The ancient Pons Palatinus was built, or at least begun by Marcus Fulvius, who was censor in 575; and terminated by Scipio Africanus in 612, A. U. C. It was thrown down in 1598, and the present fragment is for the most part a papal structure of the seventeenth century. A little, and but very little, of the ancient Roman work is visible. A stranger is perhaps more struck by the nets turned round by the action of the water, than by the remains of the bridge. These nets are intended for large fish, and principally sturgeon, whose nature it is to push quietly against the obstacle; the weight of the fish prevents the motion of the net, till the person appointed to watch it can take possession of his prey.

Before we reach the theatre of Marcellus, we have to hunt out the ruins of three temples, which stand close together, almost indeed touching in some parts, but not placed symmetrically, or even parallel to each other. One of these is supposed to have been the Temple of Filial Piety, erected on the site of the prison where the story recorded by Pliny occurred, of a father nourished by the milk of his daughter; but nobody has yet attempted to determine which of the three. The present church of S. Nicola in Carcere, seems to preserve in its name some memory of the tradition, and this occupies the whole space of the middle temple, while parts of the peristyles of the two others are built up in its side walls. The middle one was a hexastyle, peripteral temple, of the Ionic order, built of peperino. Of that on the right of the spectator, we may see a range of five of the columns of the flank. They have a sort of Corinthian capital, but I would not venture from the little that is seen of the architecture, to guess at the date. The third was Doric, and much smaller.

We may next visit the Theatre of Marcellus, of which but little is seen, as it is encumbered with the formless mass of the Orsini Palace, and that little is occupied by a range of dirty shops. Enough remains to show that the architecture has been such as we might expect from the date of its erection. It formed for a long while the model of the Doric order, and is still considered as the finest example remaining of what is called Roman Doric, although the introduction of dentils in the cornice is esteemed very licentious. The Roman Doric, however, is an order of which we have so few ancient examples, that our choice is extremely limited. Of the Theatre of Pompey nothing is to be seen but a few vaults, and a range of houses, whose circular form points out that they have been erected on the ancient foundations.

The Portico of Octavia is a fine fragment, in an awkward, dirty, disagreeable situation. The order has not much ornament, but it is well proportioned. The effect is spoiled by the brick walls and arches, which have been erected to supply the place of the deficient columns, and some of these are as early as the reign of Severus. Several of the antefixæ of this roof still exist, but I believe they have fallen down and been put up again, so that we cannot determine what was their precise situation.

This portico anciently enclosed two temples, and with a long neck you may contrive to see the Composite capital of a column of one of them, which is supposed to be the most ancient specimen of the order in existence; I can venture no criticisms on what is so imperfectly visible.

There are some vestiges of the baths of Agrippa in stables behind the Pantheon, upon which I shall not stop to speculate, nor upon the more conspicuous Arco de’ Ciambelli, which is also supposed to have belonged to them, but transport you at once to the Pantheon itself, the most perfect and majestic edifice remaining, of the good time of Roman architecture. I must according to my plan, begin with what may be learnt or guessed relating to its history. For a long while, the prevailing opinion was, that the body of the edifice was of the time of the republic, and the portico an addition made by Agrippa: indeed there were some very mysterious dreams about the purpose and antiquity of this temple. I have already mentioned the investigations of M. Achille Le Clerc; the cell seems to have formed a complete circular building itself, not essentially attached to any thing, except at the back, where some remains, attributed to the baths of Agrippa, appear to be united with it, but there is no trace of a door, or other opening, to form a communication between them. In front, the two masses of brickwork which at present support the turrets, are added to bring out a straight line to receive the portico, leaving between them a space which is covered with a vault, and which may be termed the pronaos. To these the portico is added, but there is no intimate union either between the circular part and these projections, or between the latter and the portico. A brick wall, rising considerably above the roof of the portico, is supported on the arch over the pronaos, which occupies the space continued from the three middle intercolumniations of the portico; and on this wall there is a pediment, or part of a pediment, for the top is cut off, and it seems uncertain if it ever formed a complete triangle; and it is this pediment, and the detached construction of the different parts, which have afforded the antiquaries a motive for believing the present portico a posterior addition. If, reasoned M. Le Clerc, the building was originally finished without a portico, we shall find traces of the method of completing it without one either on the circular cell, or on the face of the two additional masses of brickwork. He examined both with the greatest care, and in both was perfectly satisfied, from the interruption of the mouldings, and also from the offsets of the work, that no finishing could ever have taken place on either, but that the present portico, or something analogous to it, must have existed from the first.

