1660 Spiegelboog (reconstruction)

The 1660 Joost van Breen spiegelboog (mirror-staff) reconstruction.
Figure 1: The 1660 Joost van Breen spiegelboog (mirror-staff) reconstruction.
After finishing the cross-staff replica I wanted to reconstruct a navigational instrument that no longer exists: the spiegelboog (mirror-staff). The spiegelboog was invented by Joost van Breen (he sometimes spelled his own name as "Joos', so without a 't') in the 17th century.1

On September 25th of 1660 he patented the instrument together with the work in which he described it (see figure 2).

The instrument has similar features as the cross-staff, but uses a mirror on a sliding horizon vane and two vanes on a fixed cross for observations of weak celestial bodies and the sun under hazy conditions. The vanes on the cross can be set at three different positions, simulating the three different vane lengths as known from the cross-staff. The shadow vane could also be used as a cross for very low altitude stars (see figure 10).

In addition to that the mirror can be removed (see figure 4), allowing to use the instrument as a shadow instrument, similar to the cross-staff when used in the Dutch fashion.

The spiegelboog (mirror-staff) sketch by Joost Van Breen
Figure 2: The spiegelboog (mirror-staff) sketch by Joost Van Breen
Although the instrument has been used for about 100 years by Dutch navigators, none of them survived. Initially they were dedicated instruments, but later vane sets were made available to turn a cross-staff into a spiegelboog. As none are left I made this reconstruction using the information Van Breen left in his book work Stiermans Gemack, which he published back in 1662.

After a year of research, building the instrument took me about two months. Materials used on this reproduction are ebony for the staff and pear wood for the vanes. In addition to that glass, copper and brass are used for the mirror, mirror-frame, pins and springs.

In total two reconstructions were made, the second of which (instrument no. 2) now is in the collection of the Het Scheepvaartmuseum, Amsterdam (inv.no. 2008.0299). On the 22nd of October 2005 I finally presented a paper on the Reconstruction of the spiegelboog at the 'Who needs scientific instruments?' conference at museum Boerhaave in Leiden, The Netherlands.

An article on the instrument has been published in bulletin 101 of the Scientific Instrument Society.1

Notes

[1]: For a full description of the instrument see: N. de Hilster, 'The Spiegelboog (mirror-staff): a reconstruction', in: Bulletin of the Scientific Instrument Society No. 101, (2006), pp.6-16.

If you have any questions and/or remarks please let me know.

The horizon vane of the spiegelboog (mirror-staff) with the mirror attached.
Figure 3: The horizon vane of the spiegelboog (mirror-staff) with the mirror attached.
 
When the mirror is removed the spiegelboog can be used as a shadow instrument.
Figure 4: When the mirror is removed the spiegelboog can be used as a shadow instrument.

The mirror of the spiegelboog is engraved with the inventor's and maker's names.
Figure 5: The mirror of the spiegelboog is engraved with the inventor's and maker's names.
 
The horizon vane is clamped against the staff using a similar spring as in the demi-cross.
Figure 6: The horizon vane is clamped against the staff using a similar spring as in the demi-cross.

The sight vane has a narrow slit visor to assist proper aiming.
Figure 7: The sight vane has a narrow slit visor to assist proper aiming.
 
The visor of the sight vane can be rotated away.
Figure 8: The visor of the sight vane can be rotated away.

The shadow vane of the spiegelboog (mirror-staff) at vane setting 3.
Figure 9: The shadow vane of the spiegelboog (mirror-staff) at vane setting 3.
 
The shadow vane doubles as a cross for forward observations.
Figure 10: The shadow vane doubles as a cross for forward observations.

The signature on the staff of the spiegelboog (mirror-staff).
Figure 11: The signature on the staff of the spiegelboog (mirror-staff).
 
A close-up of the adjustment screw of the spiegelboog (mirror-staff).
Figure 12: A close-up of the adjustment screw of the spiegelboog (mirror-staff).

Celestial Navigation Coastal Navigation Distance measurement
1580s Mariner's astrolabe 1590 Hood's cross-staff 1618 Demi-cross 1623 hoekboog 1660 spiegelboog 1661 Kronan cross-staff 1720 Hasebroek cross-staff 1734 Davis quadrant Early 19th c. ebony octant Late 19th c brass octant 1941 U.S. Navy quintant Hirado navigation set