A Great Height

The most enlightened theory–conjecture, rather–about the Rocky Mountains at the beginning of the 19th century predicted that they would be found to reach only about 3250 feet above their base, which might lie 3000 feet above sea level.1 It was based on the estimates of the Canadian surveyor, Peter Fidler, who explored the southern Canadian Rockies in 1792. According to the popular principle of bilateral symmetry, which then seemed to be evident everywhere else in nature, it was believed they would average no higher than the Appalachians or the Blue Ridge in Virginia and the Carolinas, say 4,000 feet above their base. Lewis had no way of measuring precisely how high he stood above sea level, yet sufficient evidence was before his eyes–snow on the surrounding mountain peaks even in August2–as he reasoned his way to a logical conclusion about what lay before him.

the mountains do not appear very high in any direction tho' the tops of some of them are partially covered with snow. this convinces me that we have ascended to a great hight since we have entered the rocky Mountains, yet the ascent has been so gradual along the vallies that it was scarcely perceptable by land. I do not beleive that the world can furnish an example of a river runing to the extent which the Missouri and Jefferson's rivers do through such a mountainous country and at the same time [be] so navigable as they are. if the Columbia furnishes us such another example, a communication across the continent by water will be practicable and safe. but this I can scarcely hope from a knoweldge of its having in it comparitively short course to the ocean the same number of feet to decend which the Missouri and Mississippi have from this point to the Gulph of Mexico.—August 10, 1805

He had only three sets of figures on which to base his conclusion. First, he knew the longitudes of the mouth of the Missouri, the mouth of the Columbia, and the mouth of the Knife River at the Mandan and Hidatsa villages in North Dakota, which had previously been calculated. Second, he had his own records of the latitudes he had measured at the northernmost and southernmost points in their itinerary to date. He and Clark made celestial observations to secure data from which the longitudes of 111 different locations along the trail were to have been calculated after the expedition was over, but those solutions were never worked out. Third, he had at hand Clark's daily estimates of distances traveled.3

Deluc's "portable" barometer

drawing of Deluc's portable barometer

Science & Society Picture Library

Why didn't the expedition carry an altimeter? The barometer had been invented in Europe about the middle of the 17th century, and the impetus for its further development emerged with the recognition of two practical applications, the study of the relationship between atmospheric pressure and changes in the weather, and the measurement of the heights of mountains.

About 1772 the Swiss physicist Jean Andre Deluc (1727–1817) invented a portable barometer expressly for the latter purpose. But despite his and a number of others' refinements and gimmicks,4 in 1803 the most accurate and dependable barometer still basically consisted of a 34-inch glass tube, a bowl, and a vial of mercury, and was far too fragile to be practical on an expedition such as this one. It was another forty years before Andrew Pritchard of London invented the truly portable and considerably more durable aneroid barometer.5

Even if Lewis had packed a barometer, the measurements of mountain elevations would have been of dubious significance. The concept of "mean sea level" (MSL), a hypothetical point of interface between air and the surface of an imaginary "global ocean," did not begin to emerge until the early 1840s.6 However, a worldwide standard for the measurement of elevations on land required a long-term, systematic record of the rise and fall of tides and studies of the effects of winds and currents. The oldest continuously operating tide gauge in the world was established at Aberdeen, Scotland, in 1862, and the accumulation of significant amounts of global data was complete by the early 20th century–just in time for the nascent Aeronautical Age. (Meanwhile, the building of railroads relied not upon measured altitudes but upon angles of incline–gradients.) But MSL is an arbitrary standard, and is difficult to apply globally. For instance, MSL on the Pacific Coast of North America ranges from 20 to 30 feet higher than on the Atlantic Coast.

1. John L. Allen, "Geographical Knowledge and American Images of the Louisiana Territory," Western Historical Quarterly, II (April 1971), 49. Thomas Jefferson, in Notes on the State of Virginia (1781–82) the mountains bordering the piedmont: "The height of our mountains has not yet been estimated with any degree of exactness. The Alleghaney being the great ridge which divides the waters of the Atlantic from those of the Missisipi, its summit is doutless more elevated above th ocean than that of any other mountain. But its relative height, compared with the base on which it stands, is not so great as that of some others, the country rising behind the successive ridges like the steps of stairs. The mountains of the Blue ridge . . . are thought to be of a greater height, measured from their base, than any others in our country, and perhaps in North America. From data, which may found a tolerable conjecture, we suppose the highest peak to be about 4000 feet perpendicular, which is not . . . one third of the height which would be necessary in our latitude to preserve ice in the open air unmelted throughout the year." Merrill D. Peterson, ed., The Portable Thomas Jefferson (New York: Penguin, 1975), 49.

2. It is unusual today for snow to remain the year around on any but the northern aspects of the highest peaks. In Lewis and Clark's time, many parts of the northern hemisphere were still under the influence of the "Little Ice Age," which may have begun in the Northwest in the 1400s. Many glaciers in the Northwest were reaching their maximum extents in the early 1800s. The trend began to decline by the mid 1800s. Information of Robert Bergantino, October, 1998.

3. On 11 August 1805 Clark passed the 3,000-mile mark from the mouth of the Missouri, according to his daily estimates of distances traveled, and named a large island upstream from Beaverhead Rock "3000-Mile Island." Moulton, Atlas map 66. In the late 19th century, before any dams were built on the Missouri, the distance to the vicinity of that island (which has long since been washed away) was probably about 2700 miles.

4. Owen's Dictionary, which the expedition carried, described and illustrated several earlier 18th-century inventions—a "horizontal or rectangular barometer," a "diagonal barometer," a "wheel-barometer," and a "pendant barometer"—and explained their faults. There were others, but they were all "so difficult to make, so faulty when made, and so troublesome to use," that they weren't even worth discussing. A New and Complete Dictionary of Arts and Sciences; comprehending all the branches of useful knowledge, (London: Printed for W. Owen, 1764).

5. W. E. Knowles Middleton, The History of the Barometer (Baltimore: Johns Hopkins Press, 1974), 85, 132.

6. J. C. Ross, A Voyage of Discovery and Research in the Southern and Antarctic Regions, During the Years 1839–43 (London: John Murray, 1847), 22–32. Ross designated an arbitrary "Zero Point of the Sea" at his Antarctic base. Congress established the U.S. Coast Survey, the country's first civilian scientific agency, during Thomas Jefferson's second administration. In 1878 it was reorganized and renamed the Coast and Geodetic Survey, and once more restructured in 1970 as the National Oceanic and Aeronautical Administration (NOAA), including the National Ocean Service (NOS) and the National Geodetic Survey. The first person considered as chief surveyor of the USCS was Ferdinand Hassler, the West Point mathematician to whom Lewis had consigned his celestial navigation records. See Captain Albert E. Theberge, "The Hassler Legacy: Ferdinand Rudolph Hassler and the United States Coast Survey," at http://www.lib.noaa.gov/noaainfo/heritage/coastsurveyvol1/HASSLER1.html