Jasper County Democrat, Volume 2, Number 22, Rensselaer, Jasper County, 9 September 1899 — LONGEST PLUMB LINE. [ARTICLE]

LONGEST PLUMB LINE.

b Doubt!*- That Which Swtap in the Washington Momunent Daubtless the longest plumb line in existence is that suspended in the Washington monument, w*hich has a free swing of more than 510 feet. It is of hard drawn copper wire, three-axty-fourths of an inch in diameter, and is suspended, for protection, in a galvanized iron pipe. The wire is located in a plane cutting the center of gravity of the monument, which is 174 feet 10| inches above the door sill. The plummet was set June 12,1887, with the top end of the wire fastened to an adjustable brass screw in an I beam about ten feet from the west wall. - In the plummet house (which is riveted to the vertical protecting pipe), at the base, is a hollow iron pedestal, one side of which is fitted with a door for access to the interior, where stands a bucket of water in which the “bob,” a 25-pound brass spheroid, swings. On top of the pedestal are two telescopes, one on the north side facing south and one on the east side facing west. Both are moved by micrometer screws with a travel along scales graduated to inches and twentieths, which, with the screws, give readings to thousandths of an inch. These readings are taken daily, the maximum defleetion ever observed being 0.14 inch, which by calculation shows that the top of the monument moves about three times as far as the center of gravity. As carefully taken data would prove of great value to engineers who undertake the erection or custody of very tall shafts, it is proposed to make the readings automatic and continuous by means of a kind of improved pantograph enlarging 1,000 times. These, with the heat records taken at the top and at the bottom, both inside and out, would constitute a record of great value, showing what scientists call the “breathing” of this 81,120 ton mass of stone.

No less interesting was the location of the shaft, or what is recorded thereof. The bench mark, known as the Jefferson pier, was built on the first meridian kne of the United States, in 1793. It is at the intersection of a line drawn N.-S. through the center of the executive mansion and another E.-W. through the capitol. No considerable record exists of the establishment of these lines, although it ia tradition that President and Mrs. Jefferson were present at the time, and that Mrs. Jefferson gave her thimble to be set in the top of the wooden monument to receive the scratched cross. From this monument, and another located 90 degrees therefrom, levels were taken, with which subsequent levels were compared to determine whether the monument settled evenly. All the levels taken show that the fottndation has remained horizontal, although subject to a gradual diminishing settling, which, in the centennial year (1876), was 8.82 inches.