Hi John,
Not necessarily.  The geometry used for mirror mounts in optical bench 
work is that the fixed point is at the intersection of two lines at 
right angles to one another.  The two screws are then on each of the 
lines respectively.  That way the screws do not interact, i.e., they 
cause only orthogonal tilt on their particular axis.  So translating 
that to a seismometer, put the fixed pin near the pivot point of the 
boom and offside in the short dimension (perpendicular to the boom (if a 
  Lehman style, but similar rules for a vertical) -- probably in one 
corner) Place one screw way down the base on a line through the fixed 
pin and parallel to the boom.  This will become the leveling screw. 
Place the other screw on a line at right angles to the fixed point and 
the just placed screw (probably in the corner across the pivot.  This is 
now the tilt.  The only other trick is to put enough weight on the base 
so that the center-of-mass is within the triangle formed by the fixed 
point and two screws (or use a long, triangular shaped base where all 
the parts are within the triangle.  Mirror mounts typically use springs 
to hold the mounts together, rather than gravity)  So even if you use 
three screws, this geometry will make life much easier when adjusting as 
there won't be axis interactions when doing the final adjustments using 
only the two screws at the ends of the right angle. Refinements are to 
make the design fully kinematic by using a tetrahedral divot for the 
fixed point ball end to rest in, a tri-angular groove aligned along the 
line of the fixed point and the screw ball end, and finally a flat plate 
for the final screw ball end.  This constrains the whole fixture so it 
doesn't slide or twist on the resting base.
Charles Patton

John Lahr wrote:
> Hi Kay,
> 
> The problem with a fixed pin at one end is that to change to level in 
> the long direction, both of the other screws would need to be turned.  
> This would probably mess up the tilt in the short direction.
> 
> With three screws, the short-direction tilt and the long-direction tilt 
> can be adjusted independently.
> 
> Cheers,
> John
> 
> At 12:22 PM 4/2/2008, you wrote:
>> Folks,
>>
>> A quick question regarding the leveling screws for a vertical 
>> seismosmeter...
>>
>> If you look at the three leveling screws on the AS-1 seismometer at 
>> http://www.amateurseismologist.com/, 
>> you see that there are two screws on the end with the vertical support 
>> and one screw on the end near the coil.  It is obvious why only three 
>> screws are used.  But is their a reason for putting the single screw 
>> near the coil and not near the vertical support.  Also, wouldn't there 
>> be greater stability if the single screw was actually a fixed pin, 
>> leaving the other two to be adjustable screws?
>>
>> Just wondering before I drill the base in my next vertical seismometer.
>>
>> Thanks,
>> Kay Wyatt
>>
>>
> 
> 
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