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[OM] Re: Level photography ;-), was [photo] Fall Mist

Subject: [OM] Re: Level photography ;-), was [photo] Fall Mist
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Sat, 20 Nov 2004 16:25:43 -0500
At 12:30 PM 11/20/04, Chris Barker wrote:

>But the inner ear's balance mechanism has not measure of gravity per
>se, by my understanding.  It is possible I have forgotten something
>over the years, but my aeromedical training (grand name for survival
>skils in the air) mentioned the semi-circular canals, the fluid that
>they contain and the tiny hairs which bend with the fluid's movement as
>the head moves ­ measured in 3 axes.  In this way you can feel movement
>from a position; once the fluid stops moving you have reached a new
>starting point for movement.   But there is nothing to tell you which
>way is up except for other cues: visual, external feel and of course
>the way your bloo's pressure feels in your head (if you are inverted
>the blood rushes to your head).  Tell me if I'm wrong, because I need
>to know these things! ;-)
>
>A diagram is at this url: http://oto.wustl.edu/cochlea/intro1.htm

You are quite correct that it is a form of tri-axial accelerometer and 
therefore cannot sense anything unless there is an acceleration.  Gravity 
is a constant force; i.e. it induces constant accleration in the absence of 
another equal, counteractive force.  In the _changing_ motion that most 
people experience simply moving about self-propelled on the Earth's 
surface, which requires acceleration to produce it, the inner ear 
accelerometer (as with all accelerometers) senses the vector sum of all the 
forces when they do not add to zero.  Moving about while making the 
photograph gives a sense of "up" and "down" . . . and therefore 
"horizontal" as orthogonal to that.  You are correct that it (the cochlea) 
is not, and cannot be used alone; its data must be compared to and 
integrated with that from other senses.

There is also data from musculo-skeletal system you mention . . . forces 
exerted by muscles and felt by bone joints and internal organs . . . and 
some portion of the muscle forces must counteract gravity lest we fall 
over.  The human brain, in all its magnificence, integrates the data from 
all sources to get a sense of "up" and "down" relative to local 
gravity.  If the cochlea "accelerometer" fails, physical balance (a 
counteraction to gravity to keep from falling over) gets all screwed up . . 
. the brain has extreme difficulty without it, left only with what the 
other senses provide.  That's why I cited the inner ear as "notable" but 
not exclusive (my Bad for not stating this outright).  When properly 
functioning it seems to enable considerable precision under most 
conditions.  I'm certain your aviation experience has been amply able to 
create conditions that cause complete sensory confusion about "up" and 
"down" as defined by local gravity, and is why certain cockpit 
instrumentation is crucial to remaining aloft in defiance of local gravity 
. . . to prevent having a Bad Day.

BTW, gravitational mass is equivalent to inertial mass, and therefore 
gravitational and inertial forces and accelerations are equivalent.  This 
is the principle postulate of General Relativity (as an extension of 
Special Relativity).

-- John Lind

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