DIVIDE (2).

This triangulation station is not on the meridional portion of the line, but on the crest of the Bitterroot Mountains, farther south, and was located in connection with the survey of the Bitterroot Forest Reserve.

It is on the main summit of the Bitterroot Range, on the boundary line between Montana and Idaho, 20 miles (air line) west of Grantsdale. It can be reached from the Lost Horse Pass (camp being at Twin Lakes) by following the northwest face of the mountain northeast from the Twin Lakes to the crossing of Moose Creek, thence up the creek to the Meadows. The peak lies to the northwest, about 2,000 feet above the meadow.

Station mark: A rock cairn 5 feet in diameter and 8 feet in length. [Latitude, 46° 11' 32".1. Longitude, 114° 28′ 12′′.2.]

The transit used was one manufactured by Young & Sons, of the type known as No. 10 mountain transit, the maker's number being 6838. The horizontal circle was 4 inches, and this circle, as well as the vertical circle, was graduated to read with verniers to 1'. The instrument was provided with a compass attachment, fixed stadia wires, and a tripod with extension legs. The extension legs were at times very necessary, as may be seen from Pl. VI. Two stadia rods, of seasoned white pine, three-fourths inch in thickness, 12 feet in length, and 4 inches across the faces, were made in Spokane. The lower portions of the rods were tapered to a point and shod with steel. The design of the rod was black on a white ground. (See fig. 1.) By a series of tests the stadia interval was found to be in the ratio of 1 foot on the rod to 100 feet horizontal distance, as nearly as could be determined, and the rods were graduated to even feet and tenths. The extreme divisions of the rods included 10 feet, and each tenth was divided into five parts, so that hundredths could be easily read by estimation. No numbers were placed on the rods. In practice the rods were held vertically, proper corrections being applied for reducing all sights to the horizontal.

There was no expectation of obtaining exact independent results for distances measured with the transit and stadia upon the basis of the stadia wires in the transit being set in the ratio of 1 to 100 and the

rods being divided so as to read to hundredths. In point of fact, probably neither of these conditions existed, even when the instrument was fresh from the hands of the maker and when the rods were newly graduated at Spokane, and much less likely is it that they existed when the instrument was subject to rough field usage and the rods were used under entirely different and variable circumstances. It was believed, however, that they could be relied upon in determining intermediate distances between points the exact distances between which were known, and so long as the ratio between the wire interval and the graduated portions of the rods remained. nearly constant the purpose intended to be subserved would be accomplished. Thus, when a line was run with the stadia between two points whose positions were rigidly fixed by triangulation and the resulting error was proportionately distributed, sufficiently accurate results were obtained, much more nearly accurate than could have been obtained by chaining, especially after certain corrections had been applied.

The method used in running the random line was as follows:

If any

The line started from an astronomic azimuth, the initial point being on a high ridge and marked with a large signal. This signal was, when possible, used as a backsight, the instrument being transited in the direct and reversed position and the mean of the two points determined for the foresight being adopted. Of course in a timbered country it was generally not possible to see the backsights for any great distance, but whenever a ridge was crossed the trees were cut out (see Pls. IX and X) and a new backsight was established. In addition, frequent observations for astronomical azimuth were made, and thus additional checks were obtained. considerable discrepancy was discovered between the astronomical azimuth and the instrumental line, the latter was rerun in order to obtain a satisfactory check. Further, the line was connected with the triangulation and accurately adjusted thereto in azimuth as well as in distance. When the line was adjusted to the triangulation it was apparent that the transit man had introduced into his work a decided personal equation, which had the effect of constantly swinging his line in azimuth too far to the westward, as will appear hereafter. The stadia distances were read from both backsights and foresights, so that the length of each sight was obtained in duplicate, one distance depending on rod No. 1 and the other on rod No. 2, the mean being adopted. In the same

FIG. 1.-Design for stadia rod.

way vertical angles were checked by reading backsights and foresights; and thus levels were carried over the whole line. The height of the instrument above the ground station was determined at each point, and a corresponding height was sighted on the stadia rods. This height was obtained by having one of the legs of the tripod marked to tenths of a foot and by using the plumb bob as a measuring line. The magnetic declination was read at each transit station. The stadia line was divided into four sections:

First section. This section extended from the zero of the random line, namely, the summit of the Bitterroot Mountains, to transit station 154, and was controlled by connection with the triangulation system at both terminal points. At the summit of the Bitterroot Mountains it was connected by traverse with the triangulation station Divide, and transit station 154 was located directly by triangulation. The distance determined by stadia was 62,071 feet, and by triangulation 62,276 feet, the discrepancy being 205 feet, or 1 in 304. The azimuth correction in this section was found to be 0° 0′ 33′′, corresponding to a swing of 10 feet to the east in the random line at station 154 in order to make it a true meridian line.

Second section. This section was included between stations 154 and 800. Station 800 was connected by a short traverse with the triangulation station Buckhorn. The distances determined by stadia and triangulation were, respectively, 228,014 and 228,734 feet, the difference being 720 feet, or 1 in 318. The azimuth swing at station 800 was 71 feet to the east, corresponding to an angular correction of 0° 01' 04".

Third section. This section was between stations 800 and 949. Station 949 was located directly by triangulation. The distance by stadia was 49,132 feet and by triangulation 49,190 feet, the difference being 58 feet, or 1 in 848. The swing in azimuth at station 949 amounted to 12.5 feet to the east, corresponding to an angular correction of 0° 0′ 52".4. The positions of transit stations 0, 154, 800, 949, and 1046 correspond approximately to those of monuments 92, 78, 23, 8, and 0, respectively.

Fourth section. This section closed on the international boundary, and it was unfortunate that a location by triangulation could not be obtained near its terminus, but the nature of the country rendered such a location impracticable, there being no elevated points near by and the timber being very tall and dense. It was therefore checked by careful steel-tape measurements, the tape being held horizontally under a constant tension of about 15 pounds, a plumb bob being used at each end. This section extended between station 949 and a point 45 feet north of station 1046. The distance by stadia was 33,103 feet and by steel-tape measurement 33,219 feet, the discrepancy being 116 feet, or 1 in 286.

It is proper to add that there are two small corrections which should