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Never before had anyone attempted to measure the Altitude of an American mountain with a barometer.
William H. Goetzmann, Army Exploration of the American West

Mountain Barometer
Copyright ©2000 by Bob Graham

Frémont had two barometers on his second expedition. They were checked in St. Louis before starting out. These were glass tubes over 30 inches long filled with mercury. They were of a general pattern called mountain barometers, incased in metal tubes, which are designed for portability in survey work.

The aneroid, or holosteric, barometer did not exist in reliable form at the time. They began to be used in survey work in the 1860s, but because they are not a direct reading instrument, only as an adjunct to the mercury barometer; they must be frequently referenced to the mercury barometer, or to a known elevation. My own 1920s Short & Mason 2 1/2 inch Tycos barometer is shown at right.

goNote, early aneroid barometer use.
See Determination of Altitudes and Notes on the Barometrical Observations taken on the Carson and Johnson Immigrant Roads over the Sierra Nevada in 1855 by George H. Goddard, Civil Engineer

Frémont's siphon barometer, by Bunten, of Paris, was of the type invented by Jean André De Luc and improved by Louis Joseph Gay-Lussac. In barometers of this type, which have no cistern, the length of the column of mercury is measured between the long and short limbs of the folded glass tube. Bunten had, about 1840, made further improvements to the design, including an ingenious rack and vernier system to read the distance between the levels of the mercury in the two limbs. All was contained within a brass tube of about about 1 inch diameter.

The cistern barometer by Frye and Shaw of New York was similar to Fortin's design, which is used today in standard barometers. The height of the mercury in the cistern is adjusted to an index before the length of the column in the glass tube is measured against its vernier index.

Barometers, even mountain barometers, are fragile things. They would ordinarily be slung over the shoulder in a leather case. Both were eventually broken. The last, the cistern barometer was broken on October 21st.

Pictured above left is a typical mountain barometer designed for portability in survey work. Encased, for protection, inside a brass tube is a glass tube something over 30 inches long, sealed at the upper end and filled with mercury. At the lower end, pictured at below, is a cistern. When the barometer is set up to take a reading, the screw in the bottom of the cistern is let down, unsealing the bottom of the tube, which allows the mercury to descend in the glass tube. The mercury will descend until it reaches equilibrium with the weight of the air pressing down on the surface of the mercury in the cistern. At sea level, the mercury will stand about 30 inches above the surface of the mercury in the cistern. The volume of the cistern is adjustable using the same screw so that the level of the mercury touches the ivory index pin. This index is the zero point corresponding to the upper index and scale. After making this initial adjustment, the reading of the height of the mercury against the upper scale is referenced to this bottom index, which is zero.

After a reading is taken, in order to make the instrument portable, it is carefully tilted so that the mercury again runs up to the top of the tube. This must be done slowly so that the mercury does not slam against the sealed upper end and break the tube. Then the bottom screw is used to lift the leather pocket until it contacts the bottom of the barometer tube. The mercury is thereby locked into the tube. As a further safeguard, the barometer is then inverted, and is always carried upside down. Should air enter the tube, the vacuum formed when the mercury descends would be contaminated, and no accurate reading could be taken.

On Frémont's 1842 Expedition survey of the Oregon Trail, he carried two barometers. One was broken on July 25th on the Platte River, and just as he had reached the Wind River Mountains, the second was broken in crossing a stream below Boulder Lake. Frémont recorded his great disappointment.

"A great part of the interest of the journey for me was in the exploration of these mountains, of which so much had been said that was doubtful and contradictory; and now their snowy peaks rose majestically before me and the only means of giving them authentically to science, the object of my anxious solicitude by night and day was destroyed."

