222 SCIENTIFIC APPARATUS. 



sea thermometers, the bulbs being enclosed in an outer envelope 

 of glass, and the space between the bulbs being partially filled 

 with a liquid (mercury in Negretti's, spirit in Miller's) in order to 

 yield when the thermometer is exposed to severe pressure at great 

 depths, and prevent the compression of the inner bulb, which 

 would alter the indications. 



Maximum thermometers, in this country, are usually made either 

 on Phillips's or Negretti's plan. In the former, the index is a 

 small portion of the mercurial column, separated from it by a 

 minute air bubble. In the latter, it is formed by the column 

 itself. The tube is artificially contracted just outside the bulb, so 

 that the mercury, if once forced out by the action of heat, cannot, 

 by its own cohesion, make its way past the contraction and re- 

 enter the bulb. 



Of minimum thermometers, the most commonly employed form 

 is that of Rutherford, which is a spirit thermometer. The index 

 is metallic, and is enveloped in the liquid. When this recedes it 

 draws the index with it, but when the liquid advances it flows past 

 the index, which therefore marks the lowest temperature reached. 



Casella's minimum is a mercurial thermometer provided with a 

 lateral chamber. When the instrument is once set, this chamber 

 being empty, if the temperature rises the mercury passes more 

 easily into the chamber than out along the tube; while if the 

 temperature falls the mercury returns into the bulb of the instru- 

 ment. The end of the thread of mercury in the tube, therefore, 

 marks the lowest temperature to which the thermometer has been 

 exposed. 



Thermometers are made self-recording either by photography 

 or electricity. 



The former method may be applied in various ways. At Kew 

 the column of mercury is broken by an air speck, and the position 

 of this speck is photographed. At Greenwich the principle is 

 somewhat similar to that of the Kew barograph ; the unoccupied 

 space of the thermometer tube is photographed. 



