NEWTONIAN CONSTANT OF GRAVITATION. 9 
connected with the construction of the balls I shall enter later. The central tube T 
is held accurately in its place by a cylindrical fitting and the hollow screw 8. This 
tube, up to the window just above the lid, is made of thick copper; at the window 
level it is united by the window casting to the upper tube of the same size, which is 
made of brass, and this carries at its upper end the torsion head surmounted by the 
bell jar J with a central stop-cock. The torsion head admits of a variation of level 
of about 2 inches and of horizontal adjustment by means of three screws. The 
window casting forming the centre of the tube does not touch the lid, there being a 
space of about 35 of an inch between them. The equality of this all round is 
an excellent test of the accuracy of this part of the construction. The window is 
shown half size in figs. 8, 9, and 10. Fig. 8 is a front view, the upper part being in 
section, fig. 9 is a side view, and fig. 10 a section through aa. The thick cylindrical 
casting is cut through front and back so as to form two flat square faces FF 2 inches 
in the side each, and over these 2 inches the casting is cut right through, forming 
a square chamber in which the beam mirror hangs, and certain operations can be 
carried on. Four milled heads h,, , are employed in making the transfer of the 
smaller balls to and from the beam mirror, of which an enlarged view is shown in 
fig. 7. This operation is performed as follows: the two heads h, are fixed to 
the same cross axle, and when turned through a right angle cause two arms 
with V notches at their ends to pick up the beam (fig. 7) by its upper cross 
arms. In this way the beam can be raised or lowered a little or let down so 
as to hang from its torsion fibre. The small balls hang by quartz fibres from the 
hooks and eyes seen in fig. 7. When not on the beam these hang by their eyes from 
the points projecting from the cranked ends of the pins operated by the heads h, hg, 
which can be turned or pushed in or drawn out. By combining the movement of 
the heads /, and h, one of the hooks and eyes can be transferred to the V at the end 
of the upper arm of the beam mirror resting there by its hook. In the same way 
the other one is transferred. To prevent risk of the tipping of the beam and 
fracture of the torsion fibre during this operation, a weight is first hung on to the 
lower central hook of the beam and removed when the double operation is complete. 
The ends of the mirror have very fine V grooves ground in them, so that the quartz 
fibres hanging from the hooks may lie in these grooves and so be held definitely in 
position, both with respect to their distance apart and circumferentially with 
respect to the mirror. A cylindrical counter-weight K, fig. 7, of known very small 
moment of inertia, but of exactly the same weight-as the small balls with their hooks 
and fibres, can be hung upon the central hook of the beam, when the balls are 
removed to the side hooks, so that the fibre may be stretched to the same extent 
and therefore have the same torsional rigidity when the periods are being taken with 
or without the small pair of balls. 
A series of windows are provided to fit upon and make an air-tight joint with the 
plane-faces FF. Two are mere squares of plate-glass of the exact size needed. These 
MDCCCXCY.—A. c 
