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THE ROYAL SOCIETY OF CANADA 



a thin layer of vacuum tap grease was placed on the flange before the 

 disc was put in position for the purpose of making an air-tight contact. 

 The flange had a width in each case of about 4 millimetres. 



It was expected at first that the presence of grease between the 

 flange and the disc would give erratic values of the scale readings, 

 and hence erratic values of w, but it was found that after the disc 

 had been subjected to a large pressure difference and then brought 



IN MM5 



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OF MERCURY 



back to its unstressed condition any subsequent cycle of variations of 

 p gave a w-p curve for decreasing p which was practically coincident 

 with the w-p curve for increasing p. A typical example is shown in 

 the case of curve 4 on chart 1. The slightly larger value of w on 

 descent as compared with the value of w for the same value of p on 

 ascent may be due to one or both of two causes, viz., back lash and 

 elastic fatigue of the steel. If the discrepancy is due to elastic fatigue 



