100 Domestic Science 



Exercise for Student. 

 Why is it necessary to have two scales on which to take readings ? 



60. In the more elaborate instruments, such as 

 Fortin's standard barometer, arrangements are ma^de 

 whereby the variations of level of the surface of the 

 mercury exposed to the air, due to the changes in the 

 atmospheric pressures, are compensated for by bringing 

 this surface back to a definite position before readings 

 are taken. Thus only one scale is required situated 

 beside the upper level of the mercury. 



Hall barometers are commonly of the U-tube type, 

 but are provided with simple mechanism, actuated by 

 the fluctuations in level of the mercury in the open 

 limb, whereby a long pointer is made to travel round a 

 circular scale, graduated in inches and decimal fractions 

 of an inch. By this means readings of the height of 

 the mercury column may be made directly from the 

 circular scale. 



61. A barometer of quite a different type is the 

 " Aneroid ". In this form the variations of air-pressure 

 are utilised to alter the volume of a box made of metal, 

 with a thin corrugated lid. The air having been 

 removed from the metal box, the lid is kept from 

 collapsing by a strong spring ; the movements of the 

 lid of the box inwards or outwards, corresponding to 

 increase or diminution in the pressure of the atmo- 

 sphere, cause the end of a long pointer to travel round 

 a circular scale similar to that described above. Since 

 the movements of the lid are extremely small, a system 

 of multiplying levers is made use of to render them 

 readily measurable. These instruments are made of 

 such delicacy as to show distinctly the difference of 

 pressure due to carrying the barometer up a short 



