50 PROFESSOR C. V. BOYS ON THE 
lead balls from the + to the — positions. Find also with large amplitude, the exact 
period with and without the counterweight, using the drum and following all the 
details given in operation 10. At this stage it is convenient to set up the 
cathetometer and measure the stretching of the torsion fibres by observation on the 
bottom hook of the mirror. This is not necessary for the purpose of finding G, but 
it is of interest as bearing upon the elastic properties of quartz fibres. 
Operation 12. 
Turn the lid round to the neutral position. Place the steel bands on the flat- 
rimmed pullies. Pin to the ball holders, and hang on at the other end the 
counterweight. Raise the balls about $ inch, turn them individually through 60° and 
let the geometrical clamps down through the triangular orifices of the lid pillars, 
until the balls rest on the india-rubber rings on the base. Raise the lid, leaving it 
balanced in the air by its counterpoise, and after removing the two counterweights 
and the holding pins, take away the steel bands. Let down the lid again. Partly 
balance it as before. Put screens and octagon house in position as before, and after 
a day or two take deflections, if any, when the lid and lid pillars are moved between 
the + and — positions. Three sets of six elongations at least should be taken at 
each position. 
Operation 13. 
Re-suspend the gold balls in the same position as before, and find the deflection of 
the mirror, if any produced, by moving the lid and lid pillars from the + to the 
— positions. 
Operation 14. 
Take the focussing collar off one microscope of the optical compass, and slide it on 
to the nose end of the other, so as to raise it high enough to see the side of the 
bottom hook of the beam mirror. Remove the front Jens of the object-glass, which 
reduces the magnifying power to rather less than one-half. Set the optical compass 
so that the vertical tangent to the curve of the lower hook is on the zero of the 
micrometer scale in the eye-piece, that is when the mirror alone is freely suspended. 
Hang on the counter-weight and take the scale reading. Hang on the gold balls 
instead of the counter-weight and take the scale reading again. The object of this 
is to find to what extent, if any, the axis of rotation of the beam changes when the | 
balls or the counterweight are suspended (see p. 84). 
