NEWTONIAN CONSTANT OF GRAVITATION. 29 
1892, and though the wires have carried the balls ever since, they have not broken, 
stretched, or drawn out. The wire, as it left the draw plate, was free from kinks, 
and was not allowed to be bent afterwards. As it is stretched so severely, I have 
assumed that the centre of gravity of the suspended ball is vertically below the axis 
of the wire, measured at a point nearly 2 inches below its point of support. The 
actual horizontal distance between the axes of the wires at this level can be deter- 
mined with the optical compass, with an accuracy of yo,000 inch; and therefore I 
maintain that this, the most important of the geometrical determinations, is known 
with abundant accuracy. 
The Small Balls or Cylinders. 
Owing to the small size of the attracted masses, I was able to make them of pure 
gold, a metal which possesses all the advantages of lead, besides increased density 
and freedom from oxidation or corrosion. As the inside of the central tube of copper 
in which the gold balls move is polished and electro-gilt, and as they move about an 
axis which coincides with the axis of this tube, any attractions or disturbances due to 
difference of electrical potential after contact must be of the smallest order possible. 
The method of making the gold balls is somewhat similar to that followed in the case 
of the lead balls, the difference in procedure depending upon the nature of the metal 
and the reduced size. Mr. CoLeBroox made for me pairs of hardened steel bars, with 
ends ground out and polished to true spherical surfaces, each of them just under a 
hemisphere in extent of surface. These were made in pairs for spheres of diameter, 
2, °25,and 3 inch. The quantity of gold necessary for making a ball, plus a small 
excess for waste, was placed in a hollow in a piece of Bath brick or of prepared char- 
coal, and heated with an oxy-hydrogen jet with just enough oxygen to melt the gold 
until it had run down to a clean button. When this was allowed to cool by itself, 
the surface was drawn in by the contracting centre, and a cavity and dimple were 
formed. When, however, the flame was gradually withdrawn, or reduced in size, so 
that the metal solidified from below upwards, a solid button with a perfectly smooth 
surface was easily obtainable. The button so formed was placed in one of the hemi- 
spherical moulds, held in a vice, and covered with the other, which was then given a 
smart blow with a hammer. The gold was thus compressed to an almost spherical 
form with an equatorial rib. On being turned over through a right angle, the rib 
was compressed into the gold, leaving a projecting head on each side, and these were 
finally compressed into the gold after a further twist through a right angle. The 
gold was then annealed, and beaten in the moulds with gradually reduced blows, 
being turned between each blow. When the ball was made practically perfect, it was 
weighed, and brought down by a very fine file to within 1 milligram. of the ultimate 
weight. It was then annealed and gently beaten again with rapid light blows, being 
turned between each, until a highly polished perfect sphere was the result. Rubbing 
with wash-leather and a little rouge brought it to the required weight, when it was 
