150 
DR. EVERETT ON THE RIGIDITY OF GLASS. 
which, together with the optical corrections 
-•0025 T, — -0013 F 1? — -0013 F 2 , 
make the total corrections 
+ •0059 T, +-0051 F„ +-0065 F 2 , 
T 
which are so small and so nearly equal that the corrections of p maybe neglected. We 
therefore assume as the value of Poisson’s ratio from these experiments, the mean of the 
six determinations above given, which is ‘315. 
All the foregoing experiments were conducted by myself in the Lecture-room during 
the Vacation. At the commencement of the Session, the apparatus was removed to 
another room, where experiments on the steel rod were continued, under my direction, 
by two students (Messrs. King and Walker) during the months of November and 
December. The room selected for this purpose being on the ground-floor, and paved 
with asphalt, on which the apparatus rested, was superior, as regards steadiness, to the 
lecture-room, which is on the first floor ; and I may here remark that the inconsistencies 
(such as they are) which occur in the foregoing experiments, were found to be due 
mainly to the yielding of the floor under the feet of the observer. 
On the other hand the new situation afforded less height, the scale being only 223‘5 
centimetres above the mirrors. It was also rather dark ; but this defect was completely 
remedied by using a gaslight, aided by a concave reflector, to illuminate the scale. The 
scale used was a new one, of the same kind as the old, but with the lines nearer together, 
their distances, as determined by taking the means of several measurements, being such 
that 
For torsion . . 171 scale-divisions =23'88 centims. 
For flexure . . 171 scale-divisions =23-97 centims. 
The whole length of rod subjected to torsion and flexure was 46-8, and the mirrors were 
attached at a greater distance apart than in any of the foregoing experiments, viz. 38 T 5 
centims. The telescopes were at the same height above the mirrors as before, being 
clamped to the same table which had been previously used. The weights employed 
were of lOOgrms., and the system of observing was the same as in the later observations 
above described. 
The following were the values obtained for T and F in terms of their respective scale- 
divisions, each of these values being the mean of sixteen determinations. 
i(4 
Pointer at 
0° 
Torsion 25-62 
Flexure 19-68 
2(4 
„ 
30° 
„ 25-87 
„ 19-57 
3(a). 
55 
60° 
„ 25-87 
„ 19-64 
1(4 
55 
90° 
„ .25-95 
„ 19-74 
2(4 
55 
120° 
„ 25-85 
„ 19-82 
3(4 
55 '- . 
150° 
,, 25-84 
„ 19-80 
