On July 22, 1804, while at White Catfish Camp on the Missouri River, ten miles above the mouth of the Platte, Lewis took time to enter into his journal brief descriptions of the instruments he and Clark used in making celestial observations.
A common Octant of 14 Inches radius, graduated to 20', which by means of the nonius was devisible to 1', half of this sum, or 30" was perceptible by means of a micrometer. this instrument was prepared for both the fore and back observation; her error in the fore observation is 2°+, & in the back observation 2° 11' 40.3"+ at the time of our departure from the River Dubois untill the present moment, the sun's altitude at noon has been too great to be reached with my sextant, for this purpose I have therefore employed the Octant by the back observation. the degrees ' & " [minutes and seconds], recorded for the sun's altitude by the back observation express only the angle given by the graduated limb of the instrument at the time of observation, and are the complyment of the double Altitude of the sun's observed limb; if therefore the angle recorded be taken from 180° the remainder will be the double altitude of the observed object, or that which would be given by the fore observation with a reflecting surface.
Funded in part by a grant from the National Park Service, Challenge-Cost Share Program.