NEWTONIAN CONSTANT OF GRAVITATION. i 
power of the linear dimensions, had been correct. Since it varies only as the fifth 
power, and the quietness of the air is so great in my apparatus (p. 11), there can be 
no objection on this account to the double size; but I would strongly urge that in 
such a case, a room more uniform in temperature than the one at Oxford should be 
employed. It would also be well to lay non-connecting mats on those parts of instru- 
ments on which the hands are apt to rest when the balls are being transferred, or 
other manipulative operations are being carried out, so as to reduce, as far as possible, 
access of heat, and hence the interval that must elapse before observations of 
deflections or periods can be undertaken. I do not think any ready-made room is 
likely to be found available. A disused adit, at a great distance from existent 
mining operations, would be perfect, if it could be made use of. The instrument 
could then be walled up in a room to itself, and the heat from the observer and the 
travelling lamp excluded far more perfectly than in my case. An adit would be 
convenient also, in that it would allow of the use of a greater distance from the scale 
to the mirror than could be obtained in an ordinary room. This should not be less 
than 20 metres. 
I should recommend a slight change in the upper end of the lid pillars with the 
object of giving the ball holder two adjustments, one radially, and one at right angles 
to a radius, so that eccentricities observed by the microscope of the optical compass 
could be corrected. 
I also think more pains should be taken with the beam mirror to insure its rotating 
about its own centre of gravity, both when the gold balls and when the counter- 
weight are suspended. This would remove any doubt as to its constancy of move- 
ment of inertia when made to oscillate under the three conditions, and would at the 
same time make observable eccentricity of the gold balls impossible. 
Finally, I have suffered much from the great loss of time that results from the 
accidental fall of a gold ball down the central tube. It can only be replaced after 
lifting out the torsion head, torsion fibre, and beam mirror, so that all the centering 
adjustments are lost, besides which, there is the serious risk of breaking the torsion 
fibre whenever this operation is carried out. I would make the lower end of the 
central tube funnel-shaped inside, and employ a much larger holding down screw, 
with a central hole more than large enough to allow the gold ball to escape through 
it. This could, of course, be plugged at other times. In order to put the gold balls 
into their places without removing the torsion fibre, I would have a large hole in the 
torsion head behind (away from the big telescope) the torsion rod, and this would be easy 
with the larger central tube. Then if a special overhead wheel were placed with its 
edge vertically over the centre of this hole, the gold ball could be let down as 
described in the paper and transferred, as usual, on to one of the side hooks by the 
use of a simple tool made of a bent pin. 
In every other respect the apparatus behaves so perfectly, and the operations are 
conducted with such facility, that I am unable to offer any other useful suggestion. 
