152 
DE. T. E. EOBINSON AND ME. T. GEUBB’S DESCRIPTION OE 
come into play, and we found that the doubling its actual driving-weight (200 pounds 
on a single line) accelerated its rate only It rings seconds. 
But the excellent action of the clock would be marred unless it were matched by a 
corresponding accuracy of the screw and sector through which it drives the telescope. 
These were finished with extreme care, and are believed to be as exact as many dividing 
engines. The means of obtaining an accurate screw are well known ; but it may be 
useful to describe the process of cutting the sector. 
[The arc of this, which is of gun-metal and 5-feet radius, after being duly fitted to the 
frame of the sector, had several faint arcs described near its edge concentric with the 
polar axis. It was then detached and transferred to a large wheel-cutting engine, where 
it was carefully centred by means of the faint arcs just mentioned, using a micrometer- 
microscope. The proposed run of the sector was two hours, or sixty teeth (the screw 
being intended to revolve once in two minutes). It was, however, purposely made long 
enough for seventy teeth, and so many radial lines were drawn on it, using the wheel- 
cutting engine as a dividing one. Subject to its errors, these divisions represent two 
minutes of time. 
The second step in the work was to take sixty-four of these divisions, and to determine 
their relative errors by the process of continual bisection, using a pair of micrometer- 
microscopes. The third step was to cut from the erroneous divisions, using the table of 
errors and a micrometer-microscope, a set of corrected divisions. 
The fourth step was to cut the teeth from these last divisions ; the cutting part of the 
engine being provided with a clamp and tangent-screw, and also a micrometer-micro- 
scope ; the last cutter bringing the teeth as nearly as possible to the shape required for the 
application of the endless screw. The gun-metal arc was then restored to its place in 
the instrument, and a racked screw of the calculated pitch being passed a few times 
through the whole extent of the teeth, they were brought to the precise shape required 
for working with the screw of the clock. The quantity of metal removed in this last 
process was barely perceptible. 
The telescope has nine Huyghenian eyepieces, ranging from 220 up to 1000. It has 
also a micrometer, whose wires are illuminated in a dark field, and which is also provided 
with fine steel bars as more fit for faint nebulas than wires. It is similar to a pair made 
many years ago by Mr. Grubb for the Armagh Cassegrain, and for the late Lord Posse. 
Plate VIII. fig. 32 is a down view of it; fig. 33 is mostly sectional, and fig. 34 shows the 
sliding-pieces or forks which slide in V-grooves planed in the under side of the micro- 
meter-box. The figures are half size. The micrometer-screws have twenty-five threads 
to the inch, and the number of their revolutions is read off from scales through A, A, 
glazed openings in the covering plate, fig. 32. 
In fig. 33, C is a section of the part which screws into the telescope ; to this the mi- 
crometer is attached by the ring D, so as to admit of being turned in any direction. 
E and F are rings connected by the bars G ; outside and around the lantern part formed 
by these bars, revolves a short tube H, to which is attached a conical tube I carrying the 
