Hanno included 3 attachments with his last message, but without accompanying
text.
First, as a general request, can I ask that when contributors provide
pointers or links for us to look at, a few words are added to the email text
to say what's likely to be found there, so only those with an interest will
be bothered to look.
In my case, Hanno's attachments were rather
rewarding. I found three
diagrams from patent no 1912358 of 1929, by Vannevar Bush, a name that
Americans of a certain age may well recognise. It can be readily located,
from that number, within Google Patents. The patent referred to the use of a
mirror as an artificial horizon, and a proposal for stabilising it against
accelerations. It included several pages of text, omitted by Hanno, without
which those diagrams would make little sense.
An amazing proposal, which I greatly doubt could ever have been constructed.
It shows a horizontal mirror suspended on springs within a liquid-filled
transparent sphere, that floated on a bath of mercury within another
transparent sphere. Clearly, the author relished complication. It put me in
mind of those Eastern cosmologies in which the Earth was held by a comely
maiden, who stood on the back of an elephant, which was then supported by
the shell of a giant
turtle, which swam in the ocean, and so on, combined
with the concentric crystal spheres that carry the planets in the Ptolemaic
universe. All with the idea of decoupling the mirror from the accelerations
that it was being subjected to.
Not for the faint-hearted to attempt constructing, then. It seems to have
got no further, which is unsurprising.
George.
contact George Huxtable, at
george@hux.me.ukor at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Hanno Ix" <
hannoix@sbcglobal.net>
To: <
navlist@fer3.com>
Sent: Tuesday,
August 18, 2009 4:57 AM
Subject: [NavList 9559] Re: The development of bubble sextants
Gentlemen:
Please refer to:
US Patent 1,912,358
V. Bush: Apparatus for establishing an artificial datum
Filed April 8, 1928
The inventor means an artificial horizon. Please refer to the attachments
for the drawings.
He has implemented a mechanical lowpass filter. We have discussed LP's
before. Better yet: This filter simultaneously works in 2 coordinates, roll
and pitch.
He also points out that the natural oscillating frequency of the mirror
should be as "low as possible" but he doesn't give any data. Since the roll
frequency of ships is rather low to begin with (judging from you-tube:
about. 0.1 Hz = 1 period per 10 sec for an oil-tanker ) the filter has to
have a cut-off frequency of about 0.01Hz which amounts to 1 period per 100
sec. The filter also needs to be sufficiently
damped. The reason is
simply avoiding resonant oscillations of the filter/mirror in response to
the ship's movements. To build an LP of this kind is the challenge!
The inventor has made every effort to decouple the housing of the mirror
from the body of the sextant. So to speak, he created something like a
bubble level floating within bubble level.
With a lowpass of this kind, influences from pitch/roll would be reduced by
a factor of 100, possibly more. So, a 10 degree roll would create 0.1 degree
(6 arc-min) deflection of the mirror. Smaller ship will have higher
roll/pitch frequencies than oil-tankers, maybe 0.5 Hz. Accordingly, on
smaller ships 1 arc-min oscillation of the mirror might be possible. Is that
error sufficiently low given the circumstances? It is certainly much less
than I saw once in a bubble sextant.
Perhaps most importantly, this patent points out how
to separate
accelerations of the sextant which are instantaneous from
gravitation which is constant in time.
When I made my proposal with the tubular ring the other day I had similar
ideas in mind, however I had not fully understood the interaction between
bubble and spirit.
Regards
