8 PROFESSOR C. VY. BOYS ON THE 
figure of revolution about the suspending fibre as an axis. With the hope of obtainmg 
very perfectly conducting and uniform cylinders, both for the outer case and for the 
central tube, I ascertained what sizes of Elmore tube would be obtainable, and thus 
determined the actual and final dimensions of the apparatus. When this was too 
far advanced for change to be possible, the Elmore Company informed me that they 
could not supply the sizes previously settled, and so I had to be content with a piece 
of thick triblet-drawn brazed copper tube for the centre, and a thick brass casting 
for the surrounding case. The experiments show that no appreciable disturbance has 
arisen owing to any want of perfection in the tube. The casting was turned inside and 
out without being moved from the face plate, and, except in conductivity, is as perfect 
as pure copper. In order to keep the gravitational symmetry round the axis as perfect 
as possible, I had holes drilled in the massive base round the levelling screws, so as to 
remove as much metal as they added. The important dimensions on which I finally 
decided were :— . 
Distance from centre to centre of large balls 77 plan, 6 inches or 4 inches. 
Distance from centre to centre of small balls a plan, 1 inch, about. 
Diameter of large balls, 44 inches or 24 inches. 
Diameter of small balls, °2 inch and ‘25 inch. 
Difference of level between upper and lower pairs, 6 inches. 
With these settled the rest of the design of the apparatus shown in figs. 1-15 
followed naturally enough. I think it most convenient first to describe the apparatus 
in moderate detail, without going into the reasons why I decided upon each particular, 
and afterwards to show how the design accomplishes all that is needed for an accurate 
determination of G, the Newtonian constant of gravitation. 
The Apparatus (Plate 1). 
Fig. 1 is a vertical section through the centre of the apparatus, the window alone 
being in elevation ; fig. 2 is a sectional plan through aa. Taking the structure first, 
B is a massive brass base, turned on both sides, carried by three levelling screws with 
lock nuts. C is the outer brass cylindrical casing screwed to the base B and accu- 
lately turned as already mentioned. Lisa turned brass lid mechanically fitting C, 
on which it can be made to turn by the action of the train of wheels WWW. ‘The 
edge of the flange is divided in degrees, and can be read to 75° upon the vernier V, 
fig. 3. Two tubular pillars PP are fitted into holes diametrically opposite to one 
another and 6 inches or 4 inches apart, according to the size of ball that is to be 
used. The heads of these pillars are shown half size in figs. 4, 5, 6, where it will 
be seen that at angles of 120° there are three radial V’s forming a geometrical clamp 
with either ball support. Also that on just raising the latter and giving it a rotation 
of 60° it ean be let down through the tubular pillar. As seen, the large balls hang 
from these geometrical clamps by wires, but into these particulars and into the details 
