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Show [ 148] 46 I. Note from Professor Hubbard. The instruments employed in the determination of astronomical positions were- . . A portable transit instrument, by Young, of Ph1ladel ph1a. A sextant, by Troughton. A sextant, by Gambey. Two pocket chronometers, (Nos. 438 and 443,) by Appleton. The transit instrument was made by Mr. William J. Young, of Philadelphia. The length of t~e telescope was ,2? inch.es, the diameter of the object glass 2~ mches, and the axts 16 mches between the shoulders. A circle was attached to the instrument, having a diamet~r of 11 in.ches, graduated to read t.o 10 seconds, a~d furnished w1th 3 vermers. The stand was of uon, and 4 feet m height. Of the sextants, the one by Gam bey, a new instrument, was most frequently used. The other, by Troughton, is the same that was carried in the previous exploration, and was now only used in observing at night, its divided arc heing more readily illuminated than that of the other. The index errors of both were carefully and often determined, in order that any possible change of adjust-ment might be readily detected. , The sextant observations consist o~ single altitudes of a star or the sun for time, and of Polaris or a star in the south, for latitude. They have been reduced in the usuai manner, the formulre being too well known to need quoting. All the latitudes, and the several links of the chain of longitudes connecting the primary stations, depend upon the data thus furnished. In deducing the differences of longitude, in order to obviate, so far as possible, all error arising from eccentricity of the sextant or any like cause, compari· son ha·s been made, when practicable, with observations in the ' same quarter of the heavens. The ra es of the chronometers depend entirely upon sextant ob· servations. The comparison of these rates, determined at different times and under different circumstances of climate and usage, has shown that but one o( the chronometers (No. 438) was entitled to confidence. All differences of ·longitude from the principal sta· tions have therefore been determined by this one, and the results thus obtained are, as will hereafter be seen, highly satisfaetory. The following are the observed rates, deduced, with but a single exception, from altitudes of the sun; the sign+ indicates a gaining rate: • 47 [ 148] Locality. Dates of observation. Rate of 443. Rate of 438. ' s. • s. Bent's fort .••••. 1845, Aug. 3 to Aug. 15 + 2.020 + 3.386 Camp at Salt lake " Oct. 14 to Oct. 20 + 0.883 3.317 Laguna farm ••••. 1846, Fe b. 11 to Feb. 19 - 1.754 2.146 " Mar. 30 to April14 • • • • • • • ••• 2.193 " April14 to May 22 • ••••••••• + 2.980 The whole route has been divided into three distinct lines. The first, commencing at Bent's fort, extends to the camp of January 4, 1846. The chronometers were tben for a time subjected to a rapid travel over a rough road, and their rates were thereby changed. The second line commences with the Laguna farm, between which and the camp of January 4 no observations were made, and extends to the camp of March 30-April 14, where the chronometers stopped, and another change of rate took place. The last line extends from this camp to that o'f June 7, after which date no more longitudes were determined. By combining the above rates for the- same line, giving to each a weight equivalent to the number of days elapsed between the observations on which it depends, we get the following: Rates of chronometer No. 438. s. August 21, 1845 to January 4, 1846 .......... _•• • + 3.363 February 18, 1846, to March 30, 1846.... • . • • • • + 2.175 April14, 1846, to June 7, 1846 ................ + 2.980 The transit instrument has given, by moon culminations, the longitudes of four camps with an accuracy much more than suffic!ent for ordinary geographical purposes. These camps being connected, as we have already seen, by chronometric differences, an excellent check of the whole work is thus afforded. When we remember that an error of one second of time in the observed transit of the meon induces an average error in the resulting longitude of the place of nearly seven minutes of arc, the agreeme:p.t of these independent determinations, thus referred to the same point, is unexpectedly great. The following is the method by w·hich the transit observations have been reduced: ·An estimated longitude for each of the camps in question, gave the means of computing with sufficient accuracy the "the tabular mean time of transit" of the stars observed; their places in the heavens being taken from the catalogue of the British Association. The "observed mean time of transit" was next to be obtained. Where the passage of the star over all the wires had bee~ observed, the mean, reduced to the middle wire, gave at once the time sought. For the purpose of correcting imperfect transits, a • I |
