NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bill Morris
Date: 2010 Mar 13, 16:57 -0800
You are right. What the author said was:
"A small aberration is also perceived in many sextants, arising from the elasticity of the index, when propelled along the arch by the tangent screw; for in finishing the measurment of an angle, the index is not immediately at rest; but having been bent a little forward or backward, by the propulsion of the tangent screw, according as the force has been applied, it requires a little time to regain its natural equiform state. This aberration may be perceived in many sextants, by measuring the diameter of the sun, to obtain the index error; for after the edges of the suns disc are brought with the tangent screw exactly into contact; and after a little time viewing them again to verify the observation, they will be seen either a little open, or shut in with each other, according to the previous motion given to the index by the tangent screw. And this inaccuracy is often be perceived to be a want of precision in the eye of the observer; whereas it frequently results from the energy of the index to regain its equiformity."
p 165. Coaster's Guide. Baharie, A., London, 1844.
I have an octant made for Baharie by Crichton of London and find it free from want of equiformity when obtaining the index error by means of the sun... I suspect his problem arose from stick-slip at the index arm bearing, whose screw some people just cannot resist "firming up" when they find it apparently slack. Baharie was an instrument maker as well as being the proprietor of a school of navigation and maybe should have known about this.
Bill Morris
Pukenui
New Zealand
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