Ted,
    The vertical seismometer that I showed at the broadband conference in 2=
004
http://www.iris.edu/stations/seisWorkshop04/seisWorkshop.htm, which you ref=
erenced by
http://www.iris.washington.edu/stations/seisWorkshop04/PDF/CompoundVertical=
Seismometer.pdf
  operates on the principle of the archer's compound bow.  I published a pa=
per in 2000 concerned with the physics of the compound bow, titled "Archer'=
s compound bow, smart use of nonlinearity".  It is online at http://physics=
..mercer.edu/petepag/combow.html.
    Because of force reduction at full draw, due to the eccentrics (cams), =
the bow concept can be used to lengthen the period of a similarly operating=
 seismometer.  John Nelson asked about this possibility in a mailing to the=
 listserve in 2003, to which Chris Chapman responded
http://www.seismicnet.com/psnlist/030502_055148_1.html
    For additional pictures of the prototype that I built, you may go to my=
 webpage at
http://physics.mercer.edu/petepag/cvs.html
   The sensor employed in this prototype is one of my fully-differential ca=
pacitive forms, but other types (such as coil/magnet) could be used.  As Ch=
ris has frequently pointed out, if you were to use the Faraday law sensor, =
it is important that the coil (not the magnet) be the moving part.
    I never had time to pursue this concept the way I wanted to, but the da=
ta that was presented at the IRIS-sponsored conference (first reference abo=
ve) shows that it has promise--especially if you were to build one a lot bi=
gger.  I wanted to try this with an elastic element using carbon epoxy elem=
ents in some arrangement, but never got around to it.  Incidently, the firs=
t one of these ever built was by one of my students when I was at Texas Tec=
h University, around 1997.  It was much larger and used a coil spring (appr=
oximately LaCoste zero-length type) instead of the torsion wires of the pro=
totype, but I never got around to seriously using it, since I moved to Merc=
er University soon thereafter. =20
    You may be interested to know that Erhard Wielandt, who was at the conf=
erence, encouraged me to try and get amateur seismologists interested in us=
ing the ideas.  A number of the folks there were very curious about the ins=
trument, since it had been setup on a table and made quasi-functional. =20
    Randall=
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