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Frémont's Reductions
Copyright © April 2002 by Bob Graham

Before the start of his 1st Expedition, Frémont hired Charles Preuss as a cartographer. Preuss had been recommended by Ferdinand Hassler, Chief of the Coastal Survey. Preuss was out of work, and unable to feed his family, and the job at hand required him to reduce astronomical observations from the 1839 Nicollet survey.

"That work, I told him, I could get for him. This he said he was not able to do. His profession was topography--in this he excelled, but that was all. The only thing I could devise was to get for him this astronomical work and do it myself, which I could by working in the evenings. It troubled him deeply that I should have to do this for him, , but it was the only way I could come in aid; and so it was done.

I have read in a biography that Frémont did not reduce his own observations. This apparently stems from notes in the appendices of his reports such as:

"Note from Professor Hubbard, (of the National Observatory, Washington city,) describing the instruments used by J. C, Frémont in making the astronomical observations in his third or last expedition, and the methods followed by Professor Hubbard in reducing them."--Geographical Memoir, 1848

Of course, these reductions were checked out on the return of the expeditions, in particular those referred to in the excerpt above, as they were important stations of latitude and longitude used in producing the maps. But there are many occasions noted in the reports, and in other accounts, where observations were reduced immediately as a means of navigation.

"I was acting as assistant astronomer at this time. Col. Frémont told me there would be an occultation that night, and he wanted me to assist in making observations. I selected a level spot on the snow, and prepared the artificial horizon . The thermometer indicated a very great degree of cold; and standing almost up to our middle in the snow, Col. Frémont remained for hours making observations...
"The next morning, Col. Frémont told me that Parowan, a small settlement of Mormons , forty rods square, in the Little Salt Lake Valley, was distant so many miles in a certain direction, immediately over this great mountain of snow; that in three days he hoped to be in the settlement, and that he intended to go over the mountain, at all hazards."--Saloman Nunes Caravalho

"During the day we occupied ourselves in making astronomical observations in order to lay down the country to this place; it being our custom to keep our map regularly in the field, which we found attended with many advantages."--Frémont, Report, June 16, 1842.

"Our astronomical observations do not allow us to doubt this, although I do not quite believe in the correctness of the longitude. Our latitude, as well as that given on the maps for the Bay of San Francisco, must be right. I estimate the distance from the summit to the foot of the mountain range to be thirty miles. We shall see."--Frémont, Report, February 13, 1844.

"I obtained to-night some observations; and the result from these, and others made during our stay, gives the latitude 38° 41' 57" [sic], and the rate for the chronometer 25.82."--Frémont, Report, February 14, 1844. (note: the 57 seconds printed in the narrative of the 1845 government report is incorrect--the actual determination, found in the Tables of Astronomical Observations in that report, was 38° 41' 03")

"Our latitude is 38° 41' [confirming Frémont's 38° 41' 03]"--Charles Preuss, journal, February 18, 1844

"I informed them (and long experience had given them confidence in my observations and instruments) that almost directly west, and only about 70 miles distant, was the great farming establishment of Captain Sutter...." --Frémont, Report, January 31, 1844.

go In the vouchers of expenditures for the several expeditions, in addition to the instruments purchased, are found both English and American nautical almanacs and log tables.

go As a further, more detailed, example of field reductions, see Frémont and the Determination of Coordinates, or
Longitude and the Buenaventura River.

Barometric observations were not likely reduced, other than as a rough estimate, at the time of observation. For example, Frémont's barometric register kept in the Wind River mountains in in Wyoming in 1842 was ultimately reduced with reference to the register for the same period kept by Dr. George Engelmann at his observatory in St. Louis, Missouri. Although much too far removed in distance, there were no other data on which to base the Wyoming observations. Both registers were published as tables in the 1843 Report published by the Senate.

go Read about these barometric determinations of elevation from the climb of Fremont Peak in Wyoming in 1842.

go A look at Frémont 's determinations of elevations.
go A history of Frémont 's training in mathematics, navigation, and mapmaking.
go See the article Determination of Latitude by Francis Drake on the Coast of California in 1579
go An experiment in the determination of Latitude: This is a followup to the proceeding article, in which the conclusions made therein are put to practical test that may be repeated by anyone wishing to go to the trouble.
go Also relating to the Drake landing site project, see a comparison of the Tables of Solor Declination by Martin Cortes with those of William Bourne.
go Watching the Heavens Change. How polaris has moved 2 degrees closer to the celestial pole during recorded California history, and why John C. Frémont got up at 3:00 a.m. to sight polaris in 1844--wasn't it there all night long?


©1999, 2007
Bob Graham