To frame a dome is one of the simplest branches of the art of carpentry. It was, however, till a late day, thought to require great ingenuity and scientific skill.

A dome is, in all directions from the centre of its plan, an arch: hence it is possessed of great strength; and, if properly constructed, its lightness is its greatest recommendation. The dome of the State House at Boston is a fine specimen of framing. Its span is fifty-one feet, its height from the floor nearly the same; and, with the exception of the four posts which support the lantern, no timber is used larger than three inches thick, and twelve inches wide. Every other rib is at the base of these dimensions, the alternate ones being two by twelve inches. All are placed three feet apart on the circle, and taper to about eight inches wide at the top, where they are cut against a curb, being there, about twelve incnes apart from centres.

The scarfs are similar to Fig. 2, Plate XX., and are bolted together with bolts half an inch in diameter, with plank two inches thick, spiked on each side of the ribs over the scarfing.

The rough boarding is horizontal; and, after enduring the storms of more than half a century, the structure has proved itself well adapted to its intended purpose. Were the dome larger, the size of its timbers would not necessarily have been increased, since the principles of the arch pervade the whole. A more complicated framing would have detracted from its merit as a design, since all that can be desired is accomplished by the present one; and so simple are the principles involved, that it has not been thought necessary to illustrate them by an engraving.

Where a dome rests upon a high drum, like that of the Capitol at Montpelier, it may be necessary, if the structure stands in an exposed situation, to provide a skeleton-frame of posts, girts, braces, etc, in order to strengthen the work.