Kinetic Theory of Solids. 33 



the box according to the length of the wire. It was con- 

 venient to have the vibrator swinging outside when it was 

 necessary to largely increase the moment of inertia of the 

 vibrator. To heat the chamber I used as a preliminary ex- 

 pedient a gas-flame burning inside it at one of the bottom 

 corners with vent-holes so distributed as to prevent the con- 

 centration of a stream of hot air on any one point of the 

 wire. The mean temperature of the chamber could be regu- 

 lated by the size of the flame, and was measured by a 

 thermometer the bulb of which was almost in contact with 

 the middle point of the wire. Although this was intended 

 to give only rough preliminary measurements, it worked so 

 well and was so simple and handy that I retained it through- 

 out. It was tested repeatedly in the following manner : — a 

 gas-flame was turned on sufficient to maintain a constant 

 temperature of say 70° C, as recorded by the thermometer, 

 and the period of vibration of the wire under experiment was 

 observed. The flame was then increased till a temperature of 

 105° C. was reached, when the flame was turned off, and the 

 vent-holes closed ; the temperature of the chamber then fell 

 slowly and the period of vibration of the wire was again 

 observed while the temperature fell from 71° to 69° ; the 

 difference in the two values of the period never amounted to 

 more than what would correspond to an uncertainty of 2° 

 in the reading of the temperature, and in most cases was 

 actually inappreciable. This was a degree of accuracy quite 

 sufficient for the purpose in view. 



In determining the period of vibration of wires with such 

 molecular viscosity as those of lead, tin, and zinc, there is this 

 difficulty, that when vibration is started it tends to die away 

 after a very few swings, so that accuracy in the measurement 

 of the time of a single swing is not possible without some 

 method of periodically reinforcing the swing. I found the 

 simplest and best plan of doing this to be the application of a 

 slight impulsive couple with the hands to the vibrator at the 

 middle of every nth swing. With a little practice it was soon 

 possible to make a large number of measurements of the 

 period of a lead wire not differing amongst themselves by 

 more than 1 in 300. Each determination of a period of 

 vibration was made by observing with the seconds hand of a 

 watch the time of between 100 and 300 swings. 



The rigidity is calculated from the experimental data by 

 the formula 



_ 27T 3 izy 



Phil. Mag. S. 5. Vol. 32. No. 194. July 1891. D 



