Instrument Construction
The instrument pictured was made in 1999 from brass and "mahogany" (an unknown tropical hardwood recovered from a pallet). The "mahogany" base measures 14 3/4 x 12 1/4 x 1/2 inches. The diameters of the brass circles for the crista, basilica, and tabula orbis signorum, are each 9 1/4 inches with a thickness of 1/8 inch. The brass hour circle on the tabula equinoctialis is 10 1/4 inches in diameter. The overall height of the instrument when set up for ecliptic observations at about 45° latitude is about 40 cm. All of the metal parts were fabricated from sheet or round stock with the exception of three commercial 5/16" brass washers. Nearly all of the materials were salvage, e.g. the heavy brass sheet was obtained as 4" diameter pipe, cut, annealed, and flattened with hammers, while the light sheet was from a door kick plate. The various rods and turned items were also fabricated from salvaged red or yellow round stock picked up at garage sales etc. Some construction notes follow.
Base
(tabula orizontis and tabula equinoctialis): The base was made from three 4 x 1/2" pallet slats, edge-planed
and glued up with aliphatic glue using 1/4" dowel pins for
alignment. The resulting plank was then hand-planed flat on both
sides and sanded smooth with a random-orbital sander. It was then
cut into two pieces: a 15" x 12 1/4" piece for the tabula
orizontis and an 11 1/4" square for the tabula equinoctialis.
The
equatorial circle on the tabula equinoctialis was graduated
on a thin (< 1/16") red brass circle. This circle was
first laid out with a large wing divider, then cut from red brass
sheet (from a door kick-plate) with a band saw, and filed to shape.
The dividers were then used to scribe three circles creating two
bands for graduating in hours and degrees, respectively. Next
30° and 15° intervals were marked off on the middle circle
with the dividers to serve as checks in the final graduation process
(two photo essays describe classic hand graduation of a quadrant and graduation of an astrolabe using my dividing engine).
The outer band was then graduated at one hour (15°) intervals
and numbered with 1/8" stamps (1-12 & 1-12), while the
inner band was graduated at 1° intervals, numbering every
10° (0-360°) with 1/16" stamps. The finished circle
was then centered on the tabula equinoctialis and fixed
with four bronze nails. A 5/16" diameter hole was then drilled
through both the circle and tabula for the pin which will
hold the basilica.
Basilica/tabula
orbis signorum: The circles for the basilica and
tabula were each scribed on sheets of heavy (1/8" thick)
yellow brass with large wing dividers, then cut out with a bandsaw
using a blade for non-ferrous metals. The edges where then finished
with a file. Dividers and a straightedge were used to scribe graduations
at 15° intervals around the outer edge of the basilica as noted above. These graduations were left unlabeled. The tabula
orbis signorem (top view at right) was scribed with five circles
giving, from the outside in, two narrow bands for division into
single degree and five degree intervals (0, 5, ..30 for each zodiac
sign), respectively, then a wide band labeled with the names of
the zodiac signs (and with room to add symbols or icons later),
then another narrow band divided at five degree intervals and
labeled inside the circle every 30°, going eastward from zero
(at the first point of Aries) to 360°.
The
folding mechanism is based on two parallel brass strips (1/8"
x 3/8" cross-section x 7" long) silver soldered onto
the bottom of the tabula (after its graduation) parallel
to the Cancer-Capricorn meridian and spaced 4" apart. (Due
to the large mass of brass involved, a large torch was necessary
to achieve sufficient heat, even with "extra-easy" silver
solder.) Holes (1/8" dia) were drilled through the ends of
both strips. On the end toward Capricorn 1/8" dia pins attached
the strips to a brass block (1/2" x 1/4" cross-section)
screwed onto the basilica to make a hinge. (The block was
attached with screws to allow disassembly/reassembly in trying
different arrangements.) On the end toward Cancer a brass U-bracket
fabricated from 1/8" x 1/2" brass stock was riveted
to the basilica. A brass "H" was then fabricated
from two strips of 1/16" x 3/8" of proper length to
support the tabula at 23.5° when perpendicular to the basilica and a 4" length of 3/16" round stock
soldered into holes in the strips to make the "H." The
"H" was then attached to the parallel strips on the tabula using a length of 1/8" rod through holes in
the ends to make a hinge. A second rod with a turned handle of
1/4" brass soldered on one end is used to attach the "H"
to the U-bracket to hold the tabula in the ecliptic position,
or alternatively this rod fits through a second set of holes in
the parallel strips and the holes in the U-bracket to lock the tabula in the flattened position.
A note on the folding arrangements: Hinging the tabula to the basilica to allow the tabula to reside in the plane of the basilica or at 23.5° relative to this plane (the angle of the ecliptic to the equator) turned out to be one of the more challenging design problems of the instrument. Initially I designed a simple, reinforcing hinge with a folding prop which allowed the tabula to be raised to 23.5° or alternately folded flat, both positions maintained by the force of gravity. However, in testing the instrument for the low latitudes found in the contiguous United States (25-48°) or in particular at my home in Arcata, California (42°) it was found that the instrument was unstable. The center of gravity of the tabula and crista etc. caused the assembly to open up when the equatorial and ecliptic angles were additive, and in fact for lower latitudes the assembly was unstable even with the ecliptic angle closed. Thus it was necessary to redesign the prop as a locking assembly. With the locking device it turned out that at low latitudes the tabula equinoctialis was also unstable when the two angles were additive, and thus the stylus also had to be designed as a locking device. It is apparent from the photograph of the torquetum of Martin Bylica1 and the period diagrams of torquetums that this was not a consideration in their construction. This is a result of their intended use at the higher latitudes of northern Europe, a situation I found to be true of my instrument as well - at latitudes of 50° and above locking is not required.
