Chris,

You may find that you do agree with John.  I think the key word here was 
'physical', i.e. a pendulum which has a rotational moment of inertia which 
is significantly larger than ML^2 (L being the distance from the pivot to 
the center of mass).  If the difference is considerable you do, indeed, 
sacrifice sensitivity to linear motion (vs a 'simple' pendulum) to obtain 
the long period, as you described.

Brett

At 12:20 PM 3/23/2008 -0400, you wrote:
>In a message dated 23/03/2008, johnjan@........ writes:
>My understanding of a long period physical pendulum is that sensitivity to 
>ground motion is sacrificed in order to obtain the long period.
>Hi Ted,
>
>     That is NOT my understanding in general.
>
>     But you are using a nearly balanced pendulum, with a high moment of 
> inertia but a small C of G to suspension distance, correct? This type 
> have a long period but a LOUSY sensitivity to linear ground motion. They 
> are however, sensitive to ground rotation.
>
>     Longer period pendulums like the Lehman and the Romberg are very 
> sensitive to linear ground motion and make good seismometers.
>
>     Regards,
>
>     Chris Chapman



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