ORDINARY MEETINGS. XX1xX 
under considerable stress the wax near the ends of the glass tubes gave 
way, and the space thus left in the ends of the tubes had to be packed to 
make it certain that the portion of the cord actually subjected to tension 
and torsion was the portion outside the tubes. The observations under 
the greater stresses are thus considerably less trustworthy than the 
others. 
The weights used were square plates of sheet lead with an edge of 
four inches, having a small circular portion cut from the centre, and a 
slit from the centre to the edge to adnut of their being placed on the 
holder. Their moment of inertia could therefore readily be calculated. 
The length of the cord was measured by means of a beam compass, 
reading to .01 inch, and its diameter by.a micrometer gauge reading to 
001 inch. The time of oscillation was determined by means of a stop- 
watch reading to 0.2 second. The observations given below were in all 
cases means of several individual observations,—the length and radius of 
five, and the times of oscillation, of ten. No special effort was made to 
keep the cord at constant temperature ; but the temperature of the 
laboratory varied but little. 
The course of the observations was determined by Mr. Macdonald’s 
available time rather than by the fitness of things. He kept the cord 
stretched slightly throughout the whole series by 482 grm., and from 
time to time he would apply additional weights, make the requisite 
observations, and then remove such additional weights, the observations 
requiring perhaps a couple of hours. The dates of the observations are 
given in the table below. Unfortunately the length of the cord under 
its permanent stress was not in all cases, and its diameter was in no case, 
determined immediately before the application of the additional weights, 
such observations not being necessary for the purpose originally in hand. 
The following table gives the results of the observations and the 
calculated values of the rigidity. The observations have been reduced 
to C. G. S. units, and the rigidities expressed in absolute units of that 
system. The rigidities were calculated from the formula : 
Si! 
rt tz” 
where n is the rigidity, 7 the length, and 7 the radius, of the cord, J the 
moment of inertia of the lead plates, and ¢ the time of a complete 
oscillation. In finding the moment of inertia the weight-holder of cork 
and glass was neglected, as also the small circular apertures in the lead 
plates. The volume of the cord is given in the table also. 
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