In toiling away at the puzzle of the Nikumaroro sextant box I’ve learned that Brandis made several different types of sextants around the time that the hypothetical sextant with Brandis serial number 3500 and U.S. Naval Observatory number 1542 would have been manufactured. This is the first of several posts about these different sextant types. I’ll start by discussing Brandis surveying sextants.
Brandis Catalogue No. 20, which was published sometime between 1915 and 1917, lists as item number 105 a ‘U.S. Navy Surveying Sextant’, having a radius of 6 inches and providing a measurement precision of 30 arc-seconds.
Like all other Brandis sextants made at this time, U.S. Navy surveying sextants were vernier sextants. A vernier sextant has an vernier scale on its index arm that allows users to read the sextant’s arc scale to a high precision. Brandis Catalog No. 20 tells us that the vernier scale of its U.S. Navy Surveying Sextant allowed users to read the arc scale to a precision of 30 arc-seconds. The two-headed screw that can be seen at the end of the index arm is an adjustment screw commonly referred to as tangent screw. A user determining the angle of a celestial body above the horizon  first moves the index arm by hand until the celestial body’s image, as viewed though the sextant’s telescope, is approximately on the horizon. The user then anchors the index arm to the sextant’s limb with a clamp that is connected to the index arm via the tangent screw (this clamp can’t be seen in the catalog illustration). Once the clamp has been anchored to the limb, the position of the index arm is finely adjusted by turning the tangent screw to properly align the celestial body’s image with the horizon. Bill Morris’ book The Nautical Sextant describes the Brandis index arm adjustment mechanism more clearly than I have here and provides photos of all the critical components.
In ‘What do the Numbers 3500 and 1542 Tell Us? Part Three’, we saw evidence that Brandis made some 2400 sextants Brandis for the U.S. Navy during World War I, and that at least 1000 of them were surveying sextants. Based on how frequently surveying sextants appear among surviving examples of Brandis sextants I’ve seen , my guess is that the Brandis made closer to twice that number of surveying sextants for the Navy during the war. I should say that while I've seen many Brandis sextants that look to be a match for the surveying sextant featured in Brandis Catalog No. 20, I've seen one or two sextants that have some, but not all, the features of such a surveying sextant. I'll leave discussion of these 'outliers' to a later post.
Comments, corrections, additional relevant facts, differing
are always welcome (no one will be banned, blocked, or castigated for
offering differing opinions). Send correspondance to
 Brandis Catalogue No. 20 is undated, however is contains a
testimonial letter from a customer dated December 12, 1914, so the
earliest it could have been published was some time in 1915. The firm
name on the front page is ‘Brandis & Sons Mfg. Co.’; in the post
titled ‘What Do the Numbers 3500 and 1542 Mean? Part Three’ I concluded
that that some time in 1917 the firm name changed to ‘Brandis &
Sons, Inc.’. Therefore, I think Brandis Catalogue No. 20 must have been
printed some time between 1915 and 1917.
 I believe the ‘angle above the horizon’ of a celestial body should probably be called its ‘altitude’, but I’ll stick with the landlubber’s angle here and elesewhere.
 When I try to identify a Brandis sextant’s type, things I look for are its frame pattern, whether there is an U.S.N.O. inspection certificate stating its type, and how its arc and vernier scales are divided.