Page:Memoir of a tour to northern Mexico.djvu/140

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If the elevation above the sea affords in other countries so probable a criterion for their climate and general character, that an elevation of 3,000 feet is considered equal in its effect upon the climate to nearly 10 degrees difference in latitude, an eminent proof of this rule is given in Mexico, where nature has combined, under the same degree of latitude, all variations of climate, from the tropical often to the coldest, by the mere difference in elevation above the sea.

Of the southern part of Mexico we possess already excellent profiles of the country, made by Alexander Von Humboldt, Burkhardt, and other scientific travellers; but of northern Mexico scarcely anything is known in that respect, and the series of elevations above the sea from Independence (Missouri) to Santa Fe, Chihuahua, Monterey, and the seashore, as represented in this profile, is the first one published, and will prove, as I am inclined to believe, highly interesting to every person that wishes to form for himself an opinion of the character of that country.

The calculations are based upon daily barometrical observations made by myself on the road, and cotemporaneous observations made by Dr. G. Engelmann in St. Louis, and by Mr. Lilly in New Orleans.

My barometer was a syphon barometer of 30 English inches. After having been filled with purified mercury, and boiled out several times, I compared it before my departure with Dr. Engelmann's, and found mine to be 0″.139 higher than the latter. After my return to St. Louis, another comparison proved it to be only 0″.123 higher: it had during the whole time changed but 0".016—a most favorable result, if the long transportation of it by water and land, in carriages and on pack-mules, often over the roughest road, is considered.

The mean of my barometrical observations, made on the seashore, mouth of Rio Grande, was 30".025. (the temperature of the mercury having been reduced to 32° Fahr.) By referring it to the cotemporaneous observations made in St. Louis, I calculated the elevation of St. Louis on the "city directrix," near the old market house, to be 420 feet above the sea. The city directrix is a well known and stable point, to which all the geometrical measurements in the city are at present reduced. It is supposed to be 38 feet 1 inch above the lowest water-mark of the river, and 7 feet 7 inches below the highest water-mark in 1844.

From Independence to Chihuahua I reduced my own observations to those made in St. Louis, by comparing my transient daily observations with the monthly mean of the St. Louis observations. From Chihuahua to Monterey I reduced them to the mean of my barometrical observations made in Chihuahua during summer, winter, and spring months, and containing, therefore, most likely the absolute mean of the whole year. From Monterey, where the low country begins, to the seashore, I reduced them to the observations of Mr. Lilly in New Orleans, to whom I am under obligations for a copy of his meteorological journal.

All the reductions and calculations have been made according to the known formula of Gauss.