Lewis and Clark in Montana — a geologic perspective

Octant   Octants were used principally to determine latitude by measuring the angle between the sun and the horizon. The instrument had a simple “A” shape and its legs formed an angle of 45°, hence its name (from Latin octans = one-eighth). Because the octant’s two-mirror system used the principle of reflection, it could measure an angle of 90°; thus the instrument sometimes was also called a quadrant. Most were made of hard, tropical wood.

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The index mirror was attached to the index arm, usually 12 to 14 inches long. The fore-horizon glass (half mirror-half clear glass) was affixed to the leg nearest the reflecting surface of the index mirror. A peep site was located opposite the horizon glass on the other leg of the “A”.

The octant that Lewis and Clark used was also equipped with a back-horizon peep sight and mirror. This allowed them to measure the altitude of the sun when it was greater than 60° above the horizon (early April until late August along their route.)

This octant, purchased in Philadelphia for $22, is similar to the one used by Lewis and Clark.

Sextant   The sextant, invented about 1757, had the same “A-frame” configuration as the octant, but its circular arc spanned an angle of 60°, that is, a sixth (Latin sextant) of a circle, and its index arm usually was 9–10 inches long. With its two-mirror system the sextant could measure the angular distance between objects that were as much as about 120° apart. The sextant was made of brass and had a telescope for sighting.

Photo courtesy of Smithsonian Institute

Lewis and Clark used the sextant to measure the angular distance between the moon and sun or a star for longitude. They also used it for latitude when the sun's noon altitude was less than 60°. Although the sextant could measure an angle of up to 120°, the captains could not use the sextant to find the altitude of the sun when it was greater than 60° above the horizon because they needed to use an artificial horizon, which doubled the angle to be measured.

Artificial Horizons Navigators at sea could determine the altitude of a celestial body by measuring the angle between it and the true horizon. Inland away from the sea or a large body of water, however, one rarely has a true horizon. Therefore, on land, it generally is necessary to use an artificial horizon. Lewis and Clark carried three different types of them. Each design had its own advantages depending upon observing conditions. 

Drawing by Bob Bergantino, MBMG

On bright days when the temperature was above freezing and the sun was being observed, Lewis and Clark commonly used a tray filled with water as an artificial horizon; the water made a level reflecting surface. The captains, however, used artificial horizons made from leveled mirrors when the temperature was below freezing. They also used them when observing stars because the image reflected from the mirrors was more distinct than from water.

Robert Patterson, a Philadelphia mathematician and astronomer, had devised several types of artificial horizons using either a regular (single-coated) mirror or the double-coated index mirror from a sextant. The mirror was cemented onto a wooden ball and the ball was set in a wooden frame and adjusted by three screws used as legs. It was leveled with the aid of a spirit level.

Two-Pole Chain   English and American land surveyors of the 18th and early 19th century generally used a surveyor’s chain constructed of iron links, each 7.92 inches long. Surveying chains, called Gunters chains, normally were 66 feet long and were composed of 100 links. Eighty Gunters chains equaled one statute mile, and ten square chains (example: 5 chains x 2 chains) equaled one acre. A distance of 25 links (16½ feet) was called a “pole.” Clark used the expedition’s two-pole (50-link, 33-feet) chain primarily to measure a baseline when determining a river’s width by triangulation.

Chronometer  A chronometer is a precise watch used to determine the time at which an observation is made. Chronometers usually are set to keep Greenwich Mean Time (the time at 0° longitude) because almost all nautical tables are based on Greenwich Time. Lewis and Clark’s chronometer, however, showed Local Mean Time and, because it stopped on several occasions and ran erratically at others, they needed to make special observations of the sun to determine the true Local Mean Time.

Checking the chronometer’s time  No chronometer kept perfect time; they either lost or gained time, though usually at a highly uniform rate. The captains knew that their chronometer lost time, but unlike most chronometers, its rate of loss was not consistent. To determine the chronometer’s error on Local Time, Lewis and Clark made observations called “Equal altitudes observations of the Sun.”

Photo courtesy of Pieces of Time (www.antique-watch.com)

Using the sextant and artificial horizon, they would take three measurements of the altitude of the sun in the morning, noting the time shown by the chronometer for each. In the afternoon, when the sun was at the same altitude as it had been in the morning, the times again were recorded. The average of the times, after adjusting for sun’s changing declination, gave the time that the chronometer would have shown at noon. The difference between this calculated time and 12 noon was the “error of the chronometer.” From another Equal altitudes observation the next day or some days later, the captains could determine the chronometer’s daily rate of loss during that period of time.

At a purchase price of $250 (plus 75 cents for a winding key), the chronometer was the single most expensive piece of scientific equipment taken on the journey.       

“… her balance-wheel and escapement were on the most improved construction. She rested on her back, in a small case prepared for her, suspended by an universal joint, she was carefully wound up every day at “twelve o'clock.” (Lewis, July 22, 1804) ).

Surveyor's Compass or Circumferentor   Lewis and Clark used the surveyor’s compass to record their direction of travel and determine the bearing of celestial objects such as the sun and stars. Because the bearings they observed were based on magnetic north, they often took observations of Polaris or the sun at the junction of major streams or other important points to determine the difference between magnetic north and true north. Most maps are based on true north, which does not change through time or with location.

Photo courtesy of Smithsonian Institute

The circumferentor was simply a six-inch-diameter magnetic compass, but was much more precise than the usual pocket or hiking compass and could be read to one-half of a degree. The circumferentor was equipped with front and rear sight vanes to help improve the observer’s ability to sight precisely on an object. On the underside of the compass housing was a receptacle to allow it to be fastened on a tripod or to a shaped pole that was placed firmly into the ground. Attached to that receptacle was a ball-and-socket joint that allowed the user to level the circumferentor with great accuracy. A spirit level would have been used in conjunction with the surveyor's compass to establish a level plane from which to work.

The circumferentor Lewis and Clark used was six inches in diameter and was divided into degree and half-degree increments.

Pocket Compass

Photo courtesy of Smithsonian Institute

Lewis purchased three pocket compasses for $2.50 each and also the compass shown in this picture. The pocket compasses were less cumbersome than the surveyor’s compass and were used to take the bearings when impractical to use the circumferentor and when traveling on land.

This pocket compass was one of the few items that survived the journey. After the expedition returned to St. Louis, it was purchased by William Clark. The compass is on display at the Smithsonian Institution in Washington, D.C.

Drawing by Bob Bergantino, MBMG

Spirit Level   A spirit level is similar to a miniature carpenter's level, though made with greater precision. It was used to level the surveying compass (circumferentor) for precise angular measurements and to level a mirror to produce an artificial horizon when water was not used. Clark also used the spirit level as a hand level when he measured the height of the falls and rapids of the Missouri from Lower Portage Camp to White Bear Islands (see Great Falls of the Missouri).