Commonplace · vol. 4 · no. 4 · July 2004

Tom Rea
The Pathfinder’s Lost Instruments
John C. Frémont's cavalier attitude toward his scientific apparatus
Part I | Part II | Part III | Part IV

Besides their journals, Frémont and Preuss kept logs of their instrument readings. They took readings at river confluences, important landmarks, most campsites, and many noon halts until, as we shall see, some of the instruments fell prey to accident. Frémont’s alternating care and carelessness toward them was bound up with the cavalier adventurousness that was so much a part of his character. But it seems fair, too, to connect his attitude with a larger national recklessness at the heart of Manifest Destiny.

When Frémont’s men had tried to set up the tipi the first night out from Fort Laramie, the woman had laughed at their clumsiness and since then had often helped—probably supervised—setting it up every evening. Pitching and striking tipis was women’s work, and the Lakota women were quick about it.

To measure latitude, Frémont had two sextants and a reflecting circle, essentially sophisticated protractors; they were used to measure the angle of the sun or the polestar above the horizon. But among trees or in the mountains, the horizon was impossible to locate, and so Frémont also carried with him a couple of so-called artificial horizons. These flat boxes, filled with a shallow puddle of mercury, provided a level, still, bright-silver surface in which the heavenly object would be clearly reflected. The geographer could use his reflecting circle to measure the apparent angle between the heavenly object and its reflection and divide the result by two, thus doubling the accuracy of his measurement.

Measuring longitude was trickier. Ancient geographers divided the round earth into 360 degrees of longitude, which correspond nicely with a twenty-four-hour day. That is, fifteen degrees of longitude correspond to one hour, or one twenty-fourth, of the globe’s daily rotation. If only you had a clock reliable enough to keep telling you the time at a distant, fixed spot on the globe, you could, by noting the time difference between when that clock said noon and when it actually was noon where you were—when the sun was as high in its arc as it was going to get that day—you would know the number of degrees you were east or west of the fixed spot on the globe. By Frémont’s time, the fixed spot had been established for 150 years in Greenwich, England. Spring-driven clocks that would keep reliable time on board ship or on a long journey overland—chronometers, they are called—had been available for about sixty years.

Fig. 3. Examples of the kinds of instruments used by Frémont, from left to right: a reflecting circle, a sextant, and a chronometer. Courtesy Bob Graham,

At first, chronometers were expensive and rare; Lewis and Clark did not have one. By 1842, Frémont felt able to afford three. These were a ship’s chronometer—a big one in a box, suspended with gimbals like a ship’s compass—and two smaller, sturdier, pocket-sized ones. The big one would have been the most reliable had they been at sea, but it did poorly on the trail. They left it and one other behind at Fort Laramie. They took the best chronometer with them but it, too, did not keep perfect time.

For that reason it was standard practice to use a backup system of astronomical observations involving the moons of Jupiter. The observations required a powerful telescope; Frémont’s was 120 power, fifteen times as strong as a pair of modern binoculars. The observations also demanded good weather, a lot of math, a time of the month when Jupiter was in the sky, and lengthy nighttime observations. The clocks, of course, could be read at a glance and worked about the same night or day in all weather. But they were fragile. If they were dropped, or if they got wet, a person was out of luck.

At Cache Camp, where they hid the carts, Frémont’s astronomical and chronometric observations worked out to a longitude of 106 degrees, 38 minutes, 26 seconds west of the Greenwich meridian, and a latitude of 42 degrees, 50 minutes, 53 seconds north of the Equator. This is not bad; as might be expected, it is better in its latitude than in its longitude. A modern GPS device at the spot just across the Platte River from present-day Casper, Wyoming, where he may have camped, puts his readings within about two minutes of the real latitude, but shows his longitude was eighteen or nineteen minutes off.

Fig. 4. Images of Frémont's barometer from

That still left the matter of elevations, which is what both Frémont and Preuss were trying to measure once the tipi was pitched that July afternoon. While Frémont unpacked and hung the barometer, Preuss, seeking a second opinion, started the fire. Water boils at 212 degrees Fahrenheit at sea level, and boils at lower temperatures—because of the lower air pressure—at higher elevations. The difference between the temperatures gives an indication of the actual elevation. But as with the chronometers, the expedition was having trouble keeping the thermometers safe from damage. Already they were down to just one with a scale that went high enough to measure water’s boiling point. The men had already broken their best thermometer. The one Preuss was using inside the tipi had a scale high enough to measure water’s boiling point, but it was too small to allow for graduations of much accuracy.

In any case, it is possible to imagine the tall, red-faced Preuss, thermometer in hand, watching closely as the water came to a boil, and slight, dark-eyed Frémont adjusting the screw on the bottom of the transparent, soup-can-sized canister—the cistern on the bottom of the barometer where it hung from the tripod. By turning the screw he could bring the level of mercury inside the glass beaker up just high enough to touch the point of an ivory pin set there to mark zero on the scale, allowing, each time, an accurate reading at the top.

Perhaps Preuss said something from inside the tipi about the water starting to boil. Perhaps Frémont, intent on his work just outside, answered him absently. Suddenly, as if out of nowhere, a gust of wind slammed the lodge and blew it over—Preuss, fire, water pot and all, along with, Frémont wrote later, "about a dozen men, who had attempted to keep it from being carried away." In the confusion Frémont managed to save the barometer, "which the lodge was carrying away with itself," but the thermometer broke. They had no others that would measure above 135 degrees, so from now on, elevation data would depend on the barometer alone.

