1879.] the Measurement of a Body under Strain. 159 



breadth of the bar respectively, — ~, I measure this ratio as 



xv 3 



follows 



Fig. 1. 



On one side of a rectangular bar, fig. 1, of the substance for which 

 j\i is to be measured, and about the middle of its length, a circle is 

 described of diameter nearly equal to the breadth of the bar. 

 Diameters are drawn in this circle parallel and perpendicular to the 

 length of the bar, and at their extremities fine holes are drilled 

 normal to the surface. (The dimensions of these holes are so small 

 compared to those of the bar, that they do not sensibly affect the 

 flexure.) Four fine steel wires of equal length are planted firmly in 

 these holes, and their free ends are then bent together so that they 

 may all be in the field of a microscope at the same time. The bar is 

 then placed in a frame of which fig. 2 is a section. 



Fig. 2. 



AA' BB' are pairs of rollers between which the bar is held. 

 A and A' are mounted on the outer frame aa', BB' on the frame bb' 

 which can be advanced towards AA' by the screw D ; this screw has 



