JOULE'S THERMOMETER. 8 1 



advanced so that the end of the column in one position 

 coincided with the beginning of the column in the next. 

 In each position the length of the column was ascertained 

 to the one four-thousandth part of an inch, by means of an 

 instrument invented for the purpose by Mr. Dancer."* 



" Afterwards the tube was covered with a film of bees- 

 wax, and each of the previously measured spaces was 

 divided into twenty equal parts by means of a steel point 

 carried by the dividing instrument ; it was then etched by 

 exposure to the vapour of fluoric acid. The scale thus 

 formed was entirely arbitrary ; and as it only extended 

 between 30 and 90 it was necessary to compare the 

 thermometer with another constructed in the same manner 

 but furnished with a scale including the boiling as well as 

 the freezing point. When this was done it was found that 

 the ten divisions of the sensible thermometer (occupying 

 about half an inch) were nearly equal to the degree of 

 Fahrenheit." Then comes the most remarkable statement : 

 " Since by practice I can easily estimate with the naked eye 

 one-twentieth of each of these divisions, I could with this 

 instrument determine temperatures to the one two-hundredth 

 part of a degree." 



Considering that a skill in thermometry previously 

 undreamt of, and never since surpassed, was the principal 

 means by which Joule finally attained the extreme accuracy 

 in his determination of the mechanical equivalent of heat, 

 the foregoing passages are not without interest, followed, as 

 they are, by the experimental determination of differences 



* " Of the firm Abraham and Dancer, Cross-street, Manchester. I have 

 great pleasure in acknowledging here the skill displayed by this gentleman in 

 the construction of the different parts of my apparatus ; to it I must, in a great 

 measure, attribute whatever success has attended the experiments detailed ira 

 this paper." 



