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Brad Morris wrote:

> Was that common practice with the cross staff? To align to the center?  
>   
It is difficult to speak about "...common practice with the 
cross-staff...". The instrument went through several significant 
developments over the years, making it increasingly accurate time after 
time. First users probably could measure the sun's altitude with an 
accuracy between 20 and 30 arcminutes, while with a fully developed 
cross-staff one could easily achieve just a few arcminutes. Tests I have 
done with reconstructed 17th century navigational instruments have shown 
this, have been published in SIS Bulletin no. 90 in 2006 and can also be 
found on my web site www.dehilster.info (the SIS article can be 
downloaded from there as well).

To sum it up basically there were four different ways to use the 
cross-staff while observing the sun:

1) in forward use measuring the centre of the sun
2) in forward use measuring the upper limb of the sun
3) in backward use according to the general fashion
4) in backward use according to the Dutch fashion

But to complicate things even further the former two methods were used 
with and without a smoked or coloured glass to protect the eyes, while 
the latter two methods were known with one or two aperture discs (these 
were mounted at the ends of the cross and created a small aperture for 
the sun to pass through and could be used as a peep sight, for pictures 
see again my web site). Then in backward use one could have a 
cross-staff with a bone at the horizon vane (this created a relatively 
large surface to project the light from the aperture on, also available 
from my web site) or with just a nail protruding from it and finally it 
has even been suggested to use a lens similar to the ones used with 
Davis Quadrants after about 1670. So in total there were at least a 
dozen different ways to observe the sun with a cross-staff.

In the earliest descriptions by the Portuguese and Spanish the 
cross-staff was only used for Polaris. By 1574 the English used it to 
measure the sun and Bourne described one could either observe the upper 
limb or its centre, but only when its altitude was below 50 degrees. 
Above 50 degrees one was advised to use the mariner's ring (astrolabe). 
Bourne also advises the use of a coloured piece of glass and to subtract 
15 arcminutes when observing the upper limb.

 From 1595 on the cross-staff was also used backwards a method suggested 
by John Davis. This could be done in either the general or the Dutch 
fashion. In the general fashion the horizon vane was fixated near the 
eye-end of the staff (this is where the scales start) while the transom 
would slide up and down the staff to fit the space between sun and 
horizon. In the Dutch fashion the transom would be fixated, while the 
horizon vane would be slid up and down the staff. The advantage of the 
Dutch fashion was that it was easier to to and that there was less parallax.

By 1659 the aperture disc was introduced and used as a peep sight only, 
while by 1749 two apertures were used, one as a peep sight and one to 
project a small beam of light onto the horizon vane. It is this latter 
method that gave the instrument its accuracy of just a few arcminutes.

> Was the advantage of the upper limb alignment of the Davis Quadrant the 
death knell for the cross staff? 
No, certainly not, at least not in the Netherlands. From VOC archives it 
is known that by 1731 the Davis Quadrant was not longer supplied to 
their vessels simply because of its inaccuracy, while the Octant was not 
yet introduced either (this was officially done in 1748). So the main 
instrument used on board between 1731 and 1748 was the cross-staff. In 
addition to that one had the the availability over the less often used 
spiegelboog (mirror-staff), which was the first ever reflecting 
instrument invented by Joost van Breen in 1660 (5 years before Hooke 
invented his single reflecting instrument) and in use with the VOC since 
1670.
>  Yes, the Davis Quadrant did save the mariner's eyes, but the cross staff 
could be used with glass (glasses if we are to believe Bourne) which would 
also preserve the mariner's eye.
And in the backward manner as described above.
> In the "Practical Navigator", John Hamilton Moore, 1826; there are no 
representations what so ever regarding the cross staff or the Davis Quadrant, 
jumping immediately to Hadley's Quadrant and Sextant.  When did advice 
regarding the use of the cross staff disappear from instructional manuals.  
Was the appearance of the cross staff in the "Mariner's Compass Rectified" in 
1780 an anachronism, there just for completeness?  Perhaps this edition of 
the "Mariner's Compass Rectified" itself was simply an anachronism, 
representing a tired reprint of older instruction?
>
>   
This part is dealt with in "The Cross-staff, History and Development of 
a Navigational Instrument" by W.F.J. M�rzer Bruyns (Zutphen, 1994), p. 
34 (some of the facts stated above are also derived from this work, 
others from his "Schip Recht Door Zee" and from my own research). He 
states that in the period 1700-1750 the instrument was the one "...most 
used  ... for altitude measurement by Dutch seaman and probably also by 
others" and that "After 1750, due to the acceptance of the octant, the 
use ... receded. John Robertson, in 1754 ...mentioned only the Davis 
Quadrant and octant. In 1768 the Rotterdam teacher of navigation and 
examiner of the officers of the Rotterdam Admiralty, Antoni Struick, 
described the octant only because ... the cross staff should never be 
used because it was extremely inaccurate". According to M�rzer Bruyns 
after this the cross-staff was no longer listed in any first edition of 
navigational manuals.


Nicol�s

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