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9.- Azimuths for laying off the prime vertical, dc.-Continued. Observations at the intersection of the parallel 31° 20' and the 111th of longitude west from Greenwich, &c.—Continued.

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10.--Elements for marking parallels of latitude 31° 20' and 31° 47'.

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PARALLEL 31° 20'.

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Do....
Do...---------

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Length of parallel 31° 20'-in arc....

...do.ac...in yards ------
....do.....,in feet..---...

....do.--.-- in miles and feet
One degree of parallel of latitue 310 47'-.--
One minute of........do...nec.do...--
One second of..-----do..--.--.do.....

Or in feet ...--------------
One degree of parallel of latitude 310 20'....
One minute of...or..do.........do......
One second of...---..do..........do.....

Or in feet..
From parallel 31° 20' to 310 47' 27'------
One degree of meridian (middle latitude being 310 331)
One minute of ....do............do............do...
One second of ...do...-.-.-.--.do.--.--.-..do.......

Or in feet...

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31 miles + 5.37 feet. porno 121, 248.43 yards.

2,020. 807 o

33. 680 60 101.04

ASTRONOMICAL POSITIONS

ALONG

THE RIO BRAVO DEL NORTE.

D.-BOUNDARY LINE FORMED BY THE RIO BRAVO DEL NORTE.

OFFICE OF SURVEY OF THE UNITED STATES AND MEXICAN BOUNDARY,

Washington, D. C., May 22, 1856. SIR : In my communication of February 9, 1856, I presented to you the final results for the longitudes of the astronomical station, established under your direction in the year 1853, near the mouth of Rio Bravo del Norte, the same having been computed with corresponding observations made at Greenwich, Philadelphia High School, and Radcliffe observatories.

I have now to present, in a tabular form, the individual results of all the observations made by you and under your orders for the longitudes and latitudes of the primary astronomical positions on and near the Rio Bravo del Norte, established in connexion with the survey of the United States and Mexican boundary.

I.-LONGITUDES.

NGITUDES.

The positions thus determined are eight in number, being enumerated as follows in the order of place from the west :

a. Frontera.
a'. San Elceario.
b. El Paso del Norte.
C. Mouth of Cañon.
d. Presidio del Norte.
e. Fort Duncan, Texas.
f. Ringgold Barracks.
g. Mouth of Rio Bravo del Norte.

All the observations made at these places, together with the computations, as well as the elements and data used therein, are herewith presented in a tabular form, comprising the corresponding observations made at Greenwich, Cambridge, Philadelphia High School, and Radcliffe observatories.*

Attached to this is also a recapitulation of the results, from which are deduced the final longitude adopted for each station, the papers being marked a, , b, c, d, e, f, g, to each of which I will briefly call your attention. :a. Frontera.--The observations at this place extend through four lupations, commencing December 29, 1851, and ending April 6, 1852; making, as combined in table (I, the longitude=766m 134.02 west of Greenwich.

* These computations are to be deposited in the Department of the Interior, in manuscript form.

1

al. San Elceario.-Moon culminations were observed at this place simultaneous with those observed at Frontera. These observations extend through a period of five lunations, commencing January 29, and ending June 3, 1852; making, as combined in table (II.) the longi'tude of San Elceario=7h 5m 48.43 west of the meridian of Greenwich.

This result, however, as well as that above given for Frontera, has been corrected by combining with them the difference of longitude of these places, as determined by flashes of gunpowder simultaneously observed at Frontera, El Paso del Norte, and San Elceario, on the nights of February 14, 18, 19, and March 14, 1852, (see table III,) by which it appears that San Elceario is east of Frontera=1m 78.30, whilst that deduced from moon culminations, 1m 89.59; difference,==18.29.

The longitudes of Frontera and San Elceario, as determined from moon culminations, being corrected by half the difference , gives for the final adopted longitude of Frontera 7h 6m 128.37; San Elceario, 7h 5m 58.07.

b. El Paso del Norte being, as determined by flashes, 15%.94 east of Frontera, it is therefore in longitude=7h 5m 568.43 west of the meridian of Greenwich.