"As soon as the camp was formed, I set about endeavoring to repair my barometer. As I have already said, this was a standard cistern barometer, of Troughton's construction [made by Bunton]. The glass cistern had been broken about midway; but as the instrument had been kept in a proper position [inverted], no air had found its way into the tube, the end of which had always remained covered [by the leather pocket pressed against the open end of the tube by the bottom screw]. I had with me a number of vials of tolerably thick glass, some of which were of the same diameter as the cistern, and I spent the day in slowly working on these, endeavoring to cut them of the requisite length; but, as my instrument was a very rough file, I invariably broke them. A groove was cut in one of the trees, where the barometer was placed during the night, to be out of the way of any possible danger, and in the morning I commenced again."

Among the powder horns in the camp, I found one which was very transparent, so that its contents could be almost as plainly seen as through glass. This I boiled and stretched on a piece of wood to the requisite diameter, and scraped it very thin, in order to increase to the its transparency [so that the index could be seen]. I then secured it firmly on the instrument with strong glue made from a buffalo and filled it with mercury [carried for the artificial horizon] properly heated [to boil off any moisture]. A piece of skin, which had covered one of the vials, furnished a good pocket, which was well secured with strong thread and glue, and then the brass cover was screwed into place. The instrument was left some time to dry; and when I reversed it a few hours after, I had the satisfaction to find it in perfect order; its indications being about the same as on the other side of the lake before it was broken."

goAnd the barometer did make it to the top of Frémont Peak.

go My views on the route and the peak conquered in the Rocky Mountains in 1842. Includes the readings taken on the ascent. Includes the barometric observations made on the climb.

Unfortunately, it was broken again (this time beyond repair) two days later near Two Buttes:

I regretted it, as I was desirous to compare it again with Dr. Engleman's barometers at St. Louis [observatory], to which it had been referred; but it had done its part well, and my objects were mainly fulfilled.

Had he been able to compare the readings of his barometer and the reference barometer on his return, it would have enabled him to refine the determined elevation.

go Hypsometry: determining elevations.
go Frémont's methods of determining coordinates.
go A history of Frémont 's training in mathmatics, navigation, and mapmaking.
goFind out how correcting errors in the published coordinates in Frémont's 1845 Report led to the discovery of his "Long Camp" site.
goOr, to see a study of the determination of latitudes by Francis Drake, and how the errors in those determinations point to the actual location of his 1579 California landfall.
go Read the account of William H. Brewer's survey activities at Carson Pass, Luther Pass, and his ascent of Pyramid Peak in 1863 (Whitney Survey). This is from the full online text of Up and Down California. In the photo at right, Brewer is shown with the barometer case slung over his shoulder, rock hammer, and ever-present bowie knife.

A brief bibliography:

goBarometer World & Museum: Barometers to buy, restoration, Museum, books, spare Parts, FAQ, information.

Bowditch, Nathaniel, Ll. D., The New American Practical Navigator, E. and G. W. Blunt, New York, 23rd Edition, 1853.

Brewer, William H., Up and Down California, Yale University Press, 1930. (at right.)
go Complete text now available online.

Eaton, Herbert N, A.M., et. al., Aircraft Instruments, The Ronald Press Company, New York, 1926.

Frémont, Brevet Captain J. C., Report of The Exploring Expedition to the Rocky Mountains in the Year 1842, and to Oregon and North California in the Years 1843-'44, Printed by order of the Senate of the United States, Gales and Seaton, Washington. 1845.

Greely, Gen. A. W., American Weather, Dodd, Mead & Company, New York, 1888.

Knight, Edward H., Knight's American Mechanical Dictionary, J. B. Ford and Company, New York, 1874-1879.

Negretti & Zambra, A Treatise on Meteorological Instruments, London, 1864.

Nicollet, J. N., Essay on Meteorological Observations, Printed by order of the War Department, Washington, 1839.

Middleton, W. E. Knowles, A History of the Barometer, The Johns Hopkins Press, Baltimore, 1964.

Smithsonian Institution, Meteorological Tables, Washington, 1893.

Williamson, R. S., On the Use of the Barometer on Surveys and Reconnaissances; part I, Meteorology in its Connection with Hypsometry; part II, Barometric Hypsometry; D. Van Nostrand, New York, 1868.
go Complete text now available online.

interest, comments, or questions.

©1999, 2007
Bob Graham