At the bottom the wire is attached to another adjustable screw fastened to a 25-pound brass plumb bob. The "bob" is suspended in a bucket of water to prevent movement from ordinary vibration. The wire itself is protected from top to bottom by a casing of galvanized iron pipe, fastened at top to the underside of the I-beam above referred to, and secured to the diagonal braces of the stairway by circular bands of iron. Near the floor this pipe expands into a hood and is connected to the plummet house (which is also of galvanized iron) by riveting and soldering. In the interior of the plummet house is a hollow iron pedestal bolted to the floor, with a 2-inch circular opening on top for the free passage of the plumb wire. On the east side of the pedestal is a door which gives access to the interior, in which stands the bucket of water containing the plumb "bob." On top of the pedestal are two telescopes, one on the north side facing south and one on the east side facing west. These telescopes are moved by micrometer screws which cause them to travel, the north one along an east and west graduated scale, the east one along a north and south graduated scale. The readings of the north instrument increase to the west; of the east instrument, to the south. The scales are graduated to inches and twentieths of an inch, and by means of a micrometer (graduated into 50 sections, 20 threads to 1 inch) may be read to thousandths of an inch. Variations in the positions of the plummet line are determined by variations in the readings of the scales when the plummet line coincides with the center vertical wires of both telescopes. In a letter to Col. J. M. Wilson, Corps of Engineers, dated April 7, 1888, and written by Mr. G. M. Thomas, clerk, is said: "The 'plummet' was set to a point on the floor of the Monument on the 12th of June, 1887, with the following as the result of the reading: 0.926" north (instrument); 0.9795" east (instrument)." From a letter to the Chief of Engineers, United States Army, from Col. O. H. Ernst, Corps of Engineers, dated February 11, 1891, it appears that in 1888 the maximum movement was 0.15"; in 1889, 0.16"; in 1890, 0.14". December 31, 1890, the center of gravity was one-tenth of an inch (0.10") southeast of where it was January 3, 1888. The extreme positions of the center of gravity during the three years 1888, 1889, 1890 were in one direction March 19, 1888, and in the other August 2, 1890, the distance between these two positions being one-fourth of an inch (0.25"). The top of the Monument moves about three times as far as the center of gravity. |