140 
DE. EYEEETT OX THE EIGIDITY OE GLASS. 
a larger scale at fig. 5. By partially releasing the clamps, it was easy to rotate the 
.mirrors about the rod without longitudinal sliding. 
The point n, fig. 3, is supported by the flat brass ring M, which hangs by the wire N 
from one arm of the balance D, and a counterpoise is placed in the pan P just sufficient 
to keep the rod A B free from strain. 
The experiments were conducted in the Natural Philosophy Lecture-room. The box 
F F rested on the floor, the height of mirrors above floor being 270 millims. The scale 
reflected by them consisted of a large sheet of paper ruled in two directions at right 
angles to each other with lines about a tenth of an inch apart, and was firmly fixed at 
the height of 4597 millims. from the floor by stretching it on a board and screwing this 
to two joists whose primary office was the support of a cistern. The light, which was 
naturally good, was improved by using a concave mirror to illuminate the scale. Two 
telescopes, not shown in the Plate, were clamped to a firm three-legged table, their object- 
glasses being about 970 millims. above the floor. They were in fixed positions, directed 
one towards each mirror, and were as nearly vertical as was compatible with an unob- 
structed view of the reflection of the scale. Their deviations from two vertical planes, 
one parallel and the other perpendicular to the rod, were from yj, to in circular 
measure. They were inverting achromatic, of 1-J-inch aperture and 10 inches focal length, 
with cross wires in focus of eyepiece. A damper, consisting of a piece of thin card 
pressing lightly against the end A of the rod, was used on and after July 17th for the 
purpose of checking vibration. 
The mode of observing for flexure was as follows: — The mirrors having been adjusted 
so as to bring the central portion of the scale into view in both telescopes, a pair of 
equal weights were placed, one in the scale-pan P along with the counterpoise, the other 
on the point S, and readings were taken in both telescopes. Then the weight at S was 
transferred to S', and readings were again taken. The difference of readings in further 
telescope diminished by difference of readings in nearer telescope is assumed to measure 
the effect, on the portion of rod between the two mirrors, of a bending couple whose 
arm is the distance between the two points S, S', and whose power is the force of gravity 
on the moveable weight. 
The weight was then transferred first to T and then to T', both telescopes being read 
in each case. The differences were taken in the same way as above^ and the result is 
assumed to measure the effect, on the same portion of the rod, of a twisting couple 
whose power is the same as above, and whose arm is the distance between the points T, T'. 
The weight was then again transferred to S' and S, then again to T' and T, and so on 
several times, both telescopes being read in each position of the weight, \nd no change 
being made in any of the adjustments. The facility of thus passing from observations 
of flexure to those of torsion, and viceversd, gives the present form of apparatus a great 
superiority over that employed the previous year. 
It has been observed that the arms of couple in flexure and torsion are the distances 
S S', T T' respectively, which, though nearly equal, are not absolutely identical. This 
