Journals of the
Lewis & Clark Expedition

1. A predecessor of the sextant, the octant, or "Hadley's quadrant," as it was also known, was part of Lewis's pre-expedition purchases at Philadelphia. Jackson (LLC), 1:69, 82, 96; Lewis's description of astronomical instruments, July 22, 1804; Wilford, 96–97; Bedini (TT), 476, fig. 34. (back)

2. The following calculations (largely crossed out) are found on document 2 in Clark's Field Notes (Osgood incorrectly prints the facsimile as document 3). The calculations are inserted here under the date given. They probably represent the preliminary calculations for the observations by Lewis entered in the Eastern Journal. Little is known of Clark's formal education, but like many army frontiersmen he knew the rudiments of basic surveying. There was virtually no formal training for army officers in the United States in his time—the West Point military academy was established in 1802—and it is doubtful if he acquired any such knowledge during his army career. Lewis received his instruction in taking celestial observations from Jefferson, Andrew Ellicott, and Robert Patterson, who drew up a handbook of instructions for Lewis's use, the Astronomy Notebook. As far as we know, Lewis was Clark's only instructor. While the subject is too technical for discussion here some terms used in the calculations may be clarified. Refraction, the bending of light rays by the atmosphere, causes a heavenly body to appear to be higher in altitude than it actually is and must be corrected for in calculation. Semidiameter must be allowed for in observations of the sun or moon; if the observation is based on the upper limb (top) or lower limb (bottom) of the body's disk, half the angular diameter of the body must be added or subtracted to obtain the position of the body's center. Parallax error is caused by the fact that the observer is standing on the earth's surface instead of its center, on which all navigational tables are based. Declination is the distance in degrees of the heavenly body north or south of the equinoctial, the celestial equator. Loos, 6; Steffen, 14–15, 45–46; Bedini (TT), 364–65; Wylie, 111–13, 156–57. (back)

3. In Randolph County, Illinois, a few miles below the Monroe County line, and now within Fort de Chartres State Park. The settlement at the fort was the French capital of Upper Louisiana until the territory east of the Mississippi was ceded to Britain in 1763. The British occupied the post in 1765, but when the river began to encroach on it, they abandoned and destroyed the structure in 1772. Those were the ruins Clark saw and noted on December 4. Illinois Guide, 495–96; Alvord, passim.; Saucier & Seineke, 199–227. (back)

4. In Ste. Genevieve County, Missouri, just below the town of that name. (back)