Medieval Scientific and Philosophical Instruments
(Spring 2000)
Introduction:
For this workshop I have brought instructions and materials for
you to make three different instruments typical of the Medieval
period. But before we begin I would like to show you some instruments
I have made, and discuss briefly some of the techniques I have
used in making them, and relate how my methods compare to techniques
current in the late Medieval period. You will find additional
details about these instruments and techniques on my web-site.
I have brought a number of instruments that
I have made over the years for your inspection: a navigational
astrolabe, a planispheric astrolabe, and a torquetum (see the
instrument page on my
web-site). All of these instruments are fabricated from copper
and/or copper alloys. I did not bring my cross-staff or armillary
sphere because they are currently on display at the HSU Natural
History Museum in their Millenium window display.
- Navigational Astrolabe:
This is the first instrument I made (c 1985). The body was cut
out from a bronze plaque and is about 1/4" thick, as compared
to authentic examples which are more like 3/4" thick in
order to give them stability on a rocking ship in the wind. In
this early example I made graduations using a protractor with
a hole cut at the center and a steel rule firmly attached along
the 0° line. I then scribed a vertical line on the astrolabe
blank and attached the protractor with a bolt through a hole
in the astrolabe's center. I could then scribe lines with the
rule, aligning the degree marks on the protractor sequential
with the vertical line. This was a simple, reasonably accurate,
and inexpensive mode of graduation. Though I have never seen
this method noted in the literature, it certainly would not be
impossible in ancient times. Who knows, since we rarely have
records from Medieval craftsmen, a variation on it may have been
used by someone.
- Planispheric Astrolabe:
This instrument is made of copper and brass, both metals commonly
used for instruments in Medieval times. The blanks were cut from
14 gauge sheet with a power saber (or jig) saw, and then trued
up to round with a file on a wood lathe. Careful,
this can be quite dangerous, as copper alloys are "sticky"
and can grab, making the file into a deadly projectile!. And,
of course, the spinning blank can act as a blade and slice a
hand if you slip. For graduating this instrument I used a
rotary table (commonly found in machine shops, though mine was
modified from a micrometer actuated transit) with a fixed rule
held above it. The metal blank was then centered and fixed on
the table and lines scribed with a knife held against the rule.
This enables slightly cleaner and deeper graduations than the
scriber. This method of graduation would not have been used until
the late 18th century with the advent of dividing engines. To
cut out the rete I used a jewelers saw and then cleaned up with
needle files. Note that this is an unlikely scenario for early
craftsmen. High quality steel required to make the fine blades
of jeweler's saws was not available until near the 19th century.
Thus the rete's of astrolabes were probably made by drilling
holes and then cutting and shaping with files.
- Torquetum: The various circles were cut from brass stock after
lay-out with wing-dividers sharpened to round points - you don't
want any "knife-edge" as they will then tend to pull
instead of cutting a clean arc (trammel points will work equally
well, but you need something that can be locked at a dimension).
The dividers were then used to layout the radius of the circle
to give arcs at 60° intervals (note that this is theoretically
absolutely accurate, the precision attained in laying out these
divisions is limited only by your tools and your skill!). Additional
divisions at 30° intervals were then accomplished by bisection.
Bisection of the 30° intervals will then give 15° intervals,
however, at this point division by divider becomes more problematic.
This method of division was the method from ancient times through
the 18th century, and in fact was used for observatory instruments
through the 19th century. I did the remaining division using
my "dividing engine," an extension of the method described
for the planispheric astrolabe above.
- Projects. Three
simple projects were constructed during this workshop. These
projects are illustrated along with instructions for construction
and sample calculations via the links below:
-
- © R. Paselk
- Last modified 13 March 2000
Medieval Science
& Scientific Instruments