292 Prof. W. Thomson on the Elasticity [May 18, 



The same wire and same vibrator showed 



"'vibrations. 



Again the same wire with vibrator No. 2 * (time of vibration one way 

 1'236), showed in two trials 



Vibrations. Vibrations. 



Subsidence from 40 initial amplitude 1 ~. - 



to 20 ....................... / 



And continued from 20 to 10 .. ____ 90 90 



Again same wire and vibrator, 



From initial amplitude 20 to 10. . 103 vibrations (mean of eight trials). 

 This remarkable result suggested the question (?). 

 (rf) Only one comparison was made. It showed in a wire which was 

 kept vibrating nearly all day, from day to day, after several days, very 

 much more molecular friction than in another kept quiescent except 

 during each experiment. Thus two equal and similar pieces of wire were 

 put up about the 26th of April, hanging with equal and similar lead 

 weights, the tops and bottoms of the two wires being similarly fixed by 

 soldering. No. 2 was more frequently vibrated than No. 1 for a few 

 days at first, but no comparison of viscosities was made till May 15. Then 

 No. 1 subsided from 20 initial range to 10 in 97 vibrations. 

 No. 2, the same subsidence in 77 vibrations. 



During the greater part of May 16 and 17, No. 2 was kept vibrating, and 

 No. 1 quiescent, and late on May 17 experiments with the following results 

 were made : 



Time per 

 vibration. 

 No. 1. Subsided from 20 to 10 after 99 vibrations in 237 seconds . 2*4 



98 235 



98 235 



No. 2. Subsided from 20 to 10 after 58 142 



n 60 147 



57 139 



60 147 



.2-4 

 2-4 

 2-45 

 2-45 

 2'45 

 2-45 



[Addition, May 27, since the reading of the paper.] No. 1 has been 

 kept at rest from May 1 7, while No. 2 has been kept oscillating more or 

 less every day, till yesterday, May 26, when both were oscillated, with the 

 following results : 



Time per 

 vibration. 



No. 1. Subsided from 20 to 10 after 100 vibrations in 242 seconds 2-42 



2. 44 or 45 vibrations 2-495 



Moduli of Elasticity. A modulus of elasticity is the number by which 

 the amount of any specified stress, or component of a stress, must be divided 

 to find the strain, or any stated component of the strain, which it produces. 

 Thus the cubic compressibility of water being a-y^ny per atmosphere, its 

 "modulus of compressibility" or its "volume modulus of elasticity," is 

 21000 atmospheres, or 76 x 13-596 x 21000=21'7 X 10" grammes weight 



* Of same weight as No. 1, but different moment of inertia. 



