Though founded in idea on the dome at Florence, that of St. Peter’s differs from it in many most important and essential particulars (Fig. 452). In the first place,--while that at Florence is supported from the very floor upon an octagonal wall merely pierced by comparatively narrow arches, that at St. Peter’s is essentially a _pendentive_ dome, rising from four colossal piers which give it a square base, and united with the four arms of the church by arches, or rather _vaults_ of vast span. These arches, it is true, are not so wide as to reduce the pendentives which rise from between them to triangular forms, but are set so far apart as to leave a portion of the ideal circle between them, and to give the pendentives a horizontal base. This was necessary to give strength to the piers for the support of so gigantic a structure, but in no degree interferes with the pendentive character of the dome. Again, at Florence the octagonal wall rises to the very base of the dome, while at Rome the drum, from the pendentives upwards, is circular. At Florence it is pierced only by rather ungainly circular windows, while at Rome it is colonnaded within and without, and beautifully decorated within. At Florence the dome is of that doubtful kind which has straight sides, carrying up the octagonal form to the very top, while at Rome the dome is circular and perfect. Both are in some degree alike in construction, being _double_, with a space between, not two domes, as at our St. Paul’s, but one dome formed of two shells partially connected; a mode of construction well suited to the support of the crowning lantern. Both domes are founded in their section on the pointed arch. Their internal span is nearly the same, but their proportions differ greatly; for while that at Florence is internally only two of its diameters in height, that at Rome is two and a half; and while the former is externally one and three quarters of its diameter in height, the latter is two diameters--each irrespective of the lantern. Strangely enough, however, the great external defect of the dome of St. Peter’s is its want of height. It is so encumbered by the surrounding building that its height, from near points of view, is greatly lost. Like the mountain--which seems to be its prototype--though towering nobly in the distant view, it becomes as you approach it entangled among the nearer though smaller heights. This is obviated at Florence--at least from the eastern points of view--by the more favourable distribution of the subordinate buildings.