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    Re: The development of bubble sextants
    From: Gary LaPook
    Date: 2009 Aug 20, 11:33 +0200
    The Bush patent is intriguing since is needs no power and, though complex, is is much simpler than the devices actually made to perform its function, so there must be a problem with the concept somewhere. In the same era as the Bush patent, Sperry was creating gyro instruments for airplanes and, eventually, the autopilot. These all used gyroscopes to sense level. Then later, in the 1950s, inertial navigations systems were invented for use by our submerged nuclear subs which used gyroscopes, not the Bush concept. Then in the '60 inertial navigation systems were created for the Boeing 747 which finally got the sextant mounts removed from transocean aircraft. Needless to say, the B-747 used a plethora of gyroscopes since it had three completely separate inertial nav systems for redundancy. Gyroscopes are still in all aircraft although the most sophisticated now use "ring gyroscopes" with no moving parts, only light running around a circular light pipe. Gyroscopes also went to the moon.

    It is also quite likely that the sophistication of the Bush device wasn't worth the effort. The common (read cheaper) bubble sextants as well as the more expensive and less common pendulous mirror sextants provided the all the  accuracy that was practical and needed so there was no reason to "polish the cannonball" as we say in the artillery.

    I don't know what you are trying to accomplish. Many inexpensive bubble sextants are available on ebay and provide one or two minute accuracy when used on the ground and should be accurate enough to find your backyard. If you need greater accuracy then use a marine sextant with a bowl of mercury (gasp...from those worried about  HAZMAT) or some other less useful liquid to establish level and you will get accuracies well below one minute. If you need maximum accuracy then get a mercury theodolite (gasp...mercury again) and you will get accuracies measured in seconds.

    Or,  just buy a hundred dollar GPS.


    Hanno Ix wrote:

    You wrote below:

    But now, with the shunting engine, start moving the flatcar backwards and
    forwards. The pivoted mirror will respond to the acceleration forces, and it
    will start swinging. To reduce that effect, you could add a sort of dumbell
    arrangement, like a see-saw, with its centre of gravity aligned with the
    pivot. tThat would act like a trapeze artist's balance pole, to increase the
    moment of inertia. Now you have a system with a long period. Its response to
    short-period perturbations will be reduced by the enhanced inertia. You can
    make the period as long as you like by increasing the dumbell weights, or
    decreasing the weight and spacing of the ballast.

    And you are totally right - so far. However: you cannot accelerate forever in the same direction, with the same amount. Pretty soon you will reach top speed and then there will be no more acceleration - other than gravity! The mirror will dampen its oscillations and go back to equilibrium - i.e the horizontal position - while the train keeps going with constant speed from then on. Until, of course, the train slows down with neg. acceleration - causing the opposite movement of the mirror.

    The trick is to find out what in the particular application the biggest acceleration is, and how long it can possibly last. I bet shipbuilders can give us rather good estimations of the roll/pitch frequencies of vessels from size boat to size oil-tanker. I guess, as I said before, we are talking about 1/10 Hz to 1 Hz with exceptions on both sides.

    If the cut-off frequency of the lowpass is low enough, say 1/10 of the roll/pitch frequency, the acceleration will barely have caused the mirror to move before accelerations in the opposite direction occurs, thereby cancelling almost the first one.

    Howver, there is one more problem: In our design the mirror will be deflected while the acceleration goes on. If the engine not simply accelerates, but it does so differently with each expansion of the steam, you will see the jerks in the mirror movement. Solution: put another lowpass filter in front of the first one. Ugly, but easy to understand: let the pivot of your pendulum slide softly on a rail that is attached to the car and oriented in the direction of the accelarations, here front to back. Connect the pivot with a soft spring to the point where is was fastened to before. Make sure this secondary system is sufficently damped.

    Now, short jerks cannot be transfered to the pendulum anymore, only those slower accelerations will be transferred that really affect the overall speed of the train. The mirror will move much slower and in a mode commensurable with the train movement, and it will, in average, maintain now its pointer much closer to the gravity vector.

    I think I correctly described the line of thought that leads from the original spirit level to the Bush patent.

    Please, forgive my lapses of precision in my explanations. I hope you can follow the argument nevertheless.



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