364 
DR. EVERETT ON THE RIGIDITY OE IRON AND COPPER. 
destroy the effect of inequality in the length of the arms. This operation was, however, 
omitted through oversight, and it is therefore necessary, in comparing the torsional and 
flexural rigidities, to correct for this difference. 
The arm of couple, that is the distance SS' or TT' in fig. 1 of last paper, was 55*71 
centims. for torsion and 55 '83 for flexure. 
Also the units in which the numbers T and F are expressed, being tenths of scale- 
divisions perpendicular and parallel to the rod respectively, are slightly different, being 
and y|tu" cen li ms - f° r torsion and flexure respectively. 
The corrections for these two inequalities are in opposite directions. Their resultant 
T 
is a correction of *0016 ^ to be subtracted. 
T 
The values of ^ thus corrected are — 
From 30° and 120° . . . 1*272, 
„ 60° „ 150° . . . 1*278, 
„ 90° „ 180° . . . 1*273, 
which when diminished by unity are determinations of a or Poisson's ratio, subject to 
two other corrections, which we now proceed to calculate. 
For the mechanical correction the data are — 
Torsion in portion of rod between mirrors . . . *00661, 
Flexure in portion of rod between mirrors . . . *00519, 
these numbers (which are in circular measure) being obtained by dividing T and F 
respectively reduced to centimetres by 445*6, which is twice the height of scale above 
mirrors. They must now be multiplied by -fff, which is the ratio of all that portion of 
the rod subjected to torsion and flexure to the portion between mirrors, and also by the 
constant number *729 (see last paper, p. 143). The products thus obtained are *0074 
and *0058 ; hence the mechanical corrections of T and F are 
+ *0074 T and +*0058 F. 
The optical correction (which I am told was too briefly described in my last paper) is 
dependent on the fact that the ray from scale to mirror is not precisely vertical, and 
therefore not truly perpendicular to the scale, which is horizontal. It is determined by 
measuring — 
(1) Height of scale above mirrors. 
(2) Distance of vertical through centre of scale from line joining centres of mirrors. 
(3) Distances, resolved parallel to rod, of vertical through centre of scale from centre 
of each mirror. 
These three distances suffice to determine the corrections both for torsion and flexure, 
on the hypothesis that the centre of the scale comes into the field of view of both tele- 
scopes in every observation, an hypothesis which, though not strictly fulfilled, gives a 
fail* approximation to the actual obliquities. 
