NavList:

Dear Navlisters,

Earlier this week I asked Bill Morris a sextant question via email.  He responded in his customary genial and insightful way, concluding with a suggestion that posting the exchange on Navlist might draw out comments from "more experienced navigators."  A worthy suggestion this and I follow it forthwith.

Dear Bill,
I have just read your blog posts on sounding sextants.  I see that I have been using my little plastic Davis Mark 3 as a sounding sextant.   I find this plus my W&P three-arm protractor and a chart to be a useful system for navigating the lower Chesapeake Bay.  There are plenty of charted lighthouses, range lights, radio towers and so forth to offer frequent suitable targets.  I learned the technique from David Burch's Plastic Sextant book.  A question your blog posts leave open for me is what exactly is the advantage of the sounding sextant for this use over a regular nautical sextant?   No filters is a tiny benefit.  A magnifying scope would be another somewhat larger benefit.  Are there any others?  The thing I would like to see most is a pistol grip handle, as on some hand bearing compasses, that orients the sextant horizontally.  None of the ones I can find pictures of are like that.  Why not?  Is it because it would be unwieldy to set down? 

Best regards,
Patrick

Dear Patrick
 
Sounding sextants seem usually to have been quintants sometimes of small radius and often divided to only half a minute, since it would be pointless to have greater precision for the class of observation made. The index mirror was placed closer to the end of the arc to allow  better geometry when taking large angles. In ordinary sextants, by about 120 degrees, the view of the index mirror in the horizon mirror is reduced to a narrow slot, while, by placing the horizon mirror lower, a wider view is obtained right up to 140 degrees. I will make this point when I next edit the post.
 
Omission of the shades seemed to have saved about ten percent of the cost.
 
It does seem strange that even sextants designed specifically for taking horizontal angles generally did not have a suitable handle, and did not have the legs on the left hand face so they could be quickly set down without swapping hands. I have encountered a few that did have the legs on the left: one British, a French naval sextant and a WWII Japanese naval sextant, but none had provision for a handle at right angles to the frame. Part of the answer is perhaps that, unlike ordinary merchant seamen, the marine surveyor would have an assistant to "book" the results for him, so had no need to set the instrument down. Sometimes, it is easier to keep the less prominent object in view while "bringing down" the more prominent one (c.f. moon - star distance) in which case it may be more convenient to hold the sextant face down. In these circumstances, a pistol grip is scarcely more use than the ordinary one.
 
Many nineteenth century sextants did have provision for screwing in a handle (see Figure 19 in my June 2010 post). British seamen, merchant or naval, were encouraged to report on out-of-the-way places, including sketched surveys of harbours, latitutes and longitudes and sketches of the approaches, but of course, sextants were also sometimes sold to land surveyors who were more likely to take horizontal angles.
 
Thank you for your enquiry. You raise some interesting points. May I encourage you to post our discussion to NavList, as I am sure other points will occur to more experienced navigators than I am?
 
Kind regards
 
Bill

Since writing  Bill, I have discovered a picture of a sounding sextant that appears to have a grip of the sort I was asking for.  The problem of setting it down seems to have been addressed by making the case a sort of stand.  This sort of device with multiple telescopes that allow you to take all three angles at once does have an obvious advantage over a regular nautical sextant for the purpose of taking bearings.   What about drawbacks?   Has anyone used one of these?  Does anyone still make them?

Patrick