Page:Science vol. 5.djvu/165

 VKBaTTAHT SO, 1885.]

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��corrected. The area of the foundation was iDcreased 130 <%, or from G.400 square feet to 16,000 sqaare feet, and was carried down to 21.5 feet below the original siirfiice of the ground. Careful levels showed, that, during the process of underpinning, the base of the monument settled two inches.

The foDndation was further strengthened, and the pressure distributed over the whole of the new base, by placing a continuous buttress of concrete around the base, from the lop of the old foundation halfway out on the concrete base : a portion of the foundation masonrj' be- ing cut away, as shown in the sketch, to give ft good bearing. A teiTace of earth was after- wards added, to cover the rough masoniy, and to still further increase the depth to which the foundation was carried, and thus to increase the resisting- power of the ground against lat- ^^—. enil displacement.

^^Ll The new foundation was completed in May, ^^Bl880 ; and on Aug. 7 President Hayes as- ^^^■Bted iu laying the first new stone on the shaft. ^^^Bn tb« new jxtrtiou the space inside was en- ^^HpEged fVom S5 feet square to 31 .5 feet square, ^^H^ diminish the weight by lessening the tbick- ^^Bpen of the walls ; and solid granite backing, in two-feet courses to correspond with the out- side marble courses, was substituted for the irregolar rubble-work. When the wall grew ooMsiderality thinner, marble was used through- out. The tJiickness at 500 feet is 18 inches. The monument rose 26 feet in 1880, 71 feet in 18«1. 00 feet in 1882, 70 feet in 1883, and 90 ftet. to which was added the apex of 55 feet, ill IdM.

Eight iron columns rise in the interior, shown by small circles on the plan of the top. Four of them are far enough from the wall to sup- port the iron platforms anil stairways by which the moiuiment may be ascended: the other ^^four act as guides for an elevator. These ^^KjCriomnB have been connected willi the water- ^^^Hlwiug stratum below the monument, and with ^^^pliB netollio point on the apex. ^^*- ■SeTcral ways of capping the monument, or of constructing the apex to suit its exposed posi- tion, and secure permanence, were discussed. The adopted design was by Bernard It. CJreen, dvil engineer. Three stone corbels, one foot tiiick at the edge, begin to grow out from each ride of the well within the monument, at a oint thirty feet below Uie top of the wall. ■ increase in width as they ascend, until e top of the wall the middle one projects \ f«ct, nnd the side ones four feet and onc- r each. From tliem spring stone arched , which in turn support the roof-covering

��of stone slabs seven inches thick. The middle ribs rise thirty feet, and intersect on a cross- shaped keystone ; the side ribs abut against one another, and a square stone frame some seven feet lower down. The apex is termi- nated by an aluminium point.

After tile main waUs had reached their ulti- mate height, a frame carrying a derrick mast, which reached to a height of 75 feet, was ei'ected on the tops of the iron columns. An opening was left in the lower roof-course at one side ; tlie slone for the roof run out on a small balcony supported by projecting beams, and then raised to place. When all but three roof-courses were set (in all, some 11 feet in height), a platform was built around the top, supported on brackets resting on the slanting sides of the roof, and carried, in turn, on beams projecting through the apertures for observation left in the lower part of the roof, two on each side ; and the nine remaining stones were distributed on this platform. The central derrick was then removed, and a small quadruped derrick erected on the platform and over the point of the roof. Thus these stones, including a cap-stone weighing 3,300 pounds, were readily set, and the apex completed Dec. 6, 1884. A small opening near the top, after- wards closetl by a stone slab, [lermitled the retreat of the workmen who removed the scaf-

fjince the completion of the foundation, and the resumption of building the shaft, some slight settlement has taken place, increasing regularly and uniformly with each addition of a few courses of stone. After a few weeks from any suspension of building, settlement has always ceased ; .ind hanlly a perceptible movement again occurred until after some 200 tons' weight had been added, when the same process of settling was repeated. Altogether, in the addition of 400 feet iu height, and about 34,000 gross tons, 12,000 tons of which are in the earth terrace over the foundation, the settlement was two inches. The entire settle- ment, due to underpinning the foundation and completing the superstructure, is about four inches. The movements of the plumb-lines, of which there were two, — one from the height of 148 feet, and the other from 2.59 feet, — were but trifling. Changes in them were infrequent, and probably not alwaj's, if often, due to actual leaning of the shaft.

The workmen were protected against injurv fVom falling by a strong net suspended around the outside of the shaft ; and, since the resump- tion of construction by the United States, the only accident has been the i)reaking of the

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