PROCEEDINGS OF SECTION A. 185 
small polished facets of agate, three in number. All the wedge 
faces, as also the base-plate face, have been produced by grind- 
ing together three pieces of similar dimensions, so as to obtain 
an approximately true plane; polishing has in no case been 
resorted to, as being likely to affect the truth of the surface 
obtained by grinding. 
The diameter of the screw is about 2 of an inch, with a 
pitch of 25 to the inch. It was cut, ground, and polished in 
the main, after the manner recommended by Rowlands. The 
nut, which alone travels, is adjustable, and transmits its motion 
to the wedges by means of two steel guide rods parallel to the 
screw axis, and connected with the nut by means of a swivel 
ring. The screw itself is maintained in position in its bear- 
ings by means of a thrust plate of polished sapphire at one end, 
and a powerful spring at the other. The head of the screw, 
which is about 4 in. in diameter, is divided into 360 degs., 
reading by means of a vernier to 6 minutes. It is provided 
with mechanism to ensure the accurate repetition of any given 
reading automatically. 
The vertical table, or, rather, slide, for carrying the plates 
or discs to be ruled upon, consists of a sliding rod, the inequali- 
ties of which have been removed by grinding. This travels in 
agate-faced V ways. The base of the rod rests upon a glass 
plate in contact with the upper surface of the wedge at three 
points only. This plate is held in position by a glass-faced 
vertical slide, with which it is in contact at two points, which 
are agate facets. 
The diamond carriage, which supports the mechanism neces- 
sary to afford some four movements of the diamond point 
around a central axis, as well as affording the essential traverse 
movements, rests upon a platform of heavy plate-glass highly 
polished, having five contact points, which in this case are 
facets :f ivory, which is found to afford the smoothest frictional 
movement, and requires no lubrication—a point of importance. 
The traverse motion, as well as the lowering and raising of the 
diamond point at the beginning and end of the lines, is im- 
parted and controlled by a lever. 
The selection, setting, and cutting action of the diamond is 
of the utmost importance. Nearly all the stones I have used 
have been obtained from Bingara, N.S.W. 
IT have tried Brazilian and West Indian diamonds, also the 
black diamond or carbonado, none of which appear to possess 
any advantage over those obtained from New South Wales. 
Some little time ago I received from Dr. Van Heurck, of Ant- 
werp, two stones which had been specially prepared, after the 
method of Nobert, by one of the most skilful diamond workers 