Some differences in the materials and in the workmanship, the want of correspondence in the design, and the partial settlements which have taken place, are all adverse to the opinion that the whole building was executed at once, and two circumstances induce me to suppose that the cell is even posterior to the portico. The first is, that the use of burnt bricks was only recently introduced into Rome at the time of Agrippa, as appears from the manner in which Vitruvius speaks of them; and the first effort would scarcely be one of this magnitude and importance. My reasons for believing the use of burnt bricks to be adopted in Rome at so late a period, I mean to give you more in detail in a future letter. Perhaps we may use a similar argument concerning the form of the edifice, for though we have no precise idea of the date of the first introduction of domes, yet it seems probable that it must be assigned to about this period, and it would not be easy to imagine that the first erection would be on so large a scale; at the same time I must acknowledge, that we have no dome remaining of any size, which can put in a probable claim to a greater antiquity than the Pantheon. The second circumstance is, that the marble employed in the portico and pronaos, is the Pentelic, while that within is of Carrara. I do not think the passage in Pliny determines the first use of Luna, or Carrara marble in Rome; he may, perhaps, only have alluded to the discovery of a whiter and more beautiful bed, but I think we have sufficient remains to show that the employment of Greek marble in Rome, generally preceded that of the Italian. It has however, been contended, that the body of brickwork was erected before the portico: that this followed, and that all the marble finishings of the inside were added afterwards. The argument for this is the want of any correspondence between the marble finishings, and the openings and distribution of the brickwork. I will oppose to it a particularity in the construction of this brickwork. The Romans appear to have been early accustomed to the use of discharging arches; they introduced straight arches, (that is, an arrangement of bricks acting against one another as wedges, but kept in a line, nearly, or quite straight) over their openings, instead of solid stone lintels; but aware of the weakness arising from this method of employing the materials, they formed over them a semicircular arch, by which means the weight of the superincumbent wall was thrown upon solid parts which were able to bear it. In the earliest buildings which remain, and in general, even down to the time of Caracalla, this practice was used reasonably, and with moderation; but in later times, arches were employed, not for use, but for affectation; straight arches were employed where there were no openings, and discharging arches, which threw the weight rather on a weaker than on a stronger part. The brickwork of the Pantheon displays abuses of this sort, which is certainly a reason for assigning as late a date to its erection as is consistent with what we know of its history. Nevertheless, a cloud of witnesses establish the whole as the work of Agrippa, and perhaps, all that we ought to conclude from the want of correspondence in the different parts is, that different architects were employed, whose views were not altogether the same. Septimius Severus performed considerable restorations, but principally within side, and we have nothing to guide us as to the particulars of what was then executed, except the character of the work itself. This building, which seems to have contained no woodwork, except perhaps a small portion entirely concealed in the roof of the portico, is said to have suffered from fire; one proof among many that I could cite to you, that brick, stone, and bronze, were anciently combustible. After the transfer of the seat of empire to Constantinople, the Pantheon became an object, not of care, but of plunder. Constans robbed it of its covering of bronze tiles, and other depredations were made on it. About 608, Pope Boniface IV. consecrated it as a Christian church, but this did not preserve it from further spoliation. A little plaster now often serves to supply the place of the metals or marble taken away.

