NEWTONIAN CONSTANT OF GRAVITATION. 25 
gearing by a P 1 electric motor, by Curtriss, of Leeds, the current being supplied by 
a couple of E.P.S. cells. Mr. F. J. Suir has kindly allowed me to charge them 
when required at his laboratory. The same cells are connected up to the two 
time-marking circuits, and to a small electric lamp placed in the octagon house 
to illuminate the vernier and divisions close by. This is lighted by raising, and so 
making the upper contact of the key, which on depression is employed to make the 
signal marks from the telescope. Owing to the self-indication of the small electro- 
magnet of the time marker, a considerable spark would be formed at the mercury 
break in the clock, to the destruction of the contact, if it were not for two electrolytic 
cells in series, charged with battery acid and with platinum electrodes, which are 
employed as a bridge across the clock break. This sets up an electromotive force of 
polarization which prevents any current from-passing when the contact is kept 
broken, but its resistance is so small that the high electromotive force set up at the 
break is able readily to fall through them, thus practically abolishing the spark. 
I found this greatly superior in every way to a non-inductive resistance. There 
is one point connected with this break which I believe to be worth recording. To 
ensure good contact J amalgamated the platinum point with sodium amaleam, but 
immediately found that the contact lasted longer, and was more irregular than 
before. However, I left the point amalgamated for a fortnight, during which it gave 
more trouble by drawing the mercury out of the trough. I then unamalgamated it 
by holding it over a candle flame, until I concluded that the mercury was all gone. 
I did not make it very hot. Since that time the contact has never failed, which it 
occasionally used to do before. I attribute the improvement to an atomic roughening 
produced by the penetration of the mercury. Before the point was unamalgamated, 
the pendulum, as I afterwards discovered, made a second contact with a pool of 
mercury drawn out of the trough and electrically insulated, which contact mechani- 
cally disturbed its period. For this reason Experiment 4 is incomplete, as its time 
observations are untrustworthy. I took, however, the rigidity of the fibre from 
Experiment 5, and so completed the calculation. 
The Large Balls and their Supports. 
One of the difficulties in the preparation of apparatus for measuring the mutual 
gravitation of comparatively small bodies is met with in making the large balls. 
CAVENDISH employed large balls of cast lead 1 foot in diameter. Professor PoyntTine 
made the balls in his apparatus of an alloy of lead and antimony, for the sake of the 
extra hardness, which would make it easier to turn them accurately to form and would 
render them less liable to subsequent deformation. Though special precautions were 
taken to avoid cavities, and to obtain homogeneity, the large one was found to 
act differently in different directions, and he localized a cavity by observations 
in this way. Afterwards he found the centre of gravity was to one side of the 
MDCCUXCV.—A. EB 
