Geoff,
The level of the arm is unimportant.  The key is the plane of the arc 
described by the movement of the center of gravity of the garden gate 
(nominally the weight on the arm but with heavy arms, significantly 
closer to the pivot than the center of the weight.  So a more useful 
measure is how close to vertical the pivots are, but this is also not 
exact as the pivot geometry enters into the calculation.  See:
http://www.myeclectic.info/SeismoPivots/seismopivots.htm
for a longer discussion on this subject along with formulas of the 
trajectory.
Regards,
Charles Patton

Geoffrey wrote:
> Hello PSN;
> 
> Just out of curiosity has any of you guys with
> those garden gate seismometers ever put one
> of these little plastic levels on the lever
> to see just how far off level it is at
> 10 or 20 seconds. I mean here like 1/2,
> 1/4 or so a bubble across the line
> if you understand what I mean here ?
> 
> Maybe it is not even crossing the line
> but you can see possibly its off level
> and just touching the reference line ?
> 
> They got these fish eye levels that just tell you
> the horizontal and I thought that that might help set
> up your period ??
> 
> Has anyone ever seen a cheap calibrated level
> that will tell you fractions of a degree
> off level ?
> 
> Metrology, I have always thought, would be
> a good profession to be in. :-)
> 
> Any comments ?
> 
> geoff
> 
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