Modern buildings so surround the body of this edifice, that it is only very imperfectly that we judge of its merits as a whole; enough however is seen to prove the majestic and impressive character of this simple form. Its great distinction, in comparing its proportions with those of modern structures, is in the lowness of the dome, both as a whole, and in the circular part, which is left exposed. If we consider the roof of a modern church as a basement, we shall find the proportions of the drum and cupola more lofty than those of the Pantheon. In St. Peter’s, and in St. Paul’s, the architects have had recourse to a double, or triple construction in order to obtain this elevation, without producing internal deformity, and to this again a lantern is added, in order to give a still greater elevation to the design. This has been done with correct judgment, because the lower part of the edifice does not merely in picturesque effect act as a basement, but because it is united with the upper into an acutely pyramidal form, which it is important to preserve. If accessories were added to the Pantheon it would be necessary to give the general composition a very obtusely triangular outline. The Gothic artists aspired to a form more acutely triangular than those of the Italian architects. Each disposition has its beauties; the Gothic arrangement conveys the idea of power by the appearance of height, the ancient Roman, by that of extent. The modern Italians have attempted the union of the two. The obtuse triangle gives more the impression of strength and durability, and has also the advantage of producing a building of which a much greater proportion can be applied to internal use and effect. Three cornices surround the circular part. Palladio has introduced two stories of pilasters in the upper divisions. I know not what authority he might have had for this, but what we at present see is a rough brickwork, which was certainly covered in some way. The two turrets are modern, but it seems to me there must have been some additional elevation given to these parts in the original work. All our modern restorations agree in exhibiting a square mass behind the portico, which would look particularly bald and disagreeable, especially as it would be seen over the two pediments. The portico has eight columns in front, surmounted by a pediment, and is the only antiquity now remaining here, sufficiently perfect to exhibit the effect of this simple and beautiful arrangement. We have such things in the recent architecture of England, and some of them are very handsome, though in general they want depth. In Italy, I do not believe that it has ever been imitated by the moderns. We might, perhaps, find a reason for this in the very existence of such beautiful remains in the buildings of antiquity, and something must be allowed for the strong predilection for intricacy of form, which seems always, more or less, to have influenced the architects of this country. The same perfect simplicity of design is not preserved within the portico, it is divided by columns into something like three naves, and the centre of the great niche, on each side, is opposed to the centre of a column, and not to that of one of the spaces. The Romans began to spoil Grecian architecture as soon as they adopted it; substituting variety of form, and richness of material, to the simple elegance of arrangement, and the delicacy and beauty of finish which they found in their models. Simplicity may be carried to an extreme, and persons of the best and purest taste will differ as to the precise degree of it required. The interior of the portico of the Pantheon does not pass the limits of what a great number of amateurs would consider, not merely as an excusable, but as a desirable degree of intricacy; and compared to modern edifices, to the front of a Roman church for instance, or to the portico of St. Geneviève, at Paris, it is extremely simple. The columns are of granite, and it appears that, originally, those in front were of a granite with white felspar, which is usually called here oriental granite, and the internal ones of a granite, or syenite, containing red felspar, which is said to be brought from Egypt; of the reason of this difference I can give no guess. By a restoration of modern times, one of the columns of red granite has got in front. In some of the columns, the granite has scaled off by the action of the weather, parallel to the curved surface. They are all finely wrought, the bases and capitals are of white Pentelic marble, and the latter by no means all of equal excellence in point of execution. Urban VIII., who restored one of them, has inserted his bees instead of flowers in the abacus. The pilasters are of Pentelic marble, and in short all the other ornamental parts belonging to the portico and pronaos. The marble coating which once covered what is now naked brickwork, is gone nobody knows where, and the bare walls, and naked roofs, add to the grandeur of the edifice something of the melancholy of a ruin. The ceiling of the portico was of gilt bronze, but how this was disposed is a question which has been much agitated, the probable opinion is, that it formed a panelled vault over each division. Urban VIII. took away this bronze, then, as it appears, in a very decayed state, formed from it the four twisted columns which support the canopy over the high altar at St. Peter’s, and cast several cannon from the remainder. One of the nails, weighing forty-seven pounds, is said to be in England; the whole weight of nails was nine thousand three hundred and seventy-four pounds; the metal altogether weighed four hundred and fifty thousand two hundred and fifty pounds.

The marble doorway corresponds, both internally and externally, with the architecture of the portico, and not with that of the Pantheon itself: the opening is about nineteen feet wide and thirty-eight high, but within this, are pilasters of bronze which form the actual doorway. On these hang magnificent doors, also of bronze, and over them is a grating of the same metal. All these evidently form one thing, and belong to each other, and probably to the place where they are fixed: though it has been said that the original ones were carried away by Genseric, and that these were supplied from some other edifice.

I do not believe there is any person so insensible to the effect of architecture, as not to feel the surpassing beauty of this building internally; the simplicity and grace of its form, the beautiful colour of its marbles, principally of the giallo antico, and the delightful effect of its single central light, force themselves upon our admiration. The giallo antico is the most beautiful of all coloured marbles for the purposes of architecture, as its gentle glow is always harmonious, and the delicate variations of its tint are not such as to confuse the forms of the mouldings, while at the same time they are sufficient to relieve the deadness of a surface totally unornamented. Yet even here, the edifice has faults, and great ones. I do not mean by this, mere offences against arbitrary rules, but circumstances which are certainly injurious to its perfection, and to the agreeable impression on the mind of the spectator. The columns are rather too small in proportion to the size of the building; the entablature is disagreeably interrupted by two arches, and these arches on a curved surface are necessarily supine, that is, the crown falls back behind the springing, and this gives them an awkward appearance. The space above the columns, which is a modern alteration, so late as the last century, is altogether bad. Till then, an attic existed adorned with pilasters, formed not in relief, but by different coloured marbles. This was attributed to Septimius Severus, and not generally approved, but it seems to me to have been well calculated to preserve the general impression of the building, and to have contributed to give rather an increased value to the order below.