478 Prof. A. Schuster on the Scale-Value of 



and I beg to offer him my best thanks for the opportunity 

 which he has given me of examining these interesting relics. 



The two thermometers which I had at my disposal were 

 those called A and D in Joule's published papers ; they were 

 made and calibrated by Dancer in 1844. The thermometer 

 D includes both the freezing- and boiling-point, while A only 

 reaches to a little above 30° C. 



In order to show to what extent Joule trusted these instru- 

 ments. I quote the passages in his papers in which he refers 

 to them : — 



" My thermometers were constructed by a method very 

 similar to that employed by Regnault and Pierre. The 

 calibre of the tube was first measured in every part by 

 passing a short column of mercury along it. The surface of 

 the glass having then been covered with a thin film of bees- 

 wax, the portions of tube previously measured were each 

 divided into the same number of parts by a machine con- 

 structed for the purpose. The divisions were then etched by 

 means of the vapour of fluoric acid. Two thermometers 

 were employed in the present research, in one of which the 



value of each space was j^-rj , in the other ^— of a degree 



Centigrade. A practised eye can easily estimate the tenth 

 part of each of these spaces ; consequently I could by these 

 thermometers observe a difference of temperature not greater 

 than 0°005/' (Phil. Mag. [4] vol. iii. p. 481 ; Collected 

 "Works, vol. i. p. 214.) 



'• The thermometers employed had their tubes calibrated 

 and graduated according to the method first indicated by 

 Regnault. Two of them, which I shall designate by A and JB, 

 were constructed by Mr. Dancer of Manchester ; the third, 

 designated by C, was made by M. Fastre of Paris. The 

 graduation of these instruments was so correct, that when 

 compared together their indications coincided to about yoo 

 of a degree Fahr. I also possessed another exact instrument 

 made by Mr. Dancer, the scale of which embraced both the 

 freezing- and boiling-points. The latter point in this standard 

 thermometer was obtained, in the usual manner, by immers- 

 ing the bulb and stem in the steam arising from a considerable 

 quantity of pure water in rapid ebullition. During the trial 

 the barometer stood at 29*94 inches, and the temperature of 

 the air was 50°, so that the observed point required very little 

 correction to reduce it to 0*760 metre and 0° C, the pressure 

 used in France, and I believe the Continent generally, for 

 determining the boiling-point, and which has been employed 

 by me on account of the number of accurate thermometrical 



