This section is from the book "The National Capitol. Its Architecture Art And History", by George C. Hazelton, Jr. Also available from Amazon: The National Capitol Its Architecture Art and History.
The marble extensions had not far progressed before it was strikingly apparent that they would dwarf and render out of proportion the central dome of the old Capitol. Then, too, the old dome had nearly caught afire when the Library burned in 1851, which was an additional reason for building a new one. The plans were prepared by architect Walter, and approved by the President. The old brick and wooden dome was torn away in 1855, and the present magnificent dome of iron, painted white to resemble the building, " erected in its place. In this connection it is interesting to reflect upon the idea which Ruskin, the fastidious champion of pure architecture, suggests in his Lamp of Truth: " It may be perhaps permitted to me to assume that true architecture does not admit iron as a constructive material, and that such works as the cast-iron central spire of Rouen cathedral, or the iron rooms and pillars of our railway stations, and of some of our churches, are not architecture at all".
There was not a day during the Civil war when the sound of the builder's hammer was not heard at the Capitol. Even when, in May, 1861, all work was ordered to be suspended, the contractors practically continued at their own expense to put in place the 1,300,000 pounds of iron castings then upon the ground. The outside of the spherical portion of the new dome was finished in 1863, though not until the next year was it painted and the scaffolding removed. By the close of 1865, the wings and the interior of the dome were completed, and Walter's work was done.
The height of the building from the base line on the west to the crest on the new dome is 307 feet 6 inches. Rising, as it does, 287 feet 6 inches above the base line on the east front, away from all surrounding buildings, it is more imposing to the eye than the somewhat similar domes of St. Peter's at Rome,448 feet high, designed by Michael Angelo; St. Paul's in London, 365 feet in height, designed by Sir Christopher Wren ; or the dome of the Pantheon in Paris, which has a height of 258 feet. There is no dome in Europe more graceful in its lines and proportions.
Great engineering skill was required in the erection of the dome. The walls had to be trussed, bolted, girded and clamped in every conceivable way to hold in position the immense superstructure. Even furnished with the figures, it is scarcely possible for the mind to appreciate its immense weight. Walter calculated its 8,909,200 pounds of cast and wrought iron as giving a pressure of 13,477 pounds to the square foot at the basement floor, and the supporting walls as capable of holding 755,280 pounds to the same area. The pressure upon the walls of the cellar floor, exclusive of the weight of the Goddess of Freedom, is estimated at 51,292,253 pounds. The dome is composed of two shells, one within the other, which expand and contract with the 'variations in temperature; between these the stairway winds in its ascent. The greatest diameter at the base is 135 feet 5 inches. The cost of the new dome is officially given at $1,047,291.89.
The thirty-six columns which surround the lower portion of the exterior represent the thirty-six States in the Union at the time it was designed. The thirteen columns which encircle the lantern above the tholus are emblematic of the thirteen original States. This lantern is 24 feet 4 inches in diameter and 50 feet in height. Its light notifies the surrounding country for miles of a night session in either House. The American flag, floating from the staff above either chamber, is the signal by day of the session of the House beneath. Until late years, except during the sittings of Congress, no flag floated from the nation's Capitol. This oversight was first pointed out by Colonel Richard J. Bright, Sergeant-at-Arms of the Senate, through whose patriotic efforts the following clause was inserted in the sundry civil appropriation bill, approved August 18, 1894: "To provide flags for the east and west fronts of the centre of the Capitol, to be hoisted daily under the direction of the Capitol Police board, one hundred dollars, or so much thereof as may be necessary." Like provision has been made each year since. Out of respect for noted dead, the flags float at half-mast, and in a very few instances the Capitol has been partially draped in black. On gala days, flags wave in the breeze from staffs placed near the top of the dome, and a few years since, for a short time, arc lights with reflectors were there suspended for the purpose of more effectively lighting the park.