George,

Yes, I find that I am thinking only of vertical designs.  (Much less 
sensitive to local noise and ground tilting.)

As you imply, in non-feedback horizontals, you are using gravity 
instead of a spring.  In them, you must have at least a small amount 
of gravity for things to center themselves.

Brett

At 12:01 PM 3/1/2012, you wrote:
>Brett, Bob, and Geoff-
>
>At least in the Vertical direction. You have some chance of 
>balancing gravity out in the horizontal directions.
>
>George
>
>At 11:38 AM 2/29/2012, you wrote:
>>Bob & Geoff,
>>
>>I'd missed that part of the original question, and Bob is quite 
>>right.  Gravity (an acceleration) and the ground motion you want to 
>>see (which may also be viewed as acceleration) can't be separated 
>>from each other.  When you manage to cancel out the first one, you 
>>also end up canceling out the second.  Unfortunately, no way around it.
>>
>>Gravity is just a (huge) DC component of the same motion you want to measure.
>>
>>Brett


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