OCR Text |
Show APPENDIX NN. 1219 graphical statious than by the area covered, although the latter is not inconsiderable for so short a season. A measured and developed base was laid ont in the Carson Valley, and connected with the astronomical station established in 1873 at Virginia City, Nev. A description of the apparatus and method employed, by Dr. Kampf, being somewhat typical of that adopted for use upon the survey, is herewith given. DESCRIPTION OF MEASURING- BOD. The rod wad decided upon by the officer in charge in the winter of 1875-' 76, upon consultation with Dr. Kampf, and constructed by Mr. Edward Kahler. It was made of wood, 20 feet in length, strengthened by a vertical cross- piece. Each end of the rod is provided with a scale 8 inches long, subdivided to T£ ff of an inch, so that by a magnifier it can be read to thousandths. At a point near the ceuter an arc of a circle of 30° extension is fastened. An arm attached to the center of the circle, and movable by a micrometer- screw, carries a level, so that after determining the zero- point on the face of the circle the inclination of the rod can be easily read to 5 minutes. The rod is placed for measurement on two iron- plates, weighing about 30 pounds each, and provided with three strong iron pins 2 inches long. In the cente^ of the plate, on an elevated silver plane, is drawn a cross- line, which acts in the nature of the zero- point of the line. METHOD OF COMPARISON. The rod was compared daily, both before and after its use, with two steel standard rods, constructed by the United States Coast Survey, and of a normal leugth of 5 feet, at the temperature of 61°. 6 F. In place of the rod constructed by Mr. Kahler, a similar one, not quite 20 feet long, politely furnished by Mr. Adolf Sutro, of Sutro, ! Nev., consisting of very well seasoned and varnished wood, was used. A very simple apparatus was used, constructed for comparison, the standard steel rods being supported on two wooden blocks, and therefore elevated by the thickness of this support from the plane of me tsurement, two knife-blades were driven in a wooden board, 22 by 1J feet by 4 inches, being as much above the surface of the board as the polished plane at the end of the normal rod. The center of the sharp blade and the plane of the normal rod were brought into the same vertical plane, and by an assistant is kept in this position until the second rod is brought in contact with the first. Thus continuing, the fourth rod was found to reach over the knife- blade about 1J inches. A square block of wood was placed at the end, in contact with the normal rod, and by means of a small measure, 3 inches long, and divided to hundredths, the distance from the square block of wood to the blade of the knife was read, the temperature being always carefully uoted and the measurement repeated. The readings were made by Dr. Kampf and bis assistant. After determining the distance between two points on the edges of the knife-blades, the measuring- rod was placed on top of the blades with the utmost care, and the scale on both ends read. In this manner the amount of over- lapping of the rod was obtained. Observation made October 11, 1876, between knife- blades, 20 feet 1.537 inches, 55°. S Fahrenheit; reading of scales on rod, east end, 0.420 inch; west end, 0.140 inch; therefore the length of the rod was determined to be 20 feet + 0.997 inch, at 55°. 8 Fahrenheit. |