NEWTONIAN CONSTANT OF GRAVITATION. 11 
of wash-leather just fitting the tube. A piece of thread long enough to reach beyond 
the window is fastened at one end to the piece of wood and at the other end to a 
small fragment of iron wire. The thread and wire rest upon the wash-leather, and 
to make sure of this a second cylinder of wood is let down to press all in place. In 
case of accident to the little balls, a magnetized tuning-fork is let down the tubes by 
a piece of string, and the iron wire pulled up. It is then easy, by pulling the thread, 
to bring the wash-leather to the window level and so to pick out the little balls with 
forceps, i 
Fig. 15 is a vertical section of the mnermost of the series of screens employed to 
protect the apparatus from variations of temperature. I could not at first believe that 
these would be required, but each additional protection of the kind has certainly 
improved the constancy of behaviour of the apparatus, and I have now no doubt as to 
the necessity for their use. ¢, is a brass tube with inner and outer ledges split into 
two halves, so as to fit on to the upper part of the window casting shown in chain 
lines ; f, is a plain brass tube reaching nearly to the top of the central tube, and ¢; is 
a third brass tube, with an internal ledge resting on ¢,. This is large enough to 
clear the milled heads h,, h,. An opening is made in it large enough to allow the 
telescope T and all parts of the scale S to be seen from all parts of the mirror. There 
is also a smal] hole in the back, through which the tube of the window, fig. 12, can 
be screwed. The screen tube ¢, is just clear of the lid and the window tube. 
To protect the whole instrument from variations in temperature it is completely 
surrounded by the octagon house, of which a horizontal section is shown in fig. 22. 
It is double-walled, and is made in two halves of 23-inch pine boards, separated by a 
space of 1 inch. This is filled with cotton-wool. The top is flat, double, and packed 
with cotton-wool in the same way. The two halves slide together upon the table on 
which the instrument is placed, and meet, completely enclosing it, with the exception 
of a small hole in the centre of the top, through which a cord, the use of which will 
be described later, can pass ; of a narrow slit in the front, through which the scale 
and telescope may be seen from the mirror; and of two small apertures through 
one of which the vernier V may be seen by the aid of the small telescope ¢ (figs. 18, 
19), the other admitting of the driving wheel D and air tube. The connecting wire 
between D and the wheelwork above lies in the narrow space between the inner and 
outer boards and the two styles which separate them. 
By way of illustrating the state of steadiness to which I have reduced the air in 
the central tube, I may give the result of a calculation made in the case of Experi- 
ment 8. In that experiment the points of rest would have been disturbed by one 
unit if the air in the tube had been moving round at the rate of one turn in six weeks, 
z.€., at such a rate as to blow past the balls at a rate of 1 inch in 133 days. This 
follows immediately from the torsional rigidity, decrement, period, and angular value 
of one division. No uncertainty so great as this appears in the mean deflections 
obtained during the night. 