Charles Preuss
Fig. 5. Charles Preuss. Engraving in Century Magazine, 1891. Courtesty of the Colorado Historical Society.

The tipi blew over through no fault of its own, however. It was a good-sized lodge, eighteen feet across and twenty feet high, of that superb conical design which keeps a person warm in winter, cool and mosquito-free in summer. Frémont had acquired it at Fort Laramie, probably from the Lakota camped there. At the fort Frémont also had hired an interpreter, a trader named Joseph Bissonette, and also had agreed at the request of some of the Lakota headmen to allow a young Lakota man and his wife to come along. When Frémont’s men had tried to set up the tipi the first night out from Fort Laramie, the woman had laughed at their clumsiness and since then had often helped—probably supervised—setting it up every evening. Pitching and striking tipis was women’s work, and the Lakota women were quick about it.

But Bissonette had agreed to accompany the expedition only as far as Red Buttes, now just a few miles away. That afternoon at Cache Camp, he and the Indians would have been preparing to leave, and so the voyageurs must have set up the tipi on their own for the first time. They had done a poor job.

As the tipi toppled, and the men struggled with poles and flapping lodgeskins, Frémont would have unhooked the barometer from its tripod and then tilted it very carefully to allow the mercury to flow slowly to the top of the tube, preserving the vacuum inside the tube and allowing for accurate pressure readings in the future. Next he would have had to screw up the screw at the bottom of the cistern, sealing the leather against the bottom of the tube and sealing the mercury into the column. It would have been a delicate moment, among the wind gusts. If the instrument were tilted too fast, the mercury could slam into the top end of the tube and break it. If the bottom were not securely sealed, the mercury could slosh and air would get into the tube, ruining the vacuum. Then, very carefully, the barometer would be slid back into its leather case and slung from a strap over the geographer’s shoulder, upside down with the cistern at the top, ensuring that the mercury filled the tube and the vacuum stayed safely protected.

At this point, the river fishhooked south, while the trail struck out southwest across country. Geographer that he was, Frémont chose to follow the Platte instead of the trail. They headed south, upriver past Red Buttes, and camped at a grassy spot a few miles above the mouth of what is now called Bates Creek. Another day’s journey brought them to Goat Island, which they named for bighorn sheep they found there and killed. The island lay a short distance downstream from a narrows they named Hot Spring Gate, where the Platte cut through a ridge between high walls near a hot spring. Upstream from there, canyon walls closed in completely, and so the next day the men left the river, climbed over steep, barren hills, and fifteen miles later descended more gradually back down to the Sweetwater, flowing in from the west. Turning right, up the Sweetwater, they headed for the Continental Divide.

Frémont reported the Sweetwater sixty feet across, twelve to eighteen inches deep, flowing moderately—much the same as a person would find it now, at the end of a modern July. But then, buffalo grazed near the river. The men camped in a cold rain, and the hunters killed some cows; the next day they moved seven miles further up to camp a mile below Independence Rock. Here they rejoined the trail. They spent two nights, the hunters killed more buffalo, and everyone pitched in to cut the meat in strips and hang it on racks in the sun to dry. An extra day by the stream must have been a welcome respite from traveling.

By now it was August, and the weather was changing as they steadily gained altitude. Six miles above Independence Rock, they came to Devil’s Gate, where the normally placid Sweetwater cuts a dramatic, V-shaped notch through a granite spur out of the hills nearby. Again it rained. No one had any tents; they seem to have left them behind with the carts. Some nights, big sagebrush was their only shelter. There were no trees along the river, but driftwood lay scattered along its banks. With this, and what the voyageurs called bois de vache—buffalo chips—they made fires.

A day and a half beyond Devil’s Gate they got their first view of the Wind River Mountains, looking low and dark; these marked the Continental Divide and beyond them lay Oregon. On up the Sweetwater they progressed, timbered mountains miles off on their left, bare-granite rocks rising close and steep on their right. They passed buffalo, antelope, spied the only grizzly bear of the trip, found an abandoned dog and a sore-footed horse, and now and then unpacked their instruments to take more readings.

Towards the head of the valley, they came to the place where the Sweetwater, now a rocky creek, foamed out of a canyon in "wildness and disorder," Frémont wrote later. The route got steeper and the trail again left the river for easier going. As he had on the Platte, Frémont stayed with the river. They found traces of old beaver dams, falling apart now as the beaver trade had killed off all their tenants. Finally the rock walls came too close. The men followed a ravine up to a high prairie and camped by a tributary. Here, Indians had left some poles, and so Frémont, Preuss, and some of the men were able to spend a night in the tipi, which they had brought along. The next night, just a few miles from South Pass and the Continental Divide, they again took longitude and latitude, but, for some reason, no barometer readings for altitude.

Instead, Frémont estimated the pass’s elevation at seven thousand feet—not bad, though his calculation the following year of 7,490 was far better. The place is broad, flat, and undistinguished, and so dry and windy that people seldom linger. The men had come 120 miles from the mouth of the Sweetwater, Frémont reported, 320 miles from Fort Laramie, 950 miles from the settlements on the Missouri. Now they were far beyond the extent of Frémont’s written instructions. Crossing South Pass, they were leaving the drainage of the Mississippi and Missouri rivers. They were leaving the United States and entering Oregon.

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