The vault of Salamanca is not a Gothic vault in any sense, though its rib system and its hollowed cells conform with the earliest stage of apsidal vault development leading to Gothic.[48] It is a dome, and like the larger dome of St. Peter’s, it is sprung from the top of the drum; but unlike St. Peter’s dome, it is powerfully abutted by turrets and dormers built against its springing and its haunch, and it is loaded at the crown with a cone of masonry, so that from without it looks like a stumpy spire, and not like a dome.[49]
But Michael Angelo’s vault has not even such remote approach to Gothic character as the small dome of Salamanca has. Its surface is unbroken by any hollowing into cells. It is a perfect circle on plan, and its ribs, which are embedded and not salient on the inside, cannot, therefore, sustain the vault in any Gothic way. This dome has, moreover, so much of a spherical shape as to give it a stronger tendency to thrust than the dome of Florence has, and the thrusts are exerted equally on all points in the circumference of the drum. The isolated buttresses are therefore illogical, and being set against the drum only, and not even reaching to the top of the drum, they are ineffectual. Thus though the dome was bound with two iron chains, one placed near the springing, and the other at about half the vertical height of the vault,[50] it began to yield apparently soon after its completion. Fissures opened in various parts of both dome and drum which at length caused such apprehensions of danger, that Pope Innocent XI called a council of the most able engineers and architects of the time[51] to examine into the extent of the damage, and ascertain whether serious danger existed. This council concluded that the cupola was in no danger of disintegration, and the Pope, in order to restore confidence in its safety, charged Carlo Fontana, the architect, to write a book on the building and prove the groundlessness of any fears of its collapse. Thus the matter appears for the time to have been dropped. But subsequently the condition of the structure became so alarming that three eminent mathematicians, among whom was the celebrated Boscovich, were, in the year 1742, commissioned by Pope Benedict XIV to make a further examination and submit a report with recommendations for its consolidation.
The condition of the fabric at the time of this examination will be understood from Figure 30, a reproduction of the illustration subjoined to the mathematicians’ report.[52] They found the structure, as the illustration shows, rent into numerous fissures, some of which were large enough to allow a man’s arm to be thrust through them. In some places these cracks had been filled up with brick and cement, and new ones had opened in the filling.[53] At what time the ruptures had commenced could not be definitely ascertained, but the mathematicians express the opinion, for which they state their reasons, that they may have started very soon after the completion of the work.[54] That they were not due to any weakness in the substructure was shown by the fact that this remained apparently quite firm. Had the fractures been caused by any weakness in the piers or pendentives, the mathematicians say,[55] they would be wide at the base of the drum, whereas they were found (as shown in the illustration) to be small at the base and to increase in magnitude toward the top of the drum, and in the region of the haunch of the dome. This was thought by them to show that they were clearly due to weakness resulting from the form of the structure. The report states[56] that the weight of the lantern had caused the heads of the great ribs to sink, the dome to expand at the haunch and at the springing, and the wall of the drum to be pressed outward at the top. To consolidate the fabric they recommended that additional chains be placed at various levels, the old ones having, they thought,[57] burst asunder by the force of the thrusts; but this could not be verified because they are embedded in the masonry. They also recommended clamps of iron to hold in the buttresses.
CVPOLA DI S PIETRO
Fig. 30.—The dome with its ruptures.
The Marquis Poleni of Padua, a distinguished engineer of the time, was then called to examine the dome and to take such means as he might judge most effective to secure it against further disintegration. Milizia tells us[58] that Poleni, after careful examination, expressed the opinion that although the cupola had not the catenary outline which he considered the best for stability, it had nevertheless a good form, and was in no danger of ruin. He pronounced the fissures of no material consequence,[59] and attributed them to two classes of causes. The first of these he thought was haste in construction. The dome having been built in twenty-two months with materials of unequal quality, and not carefully laid, unequal consolidation resulted and caused numerous ruptures. The second class of causes he considered to consist in the action of heat and cold, humidity and dryness, lightning and earthquake. Thus far, says Milizia, the reasoning of Poleni is just, and worthy of his great mind, and he adds, “It would seem, then, that since the cupola was solid, it ought to have been left in peace, but he nevertheless advised five chains, which were placed under the direction of Vanvitelli.”[60] The first of these chains was located at the base of the drum, the second at the level of the attic, the third at the springing of the dome, the fourth above the haunch, and the fifth around the opening at the base of the lantern.
Plate III
DOME OF ST. PETER’S