888 Mr. I. Williams on a 



rotations of the lines AG and BD (fig. 2) and partly to the 

 rise of both mirrors with increase of load. From the dis- 

 placement due to the rotation, we can calculate the angles 

 through which the lines AG and BD have turned in the 

 following way : — 



Fig. 3. 



Crr^ 



B 



Let L = distance between the knife-edges, 

 B = breadth of the bar, 



S = distance of the longitudinal scale from its 

 mirror, 



Si= distance of transverse scale from its mirror, 

 D = deflexion on the longitudinal scale, 

 D 2 = deflexion on the transverse scale. 



d- L - D 



Then 







R S L 



D 



-k Dl 





Although the transverse scale was placed at a distance of 

 about four feet from the mirror, the deflexion per kilogram 

 on this scale was only 2 mm. when the corresponding de- 

 flexion on the other scale, due to the longitudinal curvature, 

 was about 20 mm. Using this method I obtained eight 

 values of a lying between # 265 and '280, the mean value 

 being "273. 



Since the deflexion corresponding to the transverse 

 curvature is so small, a very small error in reading it pro- 

 duces a serious effect on the result. For a higher degree of 

 accuracy to be attained it is clear that the transverse de- 

 flexion must be increased. In the second method which I 

 employed this was secured, and the two deflexions were of 

 the same order of magnitude. 



