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Ellicot, astronomer to the United States, having completed his trigonometric operations for the rectification of the limits on the Ohio, being on the 12th of November in the gulf of Florida, in latitude 25 degrees, and longitude 81 degrees 50 minutes, saw in all parts of the sky, "as many meteors as stars, moving in all directions.
The geographic basis of this map is a trigonometric survey by which datum points are established throughout the country; that is, base-lines are measured and a triangulation extended therefrom. This trigonometric work is executed on a scale only sufficiently refined for map-making purposes, and will not be directly useful for geodetic purposes in determining the figure of the earth.
From such a hypsometric basis the reliefs for the whole country are determined, by running lines of levels, by trigonometric construction, and in mountainous regions by barometric observation. The primary triangulation having been made, the topography is executed by a variety of methods, adapted to the peculiar conditions found in various portions of the country.
To a large extent the plane-table is used. In the hands of the topographers of the Geological Survey, the plane-table is not simply a portable draughting table for the field; it is practically an instrument of triangulation, and all minor positions of the details of topography are determined through its use by trigonometric construction. The scale on which the map is made is variable.
These barometric levellings, though far from satisfactory as compared with trigonometric, are extremely interesting in the absence of the latter.
Some such thoughts as these passed through Madden's mind as he pursued his reckoning through trigonometric tables. The light fell redder and dimmer through the ports and he hurried to finish his work before darkness required a lamp in the steamy cabin. A furnace-like breath, laden with malodorous ship smells, drifted in upon him.
M'Gillivray, and he stated that, by the joint means of the barometric and trigonometric measurement, he had ascertained the height of one of the peaks to be about twenty-five thousand feet, and there were others of nearly the same height in the vicinity. I am, dear sir, To W. Irving, Esq. Yours truly, JAMES RENWICK. Suggestions with respect to the Indian tribes, and the protection of our Trade.
We rather incline to the opinion that the highest peak is further to the northward, and is the same measured by Mr. Thompson, surveyor to the Northwest Company; who, by the joint means of the barometer and trigonometric measurement, ascertained it to be twenty-five thousand feet above the level of the sea; an elevation only inferior to that of the Himalayas.
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