THE THERMOMETER. 



a knob at each end, which allows the alcohol to pass it freely 

 from right to left. Hut when the alcohol contracts and moves 



towards the bulb, it carries with it the glass index, which conse- 

 quently remains in the position given to it when exposed to the 

 lowest temperature. 



The instrument, therefore, after the lapse of any time, indi- 

 cates the highest and lowest temperatures to which it has been 

 exposed. A small magnet is sometimes attached, to aid in bring- 

 ing the wire gently into contact again with the mercury, and in 

 cheap instruments, especially in England, the scale is engraved 

 on a slab of wood instead of a plate of glass. 



A maximum and minimum thermometer has lately been intro- 

 duced by Messrs. JSTegretti and Zambra, which is a modification 

 of the above. Having introduced into the tube a little rod of 

 glass, the tube is softened with the blow-pipe, and slightly bent 

 where the glass rod stands, so that it becomes fixed in the tube, 

 leaving nevertheless sufficient space around it for the mercury to 

 pass. Supposing, then, the instrument to be suspended with the 

 tube horizontal, and exposed to an increasing temperature, the 

 mercury passes the bend ; but when the temperature falls, the mer- 

 cury which has just passed will not return. The extremity of the 

 column will therefore indicate the highest temperature to which 

 the instrument has been exposed. This instrument is represented 

 in fig. 8. 



34. Alcohol is frequently used as a thermoscopic liquid. It has 

 the advantage of being applicable to a range of temperature 

 below the freezing point of mercury ; no degree of cold yet ob- 

 served in nature or attained by artificial processes having frozen 

 it. It is usually coloured so as to render the column easily 

 observable in the tube. 



35. Atmospheric air is a good thermoscopic fluid. It has the 

 158 



