STRAIN ON THE PHYSICAL PROPERTIES OP MATTER, 
805 
as to fit air-tight, and were only opened for the purpose of adjustment. The top of 
the long box A A was also closed by a metal cap, and every precaution taken to 
ensure that the vibrations of the wire should be undisturbed by currents of air. The 
wire near its lower extremity passed through the centre of an aperture O made in the 
top of the box S S. The bar V V of the vibrator was damped to the lower extremity 
of the wire, and to it were suspended two cylinders of equal mass and dimensions ; 
by sliding the cylinders backwards or forwards on the bar, the moment of inertia of 
the whole vibrator could be altered without changing 1 its mass. The bar Y V was 
graduated in millimetres, and the cylinders could be clamped at any required equal 
distances from the axis of the wire ; the position of the cylinders could readily be 
estimated to the tenth of a millimetre. The mode of attaching the cylinders is 
sufficiently shown in the figure, and it will be noticed that each cylinder is capable of 
being divided into two equal parts, each part having a mass of 337 ‘ 1 grins., a length 
of 30 - 195 centims., and a diameter of T298G4 centim. These cylinders, either 
complete or in halves, were used for nearly all the wires, except those of tin and lead, 
with which last lighter cylinders and bars were employed. The bar V V consisted in 
some experiments of a piece of thin, drawn, hollow brass tubing, of which the length 
was 30‘70 centims. and the diameter L420 centims. Two brass caps provided with 
screws, about 8 centims. in length and 2 millims. in diameter, fit one into each end of 
this hollow bar, and can be easily removed from or placed in it. This bar will be 
designated as the bar B 2 . In other experiments VY consisted of a thin hollow brass 
tube, closed at both ends, of length 30‘46 centims., and, except at the centre, of mean 
diameter 0'94980 centim. The central portion of the bar had soldered on to it a solid 
concentric brass ring 1 *55 centims. in length and of the same diameter as the bar B l5 
so that either bar might be fitted into the brass block K, wdiich was itself clamped to 
the wire. The bar last described will be designated as the bar ; both B x and B 3 
can just pass through a circular aperture in the block K and be clamped at their 
centre to K. When the cylinders suspended to VY are used complete they will be 
designated as the cylinders C. 2 , and when they are used in halves as the cylinders C x . 
The torsional vibrations of the wire were observed by the usual mirror-and-scale 
arrangement, which is sufficiently shown in fig. 1, where M is the light mirror 
reflecting an illuminated circle of light crossed by a vertical, fine, dark line on to a 
scale placed at the distance of 1 metre from the mirror. The scale was bent into an 
arc of a circle 1 metre in radius, and was supported on brickwork, which last, together 
with the box S S, rested on the flagstones which formed the floor of the small wooden 
house previously mentioned. So perfectly firm and free from disturbing causes of 
whatever sort was the whole arrangement that the spot of light, when at rest, would 
remain perfectly so for any required length of time.* 
* This, of course, shows that the arrangements made for securing freedom from currents of air 
answered their purpose satisfactorily. 
