The Artificial
Horizon
Most often, a box is used and filled with mercury. The mercury, of course, presents a level reflective surface. To prevent the surface from being disturbed by wind, a roof-shaped glass cover made of optically flat glass is often added. Other liquids may be substituted. Frémont's mentor Joseph N. Nicollet reported in 1833 that when the mercury had been stolen "by some cunning fellow" from his horizon, he went on with his observations substituting "sugar water." My 1853 Bowditch suggest "Barbados tar," or "very clear molasses." When making the reductions, the double altitude is divided by two. An advantage is that very accurate determinations may be made. This cannot be done at sea.
From his expedition to the Rocky Mountains in 1842 Frémont had learned the superiority of the pocket chronometer over the boxed and gimbaled marine-type chronometer for rough traveling in extreme conditions. The pocket chronometer did not have nearly the potential for accurate rate maintenance, but it would keep running. He could make time checks on the rate by solar observations and observations of the satellites of Jupiter.
Related pages:  A Short Bibliography of Essential Reading: Bourne, William, A Regiment For the
Sea (1574), Cambridge, 1963.
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When
navigating at sea, the visible horizon serves as an
astronomical horizon, but this is usually not the case on
land. It is necessary to use an artificial horizon. In it's
simplest form, this might be a puddle of water. The
reflection of the object to be sighted is located in the
puddle, and that reflection and the actual object are
brought together in the sextant sights. The result is a
double altitude.
But
the scale of the sextant takes in only 120 degrees (2 x 60).
If the summer sun at noon is above 60 degrees, which it
frequently is, the double elevation would be off the scale
of the sextant. What then?
The
instrument commonly used in the 19th century on these
occasions, or whenever the angular distance between two
objects was greater than 120 degrees, was the Reflecting
Circle, or Circle of horizons. Having a scale that is a full
circle, it is calibrated to 720 degrees (2 x 360).
Frémont
carried the following instruments: a refracting telescope by
Frauenhofer (150x) for observations of the satellites of
Jupiter in time determinations); a reflecting circle by
Gambey; two sextants by Troughton; two pocket chronometers
by Goffe and by Brockbank; one syphon barometer by Bunten;
one cistern barometer by Freye & Shaw; six thermometers,
a spyglass, and a number of small compasses. Vouchered
expenses also include several purchases of mercury. It was
used in the artificial horizon, and also enabled him to make
a remarkable field repair of his barometer in 1842.
