ACOUSTICS AND GRAVITATION. 



143 



viscous deformation, due to residual torsional strain. Figure 180 is an example, 

 observations being taken several times a day, for as many days as the essential 

 narrowness of the case permitted. 



Apart from the daily kinks, the curve shows steady progress at a rate of 

 about 0.08 of torsion per day. The wire, in other words, developes torque at 

 the rate of about 0.21 dyne cm. per day, wholly as the result of the breakdown 

 of unstable molecular groups which promote viscous deformation. How the 

 wire acquired this concealed torsional strain is hard to understand. It could 

 not have developed out of the intense tractional strain which it carries, for 

 there would be no reason to account for the particular direction of rotation. 

 The wire had been reeled on a drum with a diameter of about 8 inches ; it is 

 probable, therefore, that it was reeled helically, or in a way to impart a perma- 

 nent twist, allowing the intense tensile strain to develop a torsional strain in 

 the lapse of time. The free wire resting on a plane assumed a screw shape 



44 



with a pitch of about 2 feet. Under the heavy weight M, the steel wire is at 

 relatively low viscosity; for this implies accentuated continuous breakdown 

 of molecular groups, and therefore greater facility for the exit of the set tor- 

 sional strain. 



The reason to be given for the kinks in the curve is more uncertain. The 

 telescope and scale were attached to one of the ground walls in the basement of 

 the building; but this does not imply complete freedom from thermal and other 

 discrepancies. Again, interferences at the suspension, induced by tremors in 

 the building, might put the needle in slow vibration. Finally, the possibility 

 of radiant forces, from the closely fitting case surrounding the needle and from 

 the lead weight, can not at once be dismissed, without measurement. Prob- 

 ably all these factors enter, and the average discrepancy is about as large as 

 the gravitational attraction (on the side) to be measured. Unless immense 

 masses of lead are used on the outside, the experiment has little to recom- 

 mend it, even though it is here performed under the unfavorable conditions 

 of a steam-heated room. With regard to the graph, figure 180, it appears 

 that the increase of irregularities (which become more marked as the room 

 is more vigorously heated) is accompanied by correspondingly increased 



