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Many thanks for bringing to our attention the patent document on
Hughes and Baker�s centreless sextant. Their stated aim was to reduce
the size of the instrument and lower its cost of manufacture. They
suceeded in the former aim but presumably not in the latter, as it
seems to have sunk almost without trace. I wonder if anyone who has
seen the actual instrument noted whether it bore any serial number
that might give a clue as to whether there was any production run?

One of the main points of interest is the guiding of what the patent
document calls the index mirror carrier, e.  We would nowadays call it
the index arm. It is worth noting first that the index mirror is not
mounted where the axis of rotation would have been if there had been a
real rather than virtual axis. The mirror would be about half-way down
the index arm of a conventional sextant.

The carrier slides between two �curved and concentric� surfaces on the
upper and lower flanges, a and j. The lower curved surface, a, is vee-
shaped in section and is on the inside of the arc. A mating surface on
the carrier engages with it. The upper curved flange, j, has a slot
machined in it  and a bracket screwed to the carrier engages the
slot.

The micrometer assembly differs in detail from that of a modern
sextant, but the principle is the same: a micrometer screw engages
with a rack on the periphery of the limb and is held in engagement by
a spring. The screw can be swung down out of engagement with the rack
by overcoming the resistance of the spring, to allow rapid movement of
the index arm. In the case of this instrument, the spring seems to
have another function, to keep the sliding vee surfaces in contact and
maintaining concentricity.

George Huxtable wondered about the accuracy of guiding the index
mirror carrier. This could only be found out for certain by actual
trial, but I don�t suppose anyone is going to send  an example to New
Zealand for me to check the calibration. However, there is nothing
wrong with it in principle. Most lathes made since about 1930 have
prismatic guides that follow the narrow guide principle. That is to
say, the guiding surfaces should be narrow and as close as possible to
the point of application of the force that moves them.

The guides on the limb and carrier would have been relatively simple
to machine and the matching surfaces on the index mirror carrier would
not even have to match exactly.  If the angle of the vee on the
carrier was no greater than the angle of the vee on the limb it would
not rock at right angles to the plane of the instrument.  If the
radius of the vee on the carrier was no less than the radius on the
limb, the carrier would not  rock in the plane of the instrument. In
practice, it would not be difficult to get a very close match  I
guestimate that the vee guides are about 60 mm long and 15 mm wide,
adequate no doubt, but longer would have been better. The upper
flange, j, serves only to stop things falling apart when the
micrometer screw is disengaged and is a �keeper� rather than a guider.

The placing of the handle is unusual, but hold your closed hand up to
your face and you will see that for a lightweight instrument about 120
x 200 mm, it is placed entirely rationally.

The light path, shown in Figure 6 of the patent document is certainly
unusual in that the horizon is viewed beneath the horizon prism; and
that a prism is used instead of a mirror. The prism has an angle of
120 degrees and acts somewhat like a Porro prism except that there is
an extra internal reflection on the long face.

Hughes and Son showed a curious combination of following tradition
combined with some brilliant departures from tradition. One sees this
in the rather clunky, but entirely adequate design of Husun micrometer
sextants, contrasted with the ingenious and ergonomically advanced
design of the Mark IX series bubble sextants of WW II.  I for one
greatly regret that their centreless sextant never took off. I would
have enjoyed examining and possibly owning an example, something now
denied to me by its extreme rarity.

Bill Morris






On Jun 22, 3:10�pm, d walden  wrote:
> Like GOOGLE BOOKS, GOOGLE Patents may prove to be a very useful source of navigation information!
> H. A. HUGHES AND T. Y. BAKER.
> SEXTANT.
> APPLICATION FILED APR. 27, 1920.
> Patented July 5, 1921.
> US Pat 1,383,950
>
> �patents.png
> 57KViewDownload
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