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IX, Account of Experiments on the Flexural and Torsional Rigidity of a Glass Rod, 

 leading to the Determination of the Rigidity of Glass. By Joseph D. Eveeett, 

 B.C.L., Assistant to the Professor of Mathematics in the University of Glasgow. 

 Communicated by Professor W. Thomson, F.R.S. 



Received February 1, — Read February 22, 1866. 



These experiments were conducted in the Physical Laboratory of Glasgow University 

 during the summer vacation of 1865, upon a plan devised by Professor W. Thomson, 

 which may be briefly described as follows : — 



A cylindrical rod of glass is subjected to a bending couple of known moment, applied 

 near its ends. The amount of bending produced in the central portion of the rod is 

 measured by means of two mirrors, rigidly attached to the rod at distances of several 

 diameters from each end, which form by reflexion upon a screen two images of a fine 

 wire placed in front of a lamp-flame. The separation or approach of these two images, 

 which takes place on applying the bending couple, serves to determine the amount of 

 flexure. 



In like manner, when a twisting couple is applied, the separation or approach of the 

 images serves to determine the amount of torsion. 



The following are the details of the arrangement. 



A B (Plate XVI. fig. 2) is the glass rod, firmly held at both ends in the brass sockets 

 A, B which form the extremities of the hollow brass arms AC, B D. Each of these 

 arms is furnished with two pairs of sharply pointed cones, exactly opposite to each other 

 at E, C, F, D, of which those at E, F serve as feet for supporting the apparatus, while 

 those at C and D support the weights used for producing flexure and torsion. The two 

 distances EC, FD are exactly equal. There are joints in the arms at E, F, the axes of 

 the joints being the same as those of the cones ; and for torsion experiments these are 

 turned to a right angle on opposite sides, as shown in bird's eye view at fig. 3, where the 

 same letters denote the same parts as in fig. 2. In flexure experiments the arms are in 

 the same straight line with the glass rod, as shown in fig. 2. In both arrangements it 

 is obvious that when equal weights are hung at C and D, their effect upon the rod is 

 equivalent to a couple whose moment is the force of gravity on one of the weights mul- 

 tiplied by one of the equal distances EC, F D. In fig. 2 this couple bends the rod 

 without twisting it, and in fig. 3 twists without bending it. 



In the final series of experiments the apparatus was made to rest on three feet instead 

 of two ; one of the points, as F, being replaced by two points about an inch apart, lying 

 in a line at right angles to that joining EF. 



MDCCCLXVI. 2 D 



