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Hi Wolfgang

Thank you for helping me.  I only had access to the image from the book, not 
the relevant text.  I was able to see the parallax illustration but did not 
realize that he was using the moon as the celestial body.

In the 1780 edition of the "Mariner's Compass Rectified", Andrew Wakely; the 
mariner is still advised to align his cross staff with the center of the 
object.  The statement is "Then if you fee the Center of the Sun or Star at 
the upper end of the Crofs, and the Horizon at the lower end, the Crofs 
ftands as it ought".  The illustration linked (a jpeg this time) is 
un-mistake-ably the observation of the center of the sun.

The book then goes on to explain the Davis Quadrant and how one would observe 
the distance of the upper edge of the sun from the zenith, to which sum add 
16 m. the sun's semidiameter.  It seems that the Davis Quadrant had a an 
observational advantage over the cross staff.  No looking at the center of 
the sun, now look at the limb.

From the recommendations of "A Regiment for the Sea", Bourne; the mariner was 
advised to align the upper limb of the sun with the upper edge of the cross 
staff, but only when above 60 degrees.  Below 60 degrees, and the mariner is 
informed to align the center of the sun with the upper edge.  When I by 
chance saw the "Cosmographia", Apiani; image, it appeared to confirm the 
Bourne admonition and, as both were published within a few years of each 
other, it was a close contemporary.  Yet here, nearly two hundred years 
later, Wakely appears to offer nearly the same advice, align to the center of 
the sun.

Was that common practice with the cross staff? To align to the center?  Was 
the advantage of the upper limb alignment of the Davis Quadrant the death 
knell for the cross staff?  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.

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?

If the list would help to lend me a bit of clarity, I would appreciate it!

Best Regards
Brad







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