the late Dr. Joule's Thermometers. 491 



thin brass served to close the window partially so that there 

 was only very little, if any, circulation of water between the 

 inside and the outside of the box. 



The thermometers were read by a small microscope M 

 which could be moved parallel to itself along the upper edges 

 of the outer vessel. The microscope was mounted so that it 

 also had a free motion at right angles to the length of the 

 box. This double motion allowed it to .be moved quickly 

 above the ends of the threads of any two thermometers to be 

 compared. Latterly two microscopes were used, one for each 

 of the thermometers. The water was covered by a sheet of 

 glass, which kept the surface calm in spite of the disturbance 

 set up by the stirring. The thermometers were- supported in 

 triangular grooves cut into two adjustable brass uprights P, P. 

 Care was always taken to set them horizontally by first 

 placing a straight edge across the uprights and levelling. 

 This horizontal position is not necessary when the thermo- 

 meters are transparent, so that their divisions can be read 

 either from the front or from the back, as errors of parallax 

 are thus eliminated. But when this cannot be done the 

 reading microscope must be placed at right angles to the 

 thermometer, and then it is most convenient to have one ver- 

 tical and the other horizontal. The optic axis of M was put 

 into the vertical position by keeping at a proper distance in 

 the bath a horizontal glass-scale silvered at the back. When 

 the adjustment is correct the two images of the division 

 which is in the centre of the field of view should cover each 

 other, otherwise there is parallax. As the object of the ad- 

 justment is to avoid parallax in the reading of the thermo- 

 meters, this method answers very well if the glass-scale is 

 about 2 millini thick. It is instructive to notice how great 

 the danger of error due to parallax is when sufficient care is 

 not taken to read only in the centre of the field of view. 



The Method of Comparison and Reduction. 



Two observers were found necessary to carry out a satis- 

 factory comparison, one calling out the time at regular in- 

 tervals and taking the notes, the other reading the thermometer. 

 An example will show the method adopted. 



On June 20, 1893, a comparison was made between Joule A 

 and a Tonnelot Standard No. 4929 . Both thermometers were 

 kept in the bath at a temperature of 18° for several hours, 

 then their freezing-point was determined and found as 

 follows : — 



Tonnelot, No. 4929 : 0'0082 (mean of 4 observations). 

 Joule A : 23*23 ( „ g ). 



