Letter from CHARLES MARSH to GEORGE PERKINS MARSH, dated October 11, 1857.

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Woodstock Vt.Oct. 11 1857

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My dear Brother

I find the mode of computing heights by Guyots tables much simpler & more convenient than by Jacksons formula, & I think it must be more accurate--for by the observations we made here & at South Woodstock Jacksons method gave 313 ft. as the difference of elevation. by the same observations Guyots tables give 357 ft.--only about 7 ft. more than Doton made by leveling --which he informs me was so conducted as to be reliable.

A few days since I measured the hill in Barnard (upon which my sheep were) & made the height 1587 ft. above the house --
Oct. 20. Since writing the above I have repeated the obs. at the Town Hall

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South village, & Long Hill using a thermometer recd. from Green which agrees and keeps time with the dry bulb of the psychrometer The results for Guyot are--sill of Institute above sill of town hall 353 ft. summit of Long Hill above do. 1652 ft. we made the latter with the other thermo. & Jacksons formula 1680 ft.

At the same time I made an obs. at Bridgewater village which gives a height of only 154 ft. above the town hall (the station in Bridgewater being at about the same height above the river). This is much less than I had supposed, & I think there must have been an error in the Bridgewater obs. tho I was--I thought--very careful. The distance is 7 miles (I am told. I supposed it 6.) & I am sure there is sufficient fall for more than fifteen ten foot dams in the river I shall repeat the obs. when I have an opportunity --

For the purpose of ascertaining the effect

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of the different hydrometric conditions of the atmosphere at different elevations upon barometric measurements I made (Saturday & today) 5 observations with barometer & psychrometer at our station below the bridge. & 4 observations at the station upon the top of Mt. Tom (a copy of which I enclose)

The result the psychrometric correction (which I took from the table received last winter) is an elevation of 655 Ft.--with the correction it is reduced to 642 Ft--making a difference of 13 feet --

At a higher elevation the variation would I presume frequently be much greater In the last measurement of Long Hill the weather was clear & fine at the first obs. at the lower station-- but, when near the top we (Benj. & I) had a shower--it continued cloudy & showers were in sight constantly while there.

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At Bridgewater a storm was evidently brewing, which began a few hours later --

Such a change it seems to me must have affected the result of my observations materially I shall try it again using the psychrometer. I shall also make several more observations upon Mt. Tom & at the river, to satisfy myself fully --

I ought perhaps to say that there was about the same difference in the computation from Saturdays observation, (oct. 17)--when there was no apparent change in the weather--as in the one enclosed which includes the obs. made during a driving snow squall --

We had half an inch of snow today & the high hills were whitened several times last week --

Thinking you may like to test my long hill & S. woodstock computations I enclose the obs.

References in this letter:

Swiss-American, Arnold Henry Guyot (1807-1884), taught physical geography and geology at Princeton University. Under Smithsonian Institution auspices, he set up a system of weather observatories that utimately grew into the U. S. Weather Bureau. His barometric tables, published as A Collection of Meteorological Tables, with other tables useful in practical meteorology, published by the Smithsonian in 1852, were very influential. Guyot's contribution to physical geography, Earth and Man (Boston: Gould and Lincoln, 1849) inspired Marsh, despite Marsh's disagreements with some of its premises.

When the English instrument maker, James Green established a business in Baltimore in 1832, he brought with him the knowledge of the latest European technology to which he made substantial improvements. Green moved his firm to New York in 1849 and retired in 1885. His nephew, Henry J. Green, continued the business under his own name. Between 1840 and 1940 the firm manufactured most of the barometers for scientific use in this country.

A psychrometer is a kind of hygrometer with two similar thermometers - with one bulb is kept wet, the other dry. As it cools as the result of evaporation, the wet-bulb thermometer shows a lower temperature than that of the dry-bulb. The difference between the two readings constitutes a measure of the dryness of the surrounding air.