276 ON THE RIGIDITY 
In most of the measurements made, the quantity under con- 
sideration was determined as a mean of several observations. 
Lengths were measured by means of a beam compass reading 
to .0l inch. The limit of error of a mean value was found by 
comparing a number of such mean values with their mean, the 
greatest divergence being taken to be the possible error of a 
determination. It was found to be different according as it was 
the length of the cord or of a side of the Mallock triangle or 
of the arm, that was determined. In the two former eases the 
greatest divergence from the mean was .01 in, in the latter 
005 in, 
The diameters of the cords, which were approximately 
cylindrical, were found by means vf a screw-gauge reading 
to .001 inch. The possible error was found by the above 
method to be .0005 in. Owing to the difference in diameter at 
different parts of the cord, it was found necessary to have 
marks on the cord, at which the measurements were always 
made. 
The error that might be made in determining the angle of 
twist in the static method was estimated to be about a quarter 
of a degree. 
The weight of the small plummet used in twisting the cord 
was found by means of a balance weighing to .001 grm. The 
method of weighing was that of substitution, and the limit of 
error was estimated to be .0005 orm. 
The time of oscillation was found by means of a stop-watch 
divided into fifth-seconds, but capable of estimation to .1 sec. 
The lhmit of error was determined in the same way as in the 
case of length, and found to be about .04 sec. 
In the static method the formula used for the calculation of 
the rigidity was the following: n = 2 T'l/zr *¢ in which T is the 
torque in lb.-inch units, applied at lower end of cord, / is length 
of cord in inches,7r is radius of cord in inches, and ¢ is angle 
twisted through measured in radians. For the kinetic method 
the formula: n=87/1/t?r*g was used, in which / and r were 
