Fluid Pressure 



99 



example. This consists of a long glass tube bent round on 

 itself so that one limb of the tube is much longer than 

 the other. The longer limb is closed and is nearly full 

 of mercury, which also occupies a portion of the shorter 

 limb, as shown in the accompanying figure. The tube 

 is filled with mercury in such a way that the space 

 above the liquid in the long limb is empty, as is also 

 the case with the Torricellian form of 

 the instrument, this empty space being 

 generally known as the " Torricellian 

 vacuum ". Thus the surface of the 

 mercury in the long arm of the baro- 

 meter is under no pressure save that 

 of the exceedingly small quantity of 

 mercury vapour which volatilises into 

 the space at the top of the tube. On 

 the other hand, the free surface of the 

 mercury in the short limb is subject to 

 the pressure of the atmosphere. The 

 distance between the levels of the liquid 

 measures the magnitude of the air-pres- 

 sure and is found to vary from day to 

 day. To enable this distance to be 

 measured, a scale of centimetres and 

 millimetres is etched on the short limb 

 and on the upper part of the long one. Fig 30 

 Both scales are numbered from a zero 

 point about midway between them, the numbers on 

 the scale on the long arm reading upwards and those 

 on the other arm downwards. Thus the height of the 

 mercury column supported by the pressure of the 

 atmosphere is the sum of the readings taken at the 

 levels of the mercury. 



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