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SONORA LINE-MONUMENTS.

IV.—Table showing the triangulated distances, in metres and yards, between the different monuments of the survey of that portion of the boundary line running from the Initial Point, on the Colorado river, to the intersection of the parallel 31° 20′ 00′′ north latitude, and the meridian 111° 00' 00" west of Greenwich.

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Astronomical observations for establishing the 111th meridian of longitude west of Greenwich, and for determining the parallels 31° 47′ and 31° 20′ north latitude: By John H. Clark.

The four manuscript volumes which have been submitted to the U. S. Commissioner, embrace a complete tabulation of all the observations, both for latitude and longitude, made along the parallels of 31° 47' and 31° 20'. It will be seen, by a reference to these volumes, that each of the stations has been determined in latitude with a zenith telescope, (T. & S.,) by about one hundred observations, and two in longitude, with a transit of thirty-six inches focal length, by observations on the moon and moon culminating stars.

In accordance with the direction of the Commissioner, the stars for latitude were at first selected exclusively from the Greenwich Twelve-year catalogue. At Espia, however, it was found that this catalogue no longer afforded, in that portion of the heavens then observable, a sufficient number of pairs that would fulfil the conditions required for satisfactory results. It was therefore deemed expedient, in the exigencies of the survey, to rely subsequently almost wholly upon the British Association catalogue.

It was found, by combining the results of observations on about thirty pairs of stars taken from this catalogue, that any error in the final result arising from errors of polar distances was materially reduced; and although, by comparing the means of the observations on each pair, there seemed to be in some cases errors of polar distances developed, all the results were nevertheless incorporated to obtain the one adopted; nor was a single observation rejected because of its apparent inconsistency or want of parallelism with any previous or subsequent result obtained by it or by any other pair, except at one station, (Agua del Perro,) where the instrument was mounted on a rock, (no timber being attainable,) and the observations otherwise rendered objectionable by the violence of the wind. Thus the result of every pair, whether high or low, and without reference to the "general run," has been included in the "means" used to obtain the latitude adopted.

A subsequent and careful revision in the office of the computations made in the field has disclosed no considerable error.

A brief summary of the observations and results of all the stations, and a reference to the tables which are appended, are here given :

1. The observations at Los Nogales for determining the 111th degree of longitude west of Greenwich, extending through two lunations, and computed from data furnished by the Nautical Almanac, gave the result as shown by Table I, 110° 51′ 01′′.95, (7h 23m 24.13). The latitude deduced from 120 observations on 29 pairs of stars, taken from the British Association catalogue, is (see Table II) 31° 21′ 00′′.48.

2. Seventy-three observations on 23 pairs of stars, selected from British Association catalogue, give for the latitude of the station near the head of Rio Santa Cruz, (Table III) 31° 17′ 56′′.33. 3. At San Bernardino there were 57 observations on 21 pairs from British Association catalogue, and the latitude deduced therefrom, (Table IV,) 31° 19′ 40′′.38.

4. The result deduced from 97 observations on 28 pairs of stars gives for the latitude of San Luis spring (Table V) 31° 20′ 31′′.51.

5. At Agua del Perro there were 81 observations on 25 pairs of stars selected from both cata

logues, (British Association catalogue and Greenwich catalogue,) and the latitude deduced, (Table VI,) 31° 20′ 57′′.56.

6. The observations at Espia, made on 23 pairs of stars, partly from the Greenwich Twelveyear catalogue and partly from the British Association catalogue, amount to 81, and give for the latitude (Table VII) 31° 20′ 56′′.45.

7. The latitude of Carrizalillo, resulting from 72 observations on 14 pairs of stars, selected entirely from the Greenwich Twelve-year catalogue, is (Table VIII) 31° 50′ 55′′.23; longitude, (Table IX) 7h 11m 44.26 107° 56′ 03′′.9.

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8. The result for the initial point on the Rio Grande, obtained from 108 observations on 22 pairs of stars, selected also exclusively from the Greenwich Twelve-year catalogue, is (Table X) 31° 46′ 51′′.29.

TABLE I.

Results for longitude of astronomical station at Las Nogales, east of the intersection of the 111th degree of longitude and the parallel of 31° 20' north latitude.

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Longitude of Los Nogales by a mean of the above results, (the one adopted,) 7h. 23m. 24.136s.

h. m.

S.

7 23 23.81

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LATITUDE.

TABLE II.

Tabulation of results for the latitude of astronomical station at Los Nogales derived from observations made with a zenith telescope (T. & S.) on twenty-nine pairs of stars: By J. H. Clark, Principal Assistant.

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ASTRONOMY.

TABLE III.

Tabulation of results for the latitude of astronomical station at head of Santa Cruz river, derived from observations made with a zenith telescope (T. & S.) on twenty-three pairs of stars: By J. H. Clark, Principal Assistant, assisted by C. Ñ. Turnbull, Lieut. Corps Top. Eng'rs, U. S. A.

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53.41

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55.64

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TABLE III-Continued.

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TABLE III

Continued.

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