366 
DR. P. E. SHAW ON THE NEWTONIAN CONSTANT OF 
Facing the window, at 4|- m., is an astronomical telescope of 7'5 cm. objective. Over 
the telescope and perpendicular to it are two scales, each illuminated by a glow lamp 
which is adjustable by pulleys and strings to any place on the scales, whilst the scales 
themselves can be moved vertically up and down and clamped anywhere. 
Fig. 8 shows the connections for evacuating. A Gaede rotary pump and McLeod 
gauge are used. For washing out the vacuum, oxygen is obtained by heating mercuric 
oxide. C a is the carbon tube and the three taps, t u t 2 , t z , well-made and properly treated 
with rubber, grease and wax, as explained elsewhere, are reliable for any length of time. 
6. The Large Mass (M, M) System. —The two large spheres are of lead, 20 cm. 
diam., and weigh each about 47 kilos. I am much indebted to my colleague, 
Prof. C. H. Bulleid, for the great care and personal skill he put into the construction 
of these large masses. The special desiderata for them are : Freedom from air 
bubbles, freedom from iron, and accurate alignment in each, of three parallel brass 
tubes. A wooden pattern having been placed half-way in a moulding box, the latter 
was filled in with plaster of Paris. This made half the mould. The other half was 
made similarly with another box. The three brass tubes, full of plaster, were placed 
in position. The moulds were then roasted to dryness for several days before use. 
Commercial sheet lead only was used for the melting pot. As the molten lead was 
run into the mould great care was taken that cooling should proceed from below 
upwards. Hot cokes were laid on top of the mould, while the lead solidified at the 
bottom. The lead was puddled by a hot copper rod to dislodge all air bubbles. This 
process continued as solidification proceeded until finally the runner solidified. The 
spheres were of the same weight to 15 gr., so there was small likelihood of enclosed 
air. The tubes having been cleared of plaster and the spheres washed and swabbed 
with nitric acid to remove iron dirt, a stout carrier consisting of a copper rod, 1 cm. 
diam., screwing into a thick copper disc, was fitted into the central hole (figs. 9 and 10). 
The spheres are hung from the turn-table by copper wires. The two other holes in 
each sphere are fitted with tubes of asbestos and mica sheet rolled together ; into each 
is then placed a heating coil of nichrome (16 ohms resistance). The leads, l, l (fig. 9) 
for the heating coils are carried up to the turn-table. By this disposition these 
leading wires exert no influence on the hang of the spheres. The lead spheres are 
covered with two layers of cotton-wool, laid on in gores, for lagging. Over the cotton¬ 
wool is a layer of tin foil. In the experiments the whole heavy system can be rotated 
on a vertical axis with ease and smoothness on the ball-bearings shown in the figures. 
7. The Form of the Beam System. —In fig. 12 are shown six forms of .suspension, 
all of which have been exploited for these experiments. In each case the torsion fibre 
is attached to the top, and the mirror is shown as a small square. Any one of these 
forms would act well, if no tremors reached the suspended system from outside, but 
as vibrations do arrive from outside, some of them are unworkable in a vacuum vessel 
where there is no gas-damping. 
It is a novelty to have a suspended system of limiting sensitiveness used in a high 
vacuum, so that the technical difficulties are new. To explain the action, consider 
