REVERSING THERMOMETERS 47 



The calculation is made similarly for another glass 

 and " Volume at 0°." 



The Negretti and Zambra instrument is not pro- 

 vided with an auxiliary thermometer, and the glass 

 used is not marked on the stem. It is hardly worth 

 while to attempt to carry the correction to an extreme 

 degree of accuracy, as, even in the most closely 

 divided thermometers by this firm, the gradua- 

 tions are somewhat coarse, and the writer prefers 

 to determine the correction directly by reversing 

 at a known temperature, and observing the altera- 

 tion on bringing it into water of a higher or lower 

 temperature (but see Addenda, p. xvii.). 



It will be seen from what has been said above 

 that the reversing thermometers are unreliable at 

 the best, and they should always be used in pairs 

 to check one another. Their behaviour is very 

 capricious ; an instrument which has worked well for 

 years may suddenly, for no apparent reason, give 

 readings which are incorrect by several degrees, and 

 then, after an interval, may again act satisfactorily. 

 When an error of registering does occur it is generally 

 so large as to be obvious. 



In the author's opinion the Richter thermometer is 

 the better of the two models on account of its much 

 finer and closer graduation, and the ease with which 

 the correction is calculated. On the other hand, it 

 is larger, heavier, and more costly than the Negretti 

 and Zambra model. 



In spite of all its defects the reversing thermometer 

 is the only instrument available for depths greater 

 than from 700 metres to 1,000 metres. 



Reversing thermometers may be mounted either in 

 a simple reversing frame or as part of a reversing 

 water-bottle. 



