88 Dr. Schuster on a 



some fixed or easily reproducible scale. The request which 

 I made to Mr. B. A. Joule to allow me the use for a short 

 time of his late father's thermometers was met at once with 

 a most ready compliance, for which I have to offer my best 

 thanks. 



Thermometers made of hard French glass, having a 

 definite chemical composition, have been investigated with 

 great detail by M. Guillaume, of the Bureau International, 

 and have been compared directly with the air thermometer 

 by M. Chappuis. 



I have thought it advisable, therefore, to compare the 

 scale value of Joule's instrument in terms of that of a 

 thermometer made of French standard glass. 



The two thermometers which I had at my disposal were 

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

 made and calibrated by Dancer in 1844. The thermometer 

 D contains both the freezing and boiling point, while A 

 only reaches up to about 30°C. 



All observations on the equivalent were made with A, 

 the scale value of which was determined by a comparison 

 with the standard D. One division of A was found in 

 mercury by Dr. Joule to correspond to o°o , j']2\a s . F., which 

 is equal to o°042897 C. 



To determine the relation between these thermometers 

 and those now in use, it seemed sufficient to confine the 

 investigation to the thermometer A. The thermometers 

 having been calibrated so that the divisions are intended to 

 represent equal volumes of the capillary tube, the distances 

 of the divisions may serve as a test of the equality of the 

 bore at different places. Near the freezing point the length 

 of 50 divisions is 4*3 cms. ; this length increases gradually 

 until at the other end of the scale 50 divisions occupy a 

 length of 5 '3 cms. ; the tube is therefore conical, the diameter 

 at the top being about ten per cent smaller than at the lower 

 end. A careful examination of successive lines on the stem 



