36 PROFESSOR C. V. BOYS ON THE 
extreme positions, to introduce an uncertainty of position nearly four times as great. 
I cannot think that any serious discrepancy can have resulted from this, as the 
uncertainty of moment of inertia would be only a very small fraction of that found in 
the case of the beam mirror when the balls were suspended. This fault, such as it is, 
however, I intend to remove in my next experiments, from which I hope, also, to 
almost entirely exclude the small eccentricities already referred to. For this purpose 
I may either file the hook rather wider, or, as I think preferable, hang the counter- 
weight by a loop of silk, which will in no way constrain its hanging, but which will 
be rigid with respect to torsion, I wish, also, to make use of a slightly-silvered 
torsion fibre, so as to reduce the effect of electrical disturbances if they should be set 
up by the movements of the air. The square of the corrected period, with the 
counterweight on, diftered more in Experiments 5 and 8 than it should have done, 
being 1702°35 in Experiment 5, and 1698°73 in Experiment 8. In order to see to 
what extent this discrepancy may have affected the result, I have calculated the 
rigidity of the torsion fibre in these two cases with the observations of this quantity 
reversed, that 1s, 1698°73 instead of 1702°35, and vice versd. The effect upon the 
result is 1 part in 10,600, so that if all the error is in one observation only, the other 
one is on this account alone less than yg$o9 in error. As any sticking of the counter- 
weight would tend to increase the moment of inertia, and hence the period, I am 
inclined to look with greater favour on the smaller observation, but the difference, in 
any case, is considerably less than the actual differences found in the final results. 
In connection with the beam mirror, it is convenient here to describe the means 
provided for keeping it under control from the telescope without entering the shielded 
corner of the vault. 
Below the apparatus, figs. 1 and 2, may be seen a tube terminating in a bent glass 
pipe which enters the hollow screw s. This is joined to a narrow piece of composition 
piping, which is carried across the interval between the two tables by the wooden bar 
which serves to protect the light driving cord, and, at the eyepiece of the telescope T, 
terminates in a mouthpiece. Where observations of deflection and period are being 
made, air may be drawn through the tube causing a very gentle indraught through 
the tube of the window, fig. 12. This acting on one end of the mirror produces upon 
it a couple which may be employed to bring it to rest or to get up a swing of large 
amplitude. The extreme precision and delicacy of this process is best explained by 
considering an electrical analogue. The window tube, fig. 12, acts as a moderate 
resistance, the open space between the glass tube and the large hole in the screw as 
a short circuit or very low resistance, and the long tube across the room as a high 
resistance again. The electromotive force is the suction of the mouth. Owing to 
the high resistance of the long tube, but a feeble effect is felt at the glass end, and 
this is practically entirely satisfied by the low resistance leak. The available electro- 
motive force acting upon the resistance of the window tube is therefore very small, 
and in consequence the current acting on the needle is similarly minute. So delicate 
