increases the mercury rises 



a little, and falls when the pressure is removed. Air charged 



with aqueous vapour is lighter than dry air, so a fall in the 



mercury indicates a certain amount of water-vapour in the air, 



which may condense and become rain. The action of mercury 



is therefore used as a weather-glass, by which an index-point 



shows the movements of the fluid, by means of a wheel over 



which a thread passes, sustaining a float and a counterpoise. 



When the mercury rises the float goes up, and the weight falls, 



and turns the wheel by means of the thread. The wheel having 



a pointer on the dial tells us how the mercury moves. This 



weatJier-glass is the usual syphon barometer with the float on the 



surface and a weight (fig. 50). 



The Syphon Barometer is a bent 

 tube like the one already shown, with 

 one limb much shorter than the other. 



The Aneroid Barometer, so called 

 because it is " without moisture," is now 

 in common use. In these instruments 

 the atmospheric pressure is held in 

 equilibrium by an elastic metal spring 

 or tube. A metal box, or tube, is freed 

 from air, and then hermetically sealed. 

 This box has a flexible side, the elas- 

 ticity of which, and the pressure of the 

 air on it, keep each other in equili- 

 brium. Upon this elastic side the 

 short arm of a lever is pressed, while 

 the longer arm works an index-point, j 

 as in the circular barometer. When 

 pressure increases the elastic box "gives"; 

 when pressure diminishes it returns to 

 its former place, and the index moves in 

 the opposite direction. It is necessary 

 to compare and " set " the aneroid with 

 the mercurial barometer to ensure cor- 

 rectness. A curved tube is sometimes 

 used, which coils and uncoils like a spring, 

 according to the pressure on it 



There are other barometers, such as 

 the Water Barometer, which can be fixed 

 against the side of a house, and if the 

 water be coloured, it will prove a useful 

 indicator. As the name indicates, water Flg . SI ._ The Water lisuomlgft 



