NEWTONIAN CONSTANT OF GRAVITATION. 31 
was kept pressed against this face by screw pressure, and a steel plunger, accurately 
fitting the mould, with a polished plane perpendicular end completed the tool. The 
required quantity of gold, plus a small quantity for excess, was melted into a button 
as before, and placed into the mould. The plunger was beaten with a heavy hammer, 
under the blows of which the solid gold flowed as freely as the lead in the other case, 
penetrating the fissure between the cylinder and the bottom slab. A second plunger, 
made of brass, with au exactly central, fine needle point, was then pressed with light 
blows upon the gold to make a central hole, into which to solder a supporting wire, 
as in the case of the gold balls. Mr. Epser, of the Royal College of Science, was 
kind enough to calculate for me, by means of spherical harmonics, the very small 
difference between the attraction of the cylinder upon a point at the centre of the 
lead ball, and that which would be exerted if the whole of the mass of the cylinder 
were concentrated at its centre. The correction is for a greater variation of distance 
and of inclination from the equatorial plane than can have been met with —:00030 
of the whole, and this correction is accordingly applied in the one experiment (9) in 
which the cylinders were employed. 
I have not at present referred to the attractions between the suspending wires and 
the gold balls, and between the suspending quartz fibres and the lead balls. The 
attractions of the fibres and wires for one another are, of course, infinitesimal to the 
second degree. Calculation shows that in experiments 4 to 8 and 10 to 12, the 
attraction of the lead balls for the fibres is to their attraction for the gold balls as 
lis to 204,500, and in the same direction, while the attraction of the gold balls 
for the wires is to their attraction for the lead balls as 1 is to 115,000, and in the 
opposite direction. The reversal of the direction is surprising, but it is due to the 
fact that the most effective part of the fibres is absent in the case of the wires, owing 
to the large diameter of the lead balls, and, therefore, the action of the long wire on 
the upper gold ball, which is on the other side, is the preponderating influence. The 
difference of these two corrections is sgsyo9 of the main effect, and in the opposite 
direction. I should add, that though the absolute masses of the small balls are known 
with an accuracy of 1 in 10,000, this is in no way necessary. So long as they are 
practically alike, it does not matter whether their mass is known or not, as it is 
ultimately eliminated. I have, however, introduced into the numerical work the 
actual masses in order to obtain the constants of the fibres and of the mirror, and the 
viscosity of the air absolutely. 
The Beam Mirror and its Attachments. 
One of the most important parts of the whole apparatus, and certainly the most 
difficult to arrive at in perfection, is the combination of beam and mirror which shall 
support the gold balls definitely in position, and shall, in its capacity as mirror, make 
it possible to determine their position with the greatest possible accuracy. In my 
