1. The artificial horizon was a device that provided a reflecting surface that was always parallel to the natural horizon and was used when the latter was obscured. It was employed primarily to measure the angle of elevation of heavenly bodies. The expedition had three, one using water, another a glass pane mounted on a wooden ball, and the third a mirror attached to a panel. A tripod stand was provided, and a spirit level was essential for adjusting the device. Bedini (TT), 339–40, 461–62; Lewis's List and Supplies [June 30, 1803], Jackson (LLC), 1:69, 82, 96; Lewis's description of astronomical instruments, July 22, 1804; Bedini (SILC), 55, 58–59, 63.