DR. EVERETT ON THE RIGIDITY OE GLASS. 
141 
defect was easily remedied by turning the crosspiece through a right angle, so as to 
make S S' change places with T T'. 
Another source of error to be guarded against is want of perfect circularity in the rod 
operated upon. This is completely removed, if the deviation from circularity be small, 
by turning the rod itself through a right angle by means of the graduated circle H. 
This change has no effect on the torsional rigidity; and its effect on the flexural 
rigidity is such that the mean flexure in the two positions is the true mean for all posi- 
tions, inasmuch as the flexural rigidity in any position is proportional to the moment of 
inertia of a section about a horizontal diameter through its centre of gravity, and by a 
well-known theorem the sum of the moments of inertia about two rectangular diameters 
is constant. 
For greater security the rod was turned into six different positions, differing by 30° 
among themselves, so that the first and fourth positions furnished one mean, the second 
and fifth another, and the third and sixth another. In every one of the six positions 
observations of both flexure and torsion were taken ; and the operation of turning the 
crosspiece through a right angle so as to make the arms of couple for flexure and torsion 
change places, occurred between the third and fourth positions. 
The first rod experimented on, after much time spent in preliminary arrangements, was 
a flint-glass rod from the works of A. and P. CocHRAisr, Glasgow. The weights employed 
for producing flexure and torsion were a pair of lead weights of 100 grms. each. One 
of them (distinguishable by its ring) was hung in turn on each of the four arms, and 
the other was always placed in the counterpoise pan. 
The first complete set of observations in six positions were made July 17th and 18th, 
with the following results : — 
1(4 
Pointer at 135° 
Torsion 539 
Flexure 435^ 
2(4 
165° 
,, 5471 
„ 438 
3(4 
„ 
195° 
„ 546 
„ 4461 
1 ( 6 ). 
„ 
225° 
„ 548 
„ 454 
2 ( 6 ). 
255° 
„ 5491 
„ 454 
3(6). 
5? 
285° 
„ 546 
„ 4471, 
The numbers here given as representing the amounts of torsion and flexure, are ex- 
pressed in tenth parts of the scale-divisions, and are therefore approximately hundredths 
of an inch. Combining those positions which are mutually at right angles, we have the 
following means : — 
1 ( a ) ( 6 ). Torsion 543 ^ Flexure 444f 
2 (a)( 6 ). „ 548| „ 446 
3(a)(6). „ 546 „ 4471. 
The scale-divisions were somewhat longer in one direction than in the other, being 
of a millimetre for torsion and i ? - 5 - millims. for flexure. In order, then, to find the true 
mdccclxvii. u 
