DE. EVEEETT ON THE EIGIDITT OF GLASS. 187 



After a great number of preliminary experiments, in which improvements were gra- 

 dually introduced in the apparatus and mode of observation, the final observations were 

 made in the manner above described. They consisted of a set of flexure observations 

 with the rod turned into ten different positions, differing from one another by tenths of 

 a revolution, and of a set of torsion observations with the arms alternately fixed. The 

 fixing of one arm was necessary to the attainment of steadiness in torsion observations, 

 as without it the apparatus would have oscillated with a see-saw motion, and was 

 effected by inserting a flat piece of wood, W (fig. 3), underneath, and laying a weight 

 (not shown in fig. 3) on the top of the arm. The arrangement will be better understood 

 from an inspection of fig. 6, where X represents the weight laid on the arm. As the 

 experiments with right arm fixed and left arm fixed were equally numerous, the mean 

 result is free from errors arising from want of perfect symmetry in the two arms. 



The weights employed were the same for torsion as for flexure. They were lead 

 weights of 50 and 100 grammes, and were accurately tested. Every set of observa- 

 tions was symmetrical with respect to the zero or unstrained condition of the rod, con- 

 sisting either of five observations in the order 0, 50 grms., 100 grms., 50 grms., 0, or of 

 five observations in the order 100 grms., 50 grms., 0, 60 grms., 100 grms. 



The following were the numerical determinations obtained : — 



Torsion. 



Av/ioiuix. centimetres. 



Mean separation of images per gramme •053185 



Mean distance of nairrors from screen 277*3 



Flexure, 



Mean separation of images per gramme in ten equidifferent 

 positions, -04175, -04185, -04069, -04160, -04112, -04243, 

 -04062, -04100, -04132, -04203, giving a gross mean of . -04144 



Mean distance of mirrors from screen 271*8 



For both Torsion and Flexure. 



Length of glass rod between clamps 28*0 



Arm of couple 31-4 



As a specimen of the amount of consistency between different readings, as well as of 

 the method employed in their reduction, I will here transcribe the last set of flexure 

 observations — last in order of time, but sixth in order of position. The first column 

 contains the weight (in grammes) acting at each arm ; the second, the distance of the 

 images; the third, the amount of separation or approach of the images as compared 

 with the zero or unweighted distance. The mean separation or approach per gramme 

 is computed by dividing the sum of the numbers in the third column by the sum of 

 those in the first. 



2d2 