C. The astronomical station near the mouth of the Cañon," on the left bank of Rio Bravo del Norte, is in longitude=71 2m 298.06; having been determined, in the absence of moon culminations, by thirteen flashes of gunpowder, simultaneously observed at that station and San Elceario, on the night of June 21, 1852, to be 2m 368.0 east of San Elceario.—(See table IV.)

d. The astronomical station near Presidio del Norte, on the left bank of Rio Bravo del Norte, was determined by a series of moon culminations, observed during a period of two lunations, commencing July 26, 1852, and ending August 29, 1852, determining its position to be 6h 57m 398.02 west of the meridian of Greenwich.-(See table V.)

e. The astronomical station near Fort Duncan, Texas, was determined to be in longitude 61 42m 18.78 west of the meridian of Greenwich, from observed moon culminations, commencing October 19, and ending October 27, 1852, combined as per table herewith marked VI.

f. Ringgold Barracks is in longitude 6h 35m 68.19, as determined by moon culminations observed on ten nights during the months of June and July, 1853, the several results being combined as per table herewith, marked VII.

g. The longitude of the astronomical station near the mouth of the Rio Bravo del Norte has already been discussed by me, as presented to you in report of February 19, 1856, (herewith appended, marked g,) to which I refer you for the conclusion of the subject of the longitudes of the primary points on the Rio Bravo; it explains in detail the manner in which all the observations and computations have been tabulated, for which reason I have not herein before made reference thereto.

II. LATITUDES.

The two volumes herewith* (marked Latitudes,) contain, in a tabulated form, all the individual observations made by yourself with the zenith instrument, for determining the latitudes of all the primary astronomical stations on and near the Rio Bravo del Norte, from Frontera down to its mouth, with the exception of San Elceario and Fort Duncan ; of these two, the former was determined in 1851, by Lieutenant Whipple, topographical engineers, by transits of stars over the prime vertical, and the latter in 1852, by Lieutenant Michler, topographical engineers, by circum-meridian altitudes of N. and S. stars observed with a sextant.

* To be deposited in the Department of the Interior.

The tables appended present a recapitulation of all the results for the latitudes of the primary stations, giving results which have been adopted, as follows, viz:

a. Frontera, by a mean of 106 observations made by W. H. Emory, on twenty-four pairs of stars observed near the zenith, is determined by combining the results, as in table herewith, marked VIII, to be in latitude = 31° 48' 44".53.

The table marked IX, indicates the corrections derived from actual observations of the stars at the Washington Observatory, which corrections have been applied as indicated in table. IXa.

b. San Elceario* was determined by Lieutenant Whipple in 1851, by twenty prime vertical observations on eight stars, to be in latitude 31° 35' 12''.62, the individual results being combined as shown in table X.

c. Astronomical station near the mouth of Cañon is in latitude 31° 2' 26".15, as determined by Major Emory in 1852, from fifty observations on twenty-one pairs of stars near the zenith; the individual results being combined as shown in table marked XI. Table XIa, shows the corrections which have been applied as indicated in table XI, in order to introduce the stars' elements as given in the 66 Twelve-Year Catalogue.”

d. Astronomical station near Presidio del Norte is in latitude 29° 34' 7".13, as determined by Major W. H. Emory in 1852, from 121 observations on twenty-six pairs of stars near the zenith, combined as shown in table marked XII. Table XIIa, shows the corrections which have : been applied as indicated in table XII, in order to introduce the stars' positions as given in the “ Twelve-Year Catalogue.”

e. Fort Duncan (Eagle Pass) is in latitude 28° 42' 43".67, as determined by Lieutenant N. Michler, topographical engineers, in 1852, from eighty-seven altitudes of north and south stars, observed with a sextant, combined as in table marked XIII.

f. Ringgold Barracks (observatory) is in latitude 26° 22' 27".79, as determined by Major Emory in 1853, from 107 observations pon thirty-two pairs of stars near the zenith, combined as shown in table marked XIV. Table XIVa, shows the corrections which have been applied as indicated in table XIV, in order to give to the stars their positions furnished in the 66 Twelve-Year Catalogue."

g. The astronomical station near the mouth of Rio Bravo del Norte is in latitude 25° 57' 21".83, as determined by Major W. H. Emory in 1853, from 129 observations on twenty pairs of stars near the zenith, combined as shown in table marked XV.

The papers herewith have been arranged in this manner with a view to facilitate you in preparing them for such further use as you may deem best. Very respectfully, your obedient servant,

GEO. THOM,

Captain Topographical Engineers. Major WM. H. EMORY, United States Commissioner.

VI

* This place was subsequently determined by myself, not knowing that Lieutenant Whipple had observed thero ; but finding he had done so, I gave his observations priority.--W. H. E.

Vol. 1-28

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