Meriwether Lewis purchased a “tangent screw quadrant” in Philadelphia for $22. In actuality, this instrument was an octant. In the journals of the expedition, Lewis refers to it as an octant and Clark refers to the same instrument as a quadrant. Lewis describes it as
A common Octant of 14 Inches radius, graduated to 20’, which by means of the nonius was divisible to 1’, half of this sum, or 30“ was perceptible by means of a micrometer. . . .What Lewis referred to as a nonius was actually a vernier scale; the micrometer was the vernier scale’s tangent screw.
Since the octant was normally used when the sun was high in the sky, Lewis and Clark performed back observations. When performing back observations with an artificial horizon, the operator of the octant stood facing the sun and looked through the peephole on the lower left frame to the sun’s image reflected by the artificial horizon. The index mirror reflected the sun’s image to the mirror on the lower left frame where it lined up with the horizon when the index arm was in the correct position. The reading on the scale represented the complement of double the altitude before corrections.
The octant on display, on loan from the Mariner’s Museum in Newport News, Virginia, is set up for back observations. The colored shade glasses have been placed near the lower horizon mirror, but they may also be placed in front of the upper horizon mirror. The shade glasses help reduce the brightness of the sun.
This octant was made by Gilbert & Coy of London about 1800. Although it is not the octant that Lewis and Clark carried with them on the expedition, it is similar to that octant in all its features, including size.