LONGITUDE. The meridional portion of the line is defined as corresponding to the thirty-ninth degree of longitude west from Washington. Section 435, Revised Statutes, provides that "The meridian of the Observatory at Washington shall be adopted and used as the American meridian for all astronomical purposes," and this has been construed as meaning the old Naval Observatory. The latest adjustment by the United States Coast and Geodetic Survey fixes the longitude of the center of the dome of the United States Naval Observatory (old site) at 5h 08m 12.153 0.049, or 77° 03′ 02′′.300".74; hence the longitude of the meridian corresponding to the boundary line between Idaho and Montana is 116° 03′ 02′′.30. The United States Coast and Geodetic Survey has published an adjusted network of telegraphic longitude determinations, including points distributed in various localities throughout the United States. This system includes the Naval Observatory at Washington and also a station at Helena, Montana. From Helena the Coast Survey in 1888 determined the longitude of a pier in the court-house grounds at Spokane. The nightly programme at each station was to observe, with an astronomic transit, two sets of ten stars each for local time, each half set consisting of four stars having a mean azimuth factor nearly equal to zero, and one circumpolar star. Two such half sets, with a reversal of the telescope in the Y's between them, give a strong time determination. The same sets of stars were observed at Helena and at Spokane, thus giving the chronometer error for each local meridian. Between the two time sets the chronometers were compared by telegraph, thus giving the difference in time, and consequently in longitude, between the meridians within a small fraction of a second. As will be noticed in the following table, such observations and chronometer comparisons were made on four nights, when the observers interchanged places and made similar observations and chronometer comparison on four other nights, thus eliminating the effect of "personal equation." The figures relating to this description, which were kindly furnished by the Superintendent of the United States Coast and Geodetic Survey, are presented below: Resulting difference of longitude between the astronomic stations at Spokane, Washington, and Helena, Montana, as determined by the United States Coast and Geodetic Survey in September, 1888. At Spokane, transit No. 19 was mounted in the grounds of the county court-house. At Helena, transit No. 18 was mounted over the station in the northwest corner of the grounds of the United States assay office. The station was established in 1888. The court-house tower is 08.324 or 4.86 east and 0.811 south of the transit. AASpokane-Helena-21m 348.437 ± 0.009. A Helena (transit 1888), 7h 28m 088.789 ± 0.052. 117° 25' 48".39 ± 0.80. From the foregoing it will be observed that the probable error of the longitude determination of the Spokane pier is 0.80, or about 54 feet. Through a connection with the pier at Spokane the longitude of the meridian of the boundary line was established by triangulation and traverse, as will appear hereafter. It may be remarked, in connection with the foregoing, that while the meridian of the Observatory at Washington is, by law approved September 28, 1850, the American meridian for all astronomical purposes, the geodetic operations of the country generally are conducted with reference to the meridian of Greenwich as an initial point. Government maps are usually referred to Greenwich, and standard time is reckoned from it. SPOKANE BASE LINE. A site for this base line was found in the valley of the Spokane River, east of the city of Spokane, points for its expansion being located on the surrounding hills. The line was measured along a tangent of the Northern Pacific Railway, beginning about 2.5 miles east of the Spokane depot and extending eastward 5 miles. It was prepared by nailing boards 1 by 6 by 60 inches 300 feet apart along the cross-ties parallel to the rail. On each board a smaller board (1 by 4 by 12 inches) was nailed, and on the latter was tacked a strip of zinc 2 by 10 inches. At night two complete measurements were made with United States Geological Survey steel tape No. 1, under a tension of 20 pounds, temperature being taken by reading three thermometers at each tape length. The front end of the tape was marked on the zinc strip with a fine brad awl. The length of the tape used, compared with the mural standard of the United States Coast and Geodetic Survey on October 30, 1895, was found to be 300.0075 feet; on December 5, 1896, 300.0058 feet. The mean of these two, 300.0067 feet, was adopted. Adopted coefficient of expansion.... F.. .0000065 Mean temperature of first measurement 60°.35 592.25 feet.. .006 Mean elevation of the line above sea level, as given by a profile furnished by the Northern Pacific Railway.. .feet.. 1,972 The terminal points were transferred to the embankment 25 feet north of north rail, and the new points being intervisible, high tripod supports for the theodolite were necessary. AZIMUTH. The azimuth of the Spokane base line was determined by mounting 8-inch micrometer theodolite No. 300 over the west base and measuring the angle between Polaris and a mark placed at the east base. A series of observations, consisting of eighteen pointings (direct and reverse), was taken near elongation August 15, 1896, the resulting mean for azimuth west base-east base being 253° 18′ 45′′.80. With this value and the astronomic location of the cupola of the court-house at Spokane the positions of all triangulation stations and azimuths of all lines in the main belt of triangulation eastward to Divide and Scotchman stations, near the Idaho-Montana boundary line were computed. A check azimuth was observed at Divide triangulation station July 7, 1898, with the same theodolite. Twenty pointings (direct and reversed) on Polaris were obtained and referred to Scotchman station. " Azimuth of line Divide-Scotchman computed from Spokane base.. 170 18 05.57 Azimuth of same line by direct observation 170 18 10.25 4.38 Difference.. The observed value was adopted in the computation of positions. in the extension of the triangulation northward to the international boundary. Examples of record and computation for azimuth determination are given herewith: Azimuth determinations at triangulation station Divide, July 7, 1898. S. S. Gannett, observer. Time by mean time watch, 34 seconds fast on one hundred and fifth meridian time. One division of striding level = 3".66; 1 division of micrometer = 2".00. Level correction - 1 { (w + w' ) − (e + e') } tan h; d being the value of a division of the level, w + w' readings of west end of level bubble, e + e' readings of east end of level bubble, and h, the angular elevation of star, the foregoing formula reduces as follows: 3".66 X tan 47° 57 57" (1.11) =1.01. |