NEWTONIAN CONSTANT OF GRAVITATION. 67 
the decrement from °842 to ‘937, which in itself was a great advantage, so much 
difficulty seemed inherent in the method that I determined not to prosecute it. The 
values found this night for T,” were 
MEME no 6 ee SOM 
Inhydrogen . . . 35401 
The deflections owing to disturbance could not be so accurately determined as usual; 
they were 
Tinta a ere aa ero 20 
In hydrogen . . . 385238 
The only observation of real interest in connection with the hydrogen experiment 
was the effect of the gas upon the mirror. The mirror was bent to a small extent, 
causing the image of the divisions to practically disappear. A movement of the eye- 
piece outwards of about 3 inch was needed to make them appear sharp again. On 
letting the gas escape the focus went back to its old place, and this was repeated 
without variation three or four times. I imagine that the glass became convex under 
the influence of the hydrogen in consequence of the glass being penetrated by the 
quickly-moving molecules, and so becoming expanded in the front or unprotected 
side, while the silver and lacquer at the back prevented this action, much as paper or 
lace will protect glass from the cutting action of a sand-blast. The bending may have 
been produced by a contraction of the lacquer or silver, but this seems hardly con- 
ceivable. An interesting line of inquiry is suggested by this experiment, but 1 have 
not been able to do more at present. 
An examination of the results shows that they hover about two values, experi- 
ments 3, 4, 5, 6 and 12 being about 5°520 for A, and the remainder about 5°528. It 
is impossible to trace any connection between the arrangement of the apparatus, &c., 
and this small irregularity. It is necessary, therefore, to review the deflections and 
periods of each and the conditions, whether of disturbance or quiet, under which they 
were carried out. No. 3 has already been discussed, and the wonder is that it should 
agree so well with the others when the imperfect conditions are borne in mind, and 
when it is remembered that the torsion fibre in this experiment had only one-third of 
the rigidity of those used later, while the gold balls were only half as heavy. 
As already mentioned, the periods in Experiment 4 were lost, so that an absolute 
result could not be calculated. The values of 2r in the two cases only differed by 
00002 inch, an amount which is probably beyond the certainty of measurement, and 
therefore the results for 4 are merely obtained from those from Experiment 5, by 
multiplying or dividing by the ratio of the deflections. 
Deflections and periods for Experiment 4 were taken over several days. The 
deflections were :— 
K 2 
