July 28, 1842, was a hot day along the North Platte River. John C. Frémonts voyageurs raised the buffalo hide slightly around the bottom edge of his tipi when they pitched it, hoping to let a breeze blow through. Inside, Frémonts cartographer, the phlegmatic Charles Preuss, started a fire under a pot of water while, outside, Frémont set up a tripod. From a leather case he withdrew a brass tube nearly a yard long, and hung it from the tripods apex. A small glass canister, about the size of a soup can, was attached to the bottom end of the tube. The bottom of the canister was made of leather, through which protruded the head of a screw that a person could loosen or tighten with thumb and forefinger. This was a cistern barometer, state of the art for its time.A more extravagant disregard for instructions on his second expedition would lead him deep into Californiastill a part of Mexicoand would win him not censure, but fame.
The day was calm, and the instrument hung straight and still. It looked safe enough. To read what it had to tell him, Frémont would have put his hands on his knees and peered at a place near the top of the tube where the brass was cut away, revealing a glass tube cased inside. Inside the glass, the height of a column of mercury could be read on a measured scale. But first, he had to make some adjustments.
Frémont was twenty-nine, a second lieutenant in the Corps of Topographical Engineers, an intellectually elite, semi-autonomous branch of the U.S. Army in which he was one of only thirty-six officers. He was in command of two dozen men, on the first of five exploring expeditions he eventually would lead to the trans-Missouri West. He was a geographer, well trained in navigation and cartography, and benefiting professionally from some of the U.S. governments earliest support of science. Though admirers would come to call him the Pathfinder, he was not an explorer; the route he was following in 1842 had been used regularly by white trappers and traders for two decades or more. He was, however, a popularizer, and the information he brought back would eventually open that route to hundreds of thousands. It became the Emigrant Road, the main trunk of the trails to Oregon, Utah, and California. But as much as anything, he was an adventurer.
Fig. 1. Portrait of John C. Frémont by George Peter Alexander Healy, c. 1843. Oil on canvas; 30.5 x 25.5" (76.20 x 64.14 cm.) Collection of the Union League Club of Chicago.
Financing for the trip had been quietly engineered by Senator Thomas Hart Benton of Missouri, who also happened to be Frémonts father-in-law. Benton had his eye on Oregon, which at the time meant all the country west of the Continental Divide and north of the forty-second parallelnow the northern border of California, Nevada, and Utah. South of that line, in 1842, was still Mexico. Oregon was claimed jointly by Britain and the United States; still, Benton wanted to see it filling up with U.S. settlers as soon as possible. Benton was a standard bearer for what came to be called Manifest Destiny, the idea that the United States was justly fated to fill the continent. Similar thinking led to the election of an expansionist president, James K. Polk, in 1844, and in 1846 Polk led the way to war with Mexico.
Benton had more immediate, tactical aims, however. Though much of the information on how to get to Oregon was sound, it was available only from word of mouth, and from a few maps that mixed fable and guesswork with fact. Frémont, Benton knew, would return with good information, and afterward the government could issue thousands of reliable maps. Then, given the right publicity, Benton could turn the politics over to a fast-growing, land-hungry public, and count on a reliable outcome.
Frémont and his men were now a month and a half out from the Missouri settlements and approaching Red Buttes near present-day Casper, in central Wyoming. Though his instructions from Colonel John Abert of the Topographical Corps were simply to travel up the North Platte to the mouth of the Sweetwater River, Frémont seems from the start to have planned to lead his men 150 miles beyond that point, into the mountains beyond South Pass on the Continental Divide. He had Bentons tacit approval for this intention, and perhaps Aberts as well.
Fig. 2. Map by author
A more extravagant disregard for instructions on his second expedition would lead him deep into Californiastill a part of Mexicoand would win him not censure, but fame. In California again in 1846 and 47, he defied orders outright. By then it was wartime, however; he was court-martialed and left the army. His fourth expedition, a civilian attempt to cross the mountains of Colorado in the winter of 1847-48, ended in disaster and cannibalism among his men. His fifth, into Colorado and Utah, was of little consequence. His fame continued, however, and in 1856 he was drafted to run for president on the first national ticket of the Republican Party. But for now, near the North Platte, peering at the barometer, he was just curious. He wanted to know his elevation above sea level, and he planned soon to measure the true altitude of the Rocky Mountains.
Barometers measure the pressure of the column of air reaching upward into space, where there is no longer any air at all. The greater the observers elevation, the shorter and less dense the column of air above, and the less, therefore, it weighs. In Frémonts time the connection between weather events and changes in local air pressure were not well understood; barometers were rare and used primarily to measure elevation. In this case, measuring the height of the mountains would mean getting the instrument safely to the top of a peak still 150 miles off. The men had left one barometer for safekeeping at the fur-trading post at Fort Laramie a week earlier. Since then, trail travel had broken another. This was the only one left, and Frémont, just now, was being careful with it.
Earlier in the day, the expedition had crossed the braided channels of the North Platte River and camped on the north side. The two dozen men were mostly French-speaking, civilian voyageurs from St. Louis, most mounted on mules and the others driving two-wheeled carts. They took the wheels and canvas covers off the carts and concealed them in dense brush in a cottonwood grove. In sand drifts nearby, they buried everything else they could do without for a few weeks, intending to pick it up on their return journey, and they named the place Cache Camp, because they hid so much equipment there. They would be traveling light now, carrying fewer provisions, and they would have to hunt more often. This was a risk; they had been warned that morning by a band of Oglalla Lakota after crossing the river that they soon would enter a landscape empty of buffalo and ravaged by drought and grasshoppers. But without the carts, they could leave the trail, perhaps find more game for themselves and grass for their animals, and locate mountains, canyons, lakes, and river courses they would otherwise have missed.
For locating topographical features, whether well known or little known, was at the core of what Frémont had been sent out to do. Locating them meant pinpointing their latitude, longitude, and sometimes their elevation, raw material for the good maps Benton demanded. Frémont had trained in field observation and mapmaking with one of the best, the French-born geographer Joseph Nicollet, whom Frémont had accompanied on earlier expeditions to the upper Mississippi and Missouri rivers. Now in charge of his own expedition, he had brought along the German-born cartographer Charles Preuss. Preuss and Frémont both kept daily journals. Frémonts formed the basis for his first report, published the following spring. Preuss's remained unknown to scholars until the 1950s. Both are in print today, provide lively reading and a useful contrast to each other, and are the main sources for this account. Preuss also made landscape sketches, engravings of which accompanied Frémonts reports.
The scientific instruments were "good instruments," Frémont notes in the report, with uncharacteristic understatement. They were in fact state of the art for the time; given the need for portability, they were the most accurate available. As such, it is not too much to see them as emblems of the nations newly institutionalized support, by way of Congress and the Topographical Corps, for scienceespecially when the science served national expansion.