NEWTONIAN CONSTANT OF GRAVITATION. 13 
generally set it so that the flame is about 4 inch wide and $ inch high. The driving- 
wheel D is made with two heavy projections not shown in the figure, to give it con- 
siderable moment of inertia, and with the handle at a distance of an inch about from 
the axis. Any motion given to it by hand is therefore less likely to be subject to 
jerks than it would be if unweighted. A very light cord rests loosely round this 
pulley, is supported by an arm of wood projecting from the other end of the table, is 
supported again on the edge of the other table, and then lightly passes round the 
little wheel D, figs. 1 and 2. This rests upon the table, and is kept from moving 
about by a weighted foot. Two pins fastened into the wheel D engage in a hole and 
slot in the cross-piece y at the bottom of the hanging wire b. Thus when D is turned 
the motion is communicated to the wheel-work WWW through the light and loose 
cord, the wheel D, and the cross-arm and wire. The only kind of force between the 
wheel D and the wire is by the construction a couple, and this, owing to the high 
ratio of gearing in the wheel-work WWW, need be only very small to give motion to 
the lid L. The friction due to the great weight of the balls M M, and of the lid, is 
largely reduced by hooking to the two rods R R screwed into the lid guys joined to a 
cross-bar above the bell-jar, which there hangs from a single line passing round the 
centre one of five wheels secured to the arch, the edge of this being exactly above the 
axis of the instrument. The line then passes over a second wheel close to the west 
wall of the vault, and carries two weights each exactly equal to one of the balls, and 
an extra weight to partly balance the weight of the lid. When the handle of the 
wheel d is turned the lid slowly and almost insensibly creeps round, and no tremor 
appreciable with ordinary apparatus is communicated to the suspended mirror. 
Owing to the extreme sensitiveness of the apparatus, and the very great magnifying 
power, a high period tremor is set up in the mirror, about equal in amount to that 
caused by ordinary traftic in St. Giles’, about a quarter of a mile away. This dies 
away very rapidly, and I am unable to trace any anomaly to this cause. The corner 
of the vault, in which the instrument is placed, is screened off from such small 
variations in temperature as my presence and the small gas flame produce, by 
two double partitions of felt, f, fi, fpf. Furthermore, the vault itself is protected 
from variations in the temperature of the air, in the long underground passage by 
which it is approached, by two felt curtains some distance apart. 
Slits and holes, no larger than are necessary, are made in the partitions f, f, to 
allow the scale and telescope T to be seen from the mirror, the vernier V to be seen 
from the telescope ¢ and the light string to pass through. These partitions are 
temporarily lifted out of the way when a certain beam /,, shown in position in fig. 19 
but not in fig. 18, is being used. The two beams J,, /, have their upper edges planed 
true, and are so supported by levelling screws that their upper edges form one level 
straight edge. These are employed when the distance from the scale to the mirror is 
being determined in the manner to be described under the heading “The Steel Tape 
and Accessories.” The beam /, I leave in position permanently, but as 7, would be in 
the way, it is only put up when required. 
