ON THE ABSOLUTE EXPANSION OF MERCURY. 13 



curves also served as a convenient means of graphic interpolation for deducing the 

 simultaneous values of the observed quantities. 



Since all the readings could not be made simultaneously, attention was directed to 

 obtaining them in quick and regular succession. The bridge was provided with 

 mercury cup connections in place of the usual plugs, giving great improvement 

 in quickness as well as in accuracy. The thermometers could be interchanged 

 instantaneously by means of a mercury cup switch, without introducing any variable 

 contact errors such as would have been unavoidable with screw connections. The 

 telescopes for reading the mercury levels contained eyepiece micrometers divided to 

 tenths of a millimetre, fitted with vertical screw adjustments, and were focussed and 

 adjusted so that the millimetre divisions coincided with those of the standard invar 

 scale situated between the mercury columns. By siiitably shading and illuminating 

 the mercury columns, readings could easily be taken by inspection to O'Ol mm. 

 A separate handle was provided for turning the vertical column carrying the 

 telescopes through an angle of 2 degrees either way to verify the adjustment of the 

 eyepiece micrometers on the invar scale. This method appeared greatly preferable 

 in practice to the use of a filar micrometer, since it was never necessary to touch the 

 telescopes when once they had been adjusted for a run. With a little practice, 

 a single observer could take all the readings, and perform all the necessary adjust- 

 ments, without any excessive haste or exertion. 



In reducing the observations, points were selected on the curves both with a rising 

 and falling temperature difference, where the temperature conditions appeared to be 

 most favourable. Points taken on the same day at nearly the same temperature 

 always agreed so closely on reduction, that it was considered preferable to take 

 short runs of about one hour each at two or three different temperatures on the same 

 day rather than runs of long duration at one temperature. Runs taken at the same 

 temperature on different days, separated often by many months, when all the 

 conditions of observation were completely changed, afforded a much better test of the 

 accuracy of the method, and were more likely to serve for the elimination of constant 

 or accidental erroi-s than runs taken under constant conditions. Observations given 

 in the tables under the same date at the same temperature were taken with a rising 

 and falling temperature respectively, or otherwise differed materially in the conditions 

 under which they were taken. 



G. Method of Reducing (lie Observations. 



The following example, showing the reduction of a single observation, will serve to 

 illustrate the order of magnitude of the corrections involved. The corrections were 

 worked to one figure beyond the limit of accuracy of reading, except that, in the 

 case of the platinum thermometers reading to 0'002 C., it was considered useless 

 to express the temperatures beyond 0'001 C., as this represented an order of accuracy 