Crista/Turnus:The
outline circle for the crista was scribed on a rectangular
sheet of heavy (1/8" thick) yellow brass with large wing
dividers. Additional circles were scribed at 3/16", 3/8",
3/4", and 1 3/4" in from the edge. The sigmoid curves
of the neck were also laid out with dividers set to large radii.
The resultant head and neck was then cut out with a bandsaw using
a blade for non-ferrous metals, using multiple cuts to follow
the complex curves of the neck. The edges where then shaped with
the roller portion of a belt sander and finished with flat and
curved files. Heavy lines were scribed across the entire diameter.formed
between the outer diameter of the crista and the outermost
circle was then divided at 5° intervals and the band formed
by the two outermost scribed circle was divided at 1° intervals
with a 'dividing engine.' The divisions were numbered at 10°
intervals in quarters with the horizontal axis at 0°, 0°
and the vertical axis 90°, 90°.
The bottom of the crista was carefully squared off perpendicular to the vertical axis, filed flat and square, and then silver-soldered to a 2 1/2" square of 1/8" brass plate. Four decorative holes were also drilled and countersunk in the neck. Finally, two twisted strips of 1/8" brass were formed into diagonal braces and riveted with copper rivets (home-made from 12 gauge electrical wire) onto the plate and the neck to provide additional support.
The Turnus was cut from a 4" x 9 1/2" rectangle of 16 gauge yellow brass. First a 4" diameter circle was laid out on the with a dividers. A straight line was then carefully scribed through the center point along the long axis - line is critical as it will form the rule edges of the alidade for measuring angles. Two parallel lines were then laid out at 5/8" on either side of the axis to define the outside rule edges of the turnus. A small dividers was then used to layout small arcs to create an ogee at each end of the turnus. The turnus was next cut out with the bandsaw as above and carefully finished with files. The rule edges required particular attention to bring them into precise alignment with the axis of the turnus as initially laid out. Last, a bevel was filed on each rule edge to make reading graduations easier, and to improve the appearance. Two 1 1/4" x 1" rectangles of 1/8" yellow brass plate were next prepared for sights. A line was carefully scribed down the center of each parallel to the 1" edge. A 1/16" hole was then drilled 3/4" from the bottom carefully centered on this line, and a triangular notch was filed on the top exactly centered on the line. Each plates was then clamped with the line exactly aligned with the rule edge of the turnus and in contact with an end ogee and silver soldered. A 5/16" hole was now drilled through the exact center of the turnus for a pin to attach it to the basilica.
Finally,
four 1/8" diameter holes were drilled in the turnus and the base plate of the crista such that the crista would be centered and aligned with the rule edges of the turnus.
The holes were counter-sunk on the bottom of the turnus and the top of the crista base to allow expansion of rivet
heads. Four 1 3/8" lengths of 3/8" brass rod were then
prepared, and a 1/8" diameter hole bored through the length
of each. Four 1 9/16" lengths of 1/8" brass rod were
next cut for use as rivets, placed in the aligned turnus,
rods and crista and flattened out on each end with a small
ball-pein hammer.
Alidada circuli magna:The alidada was laid out and fabricated just like the turnus above, but with a smaller central circle. The sights of the alidada were based on the same dimensions as for the turnus, except that 1" extensions were laid out on the bottom of each to give arms for suspending the semis.
Semis: For the semis a 7 1/2" dia semicircle was laid out on 16 gauge red brass sheet and cut out with a bandsaw as above. A series of circles was then scribed with dividers to form 1/4", 1/2" and 1" bands for graduations at 1°, 5°, and 10° intervals. The graduations were then numbered 90° - 0° - 90° at the 10° graduations in the 5° band. Small brass pins were then silver soldered at the two ends of the horizontal (90° - 90°) edge for suspension from the alidada sights. A small brass plumb bob, turned from 1/2" yellow brass round stock, was suspended from the central hole to finish the zenith indicator.
Cabinet
Construction
In
the Spring of 2000 I determined to construct a cabinet for the
storage and transport of the Torquetum. The walls of the cabinet
were cut from 3/4" rich-grained red "Philipine Mahogony"
recycled from speaker cabinets I had made in 1969. The corners
are finger-jointed (box-jointed) for strength. The top and bottom
were cut from high quality 1/4" Teak veneer paneling recovered
from an LA law office by my brother's construction firm. The interior
pine mounts were all cut from 3/4" pine recovered from a
"pine-box coffin" I had built years ago as a prop for
a retirement party. The various pine pieces are cut to accomodate
and fit the crista, semis, and alidada such that
no movement is possible once the various tabs are turned into
place. The base of the torquetum lies on top of the pine frame
and is held in place by three heavy bronze brackets modified from
some brackets salvaged from discarded physics apparatus. The interior
lid of the cabinet is lined with a fitted bit of 1/4" polyethylene
packing foam wrapped with red velvet left over from one of my
wife's costume projects. The carry-handle for the cabinet is made
from two modified bronze brackets drilled to fit the knocker of
a salvaged brass door knocker. The hinges and latch are quite
standard, of the sort available at any US hardware store. The
cabinet with the torquetum on display is shown below:
