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Brad wrote, about his sextant

| The helical screw is mounted on a spring mounted hinge, which keeps the
screw in contact with the teeth milled into the back of the arc.
|
| When you press the release, the screw lifts off of the arc and you can
easily move the helical screw to another section of the arc.
|
| You could (with a lot of effort) use the adjusting knob to move the
vernier from one end of the arc to the other, but who would want to?

=================

What Brad has described above is the standard arrangement for engaging worm
with rack, which applies to every micrometer sextant, and to some Vernier
sextants; those with the "endless tangent screw" feature.

Other Vernier instruments used a short screw for fine positioning of the
arm, that usually worked against a spring, in a shoe which could be clamped
on to the arc in an appropriate spot, allowing a few degrees of adjustment
before the screw ran out of thread. To make big changes, to get the index
arm to roughly the right place, you freed the clamp and shifted the arm
without using the screw.

This arrangement could be frustrating if you are following an object as it's
continually rising or descending, as eventually the fine-adjustment runs out
of range, forcing you to slacken the clamp, readjust the screw to bring it
to mid-range again, then reclamp. Indeed, the standard adjusting screw and
clamp screw were often fiddly objects, which would be hard to work with
gloves on. I can see the particular advantage of the endless tangent in
those cold waters.

The endless tangent, and the standard micrometer arrangement, both avoid
this difficulty; they never run out of range. And just as Brad says, to make
big changes, you press a button or lever to disengage the worm from the
rack, which allows the arm to swing freely; then release, when the worm
clicks in to the nearest notch in the rack. It's far more convenient.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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