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Frank is right.  The name of the Portuguese navigator, and inventor of that
sextant, was Coutinho, and not, as I mistranscribed, Continho.

Also he surmised- "It appears that there's a mirror in front of the horizon
glass at a 45 degree angle that lets the navigator view the two
perpendicular spirit levels which are presumably set horizontal beneath that
mirror. Does that sound about right?" Indeed it does; Frank has made a good
shot at understanding what's involved, as will be clear by taking a look at
Ifland's excellent text and diagrams, which I attach.

It looks as if there's  a vertical strip-view of the scene through the
centre-bit of the horizon mirror and the index mirror. On one side,
presumably,  is a straight view of the horizon, and on the other side, a
reflected view of the
spirit-levels.

It must have been a very tricky business getting the sensitivity of the
pitch-sensing mirror to exactly match the motion of the horizon, so that a
Sun or star image can be aligned alongside it. And a tricky business, too,
to arrange the "special objective" to allow the eye to focus both on the
body, at infinity, and the bubble, close-up. Presumably, it was some sort of
compound lens, cut from vertical slices.

There seems to be a reflector for getting some external light in, for
daytime illumination of the bubbles, through a slot below them. Presumably,
there was battery illumination at night, which must have been an early use
of battery lighting.

George

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

========================

TUBULAR SPIRIT-LEVEL SEXTANTS

ln 1921, in one of the more spectacular events in early aviation history,
AdmiraI Gago Coutinho, navigator, and Sacadura Cabral, pilot, flew eleven
and one-half hours nonstop from Cape Verde Islands to Rio de Janeiro.
Coutinho carried a spirit-level artificial-horizon sextant of his own
design, first tested in 1919, on this pioneering flight, although it has
been suggested that his celestial observations actually were made off the
natural horizon. The rights to development of the new sextant belonged to
the Portuguese navy, which contracted with the German firm C. Plath to
produce the instrument starting in 1926 (Figure 171). ln its final form, the
sextant featured two spirit levels placed at right angles to each other, one
to indicate the horizontal and the other to deal with sextant tilt. The two
levels were viewed through a system of mirrors placed beyond the horizon
glass. The sight was taken when the bubbles were centered in the two tubes.
A disadvantage of the Coutinho sextant was that the observer had to look
simultaneously in several places within the field of view-at each of the two
bubbles and at the image of the celestial body. A sketch of the Coutinho
sextant is shown in Figure 172.

The "System Gago Coutinho," as it came to be known, proved itself again on a
flight from Lisbon to Rio in 1927 aboard the flying boat Argus, with Captain
Jorge de Castilho serving as navigator. During this night flight over the
ocean, Castilho dispensed with radio-direction finding and relied solely on
the bubble sextant for navigation. Two years later, Captain Witteman
navigated the dirigible Graf Zeppelin around the world with a Coutinho
sextant by Plath. With this high-profile performance record, the new
instrument was the hit of the 1930 Berlin Air Show. It was used through the
1930s by many of the major airlines of the world...

Tubular spirit-Ievel artificial horizons, whether straight or curved, have
several serious limitations. Two tubes are required - one parallel to the
plane of the instrument to establish the horizontal and one at right angles
to deal with tilt of the plane of the instrument. ln the early designs, the
observer had to look at three places at once: at the reflected image of each
of the two bubbles and at the image of the celestial body reflecod in the
horizon glass. A special lens was required to keep the eye focused on the
bubbles at a few inches from the eye and at the same time, focused on the
celestial body at infinio distance. Despite these disadvantages, Admiral
Richard Byrd used a tubular spirit-Ievel artificial horizon mounted on a
conventional sextant to establish his position in his flight over the North
Pole in 1926. Brandis & Sons of Brooklyn produced a few of these
instruments, but they were never widely successful.

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