234 THE BAROMETER. 



height of the column are seen to take place, which have 

 a range of about 50 mm . If the atmosphere were not 

 subject to violent movements, the height of the baro- 

 meter .at the same place would be pretty constant, and 

 it would be always the same if the atmosphere were 

 perfectly at rest. But causes, which will be considered 

 further on in the chapter on HEAT, produce great 

 atmospheric currents, gales, and winds; these sweep 

 over vast areas and disturb the equilibrium of the 

 atmosphere: hence arise the continual fluctuations of 

 the atmospheric pressure. 



The tube may be prepared from a piece of glass tubing, somewhat 

 more than 80 cm long, which is drawn out into a short point near 

 one end ; the point is then closed and rounded off over the spirit- 

 flame with the blow-pipe, turning the tube constantly between the 

 fingers of the left hand, in order to prevent the tube being melted 

 at one side more than at another. The heated end is apt to 

 crack, especially when the glass is thick, unless it be very slowly 

 cooled ; this is done by keeping the end in the flame for some time, 

 turning it constantly, and raising it gradually higher and higher 

 above the flame until it is nearly cooled. The sharp edge of the 

 open end should be rounded off in the spirit-flame. A very 

 clean and dry tube should be selected, as it is difficult to clean it 

 afterwards, when one end is closed ; a tube open at both ends may 

 be cleaned by passing through it a piece of twine (if necessary with 

 the help of a long wire) and tying to the end of the string a small 

 piece of linen ; the latter is then repeatedly drawn through the tube. 

 As in every flame vapour of water is formed, some moisture will 

 generally enter the tube while the open end is held in the spirit-flame. 

 The tube should be dried in the following manner. Heat the baro- 

 meter tube by moving it several times to and fro over the spirit- 

 lamp, or by holding it in a horizontal position before a large kitchen 

 fire ; introduce into it nearly up to the closed end a very narrow glass 

 tube, 85 or 90 cm long, and suck at the projecting end of the narrow 

 tube ; a current of air is thus passed through the barometer tube 

 which removes the moisture. 



Air-bubbles are apt to remain between the glass and the mer- 

 cury, when the latter is poured in ; they rise gradually to the top 

 after the tube has been inverted, and in virtue of the expansive 



