Letter from GEORGE PERKINS MARSH to ARNOLD HENRY GUYOT, dated October 27, 1857.

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Burlington V't Oct 27 57

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Dear Sir

The study of Phys. Geog. has for several years been a favorite pursuit, or rather I may say recreation of mine, and though I have not the honor of being personally known to you, I hope that circumstance will be a sufficient apology for the liberty I take in asking you a question or two upon the subject of barometric meas. of altitudes

In taking a few measurements in windsor county Vermont during the last summer, with my brother C M Esq of Woodstock in that co who is also much interested in the same study, we were surprised to find that the elevation of our highest cultivated ground on the spine of the G. M. was much greater than had been generally supposed, & we proposed during the next year to multiply these observations to as great an extent, in different parts of the state, as we shall find practicable.

It is, of course, In the first place, a matter of much interest to determine as accurately as

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possible the altitude of my brother's residence which will usually be the lower station for observations in that vicinity. Woodstock is 100 miles from Boston, the nearest point of the sea coast, & we have no present means of ascertaining its elevation except by RR levels, & barom. comparisons. The RR levels in general are notoriously fraudulent & totally unreliable. I beg therefore to inquire within what probable limits of error the elevation of W. above so distant a point as C. can be determined by a comp. of of the b & other [...] inst. for a single year.

Hitherto my brother has used a single barom. ([...] cistern furnished by S.I.) observing the height on starting for & returning from the upper station, & taking the mean for the lower. He will be provided with another the next season, but I beg to inquire whether, where the distance between the low. & up. stations is not more than 8 or 10 miles, & the time between 1 & 2 obs at lower stat 6 or 8 hours that method is not as reliable as simultaneous obs. by observers & with instruments. With reference to this I may observe

It has not infrequently happened that the hyg. cond. of the atmosphere, as indic by the psych. at the two stations at the respective

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times of observation (differing by an interval of two or 3 hours), has varied so much that the application of the tabular psych. cor. to the bar at both stations would make a difference of near 1 per cent in the estimates George. Should the psych cor. be applied.

The summits of many of the hills we desire to measure are absolutely bare, and of course there is some difficulty in protecting the det. th. from the influence of direct solar action & of radiated heat from the [...] rocks or ground. What is the most convenient readily portable apparatus for this purpose, & do you use any means of shielding the barom. also & its at th for the same action? I have found much inconvenience from this difficulty in using meteor. inst. in the East & have often noticed that under the clear sky of Ar P. in Turkey, a tent or other similar obstruction to the free radiation from the ground would raise the temp of the air confined by it many degrees, so that in it was very dif. to find a position for the instruments which I could be satisfied was reasonably free from the action of local disturbing causes

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.

Charles Marsh (1821-1873), Marsh's youngest brother, maintained the family farm in Woodstock until his death. He and Marsh frequently corresponded about barometric pressure, precipitation, mountain heights, and other natural and meteorological phenomena

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.