Rome, Saturday, June 29, 1851.

St. Peter's is the Niagara of edifices, having the same relation to other master-pieces of human effort that the great cataract bears to other terrestrial effects of Divine power. In either case, the first view disappoints, because the perfection of symmetry dims the consciousness of magnitude, and the total absence of exaggeration in the details forbids the conception of vastness in the aggregate. In viewing London's St. Paul's, you have a realization of bulk which St. Peter's does not give, yet St. Paul's is but a wart beside St. Peter's. I do not know that the resemblance has been noticed by others, but the semi-circle of gigantic yet admirably proportioned pillars which encloses the grand square in front of St. Peter's reminds me vividly of the general conformation of our great water-fall, while the column or obelisk in the center of the square (which column is a mistake, in my humble judgment, and should be removed) has its parallel in the unsightly tower overlooking the main cataract from the extreme point of Goat Island. Eternal endurance and repose may be fitly typified by the oceans and snow-crested mountains, but power and energy find their best expressions in the cataract and the dome. Time and Genius may produce other structures as admirable in their own way and regarded in connection with their uses; but, viewed as a temple, St. Peter's will ever stand unmatched and unapproachable.I chose the early morning for my first visit. The sky was cloudless, as it mainly is here save in winter, but the day was not yet warm, for the summer nights are cooler here than in New-York, and the current English talk of the excessive heat which prevails in Rome at this season is calculated to deceive Americans. No one fails to realize from the first the great beauty and admirable accessories of this edifice, with the far-stretching but quite other than lofty pile of the Vatican on its right and its own magnificent colonnade in front, but you do not feel that it is lofty, nor spacious, nor anything but perfect. You ascend the steps, and thus gain some idea of the immense proportions prevailing throughout; for the church seems scarcely at all elevated above the square, and yet many are the steps leading up to the doors. Crossing a grand porch with an arched roof of glorious mosaic, you find yourself in the body of the edifice, which now seems large and lofty indeed, but by no means unparalleled. But you walk on and on, between opposing pillars the grandest the world ever saw, the space at either side between any two pillars constituting a separate chapel with its gorgeous altar, its grand pictures in mosaic, its sculptured saints and angels, each of these chapels having a larger area than any church I ever entered in America; and by the time you have walked slowly and observingly to the front of the main altar you realize profoundly that Earth has nothing else to match with St. Peter's. No matter though another church were twice as large, and erected at a cost of twice the Thirty Millions of dollars and fifty years expended upon this, St. Peter's would still stand unrivaled. For every detail is so marvellously symmetrical that no one is dwarfed, no one challenges special attention. Of one hundred distinct parts, any one by itself would command your profoundest admiration, but everything around and beyond it is no less excellent, and you soon cease to wonder and remain to appreciate and enjoy.I devoted most of the day to St. Peter's, seeing it under many different aspects, but no other view of the interior is equal to that presented in the stillness and comparative solitude of the early morning. The presence of multitudes does not cloud your consciousness of its immensity, for ten thousand persons occupy no considerable portion of its area and might very easily be present yet wholly invisible to one who stood just inside the entrance and looked searchingly through the body of the edifice to find them; but there are usually very few seats, and those for the privileged, so that hundreds are constantly moving from place to place through the day, which distracts attention and mars the feeling of repose and delighted awe which the naked structure is calculated to inspire. Go very early some bright summer morning, if you would see St. Peter's in its calm and stately grandeur.

I ascended to the roof, and thence to the summit of the dome, but, apart from a profounder consciousness of the vastness and admirable proportions of the edifice, this is of little worth. True, the entire city and its suburbs lie clearly and fully beneath and around you; but so they do from the tower of the Capitol. Views from commanding heights are obtained in almost every city. The ascent, however, as far as the roof, is easier than any other I ever found within a building. Instead of stairs, here is a circular road, more like the ascent of a mountain than a Church. One single view is obtained, however, which richly compensates for the fatigue of the ascent. It is that from the interior of the dome down into the body of the Church below. The Alps may present grander, but I never expect to have another like this.

Here I had personal evidence of the mean, reckless selfishness wherewith public edifices are regarded by too many, and the absolute necessity of constant, omnipresent watchfulness to preserve them from wanton dilapidation. Five or six French soldiers had been permitted to ascend the dome just before I did, and came down nearly at the same time with me. As I stood gazing down from this point into the church below, two of these soldiers came in on their way down, and one of them, looking around to see that no one was present but a stranger, whipped the bayonet he wore out of its sheath, forced the point into the mosaic close behind as well as above us, pried out one of the square pieces of agate or some such stone of which that mosaic is composed, put it in his pocket and made off. I had no idea that he would deface the edifice until the moment he did it, and then hastily remonstrated, but of course without avail. I looked at the wall on which he operated, and found that two or three had preceded him in the same work of paltry but most outrageous robbery. Of course, each will boast of his exploit to his comrades of kindred spirit, and they will be tempted to imitate it, until the mischief done becomes sufficiently serious to attract attention, and then Nobody will have a serious reckoning to encounter. A few acts of unobserved rapine as trifling as these may easily occasion some signal disaster. In an edifice like this, there should be no point accessible to visiters unwatched by a faithful guardian even for one hour.

In the afternoon, I attended the Celebration of High Mass, this being observed by the Catholic world as St. Peter's Day, and the Pope himself officiating in the great Cathedral. Not understanding the service, I could not profit by it, and the spectacle impressed me unfavorably. Such a multiplicity of spears and bayonets seem to me strangely out of keeping in a place of worship; if they belong here, why not bring in a regiment of horse and a park of artillery as well? There is ample room for them in St. Peter's, and the cavalry might charge and the cannoniers fire a few volleys with little harm to the building, and with great increase both to the numbers and interest of the audience. I am not pretending to judge this for others, but simply to state how it naturally strikes one educated in the simple, sober observances of Puritan New-England. I have heard of Protestants being converted in Rome, but it seems to me the very last place where the great body of those educated in really Protestant ways would be likely to undergo conversion. I have seen very much here to admire, and there is doubtless many times more such that I have not seen, but the radical antagonism of Catholic and Protestant ideas, observances and tendencies never before stood out in a light so clear and strong as that shed upon it by a few days in Rome. I obtained admission yesterday to the Sistine Chapel of the Vatican, and saw there, among the paintings in fresco, a representation of the death of Admiral Coligny at the Massacre of St. Bartholomew; and if this were not intended to express approval of that horrible massacre, I would like to know what was meant by having it painted and placed there.

But to return to St. Peter's. The entrance of the grand procession from the Vatican was a very slow process. In its ranks were the Noble Guard, the Swiss Guard, the Cardinals, and many other divisions, each in its own imposing and picturesque costume. At length came the Pope, seated in a magnificent chair on a raised platform or palanquin, the whole borne on the shoulders of some ten or twelve servitors. This was a capital arrangement for us strangers, who wished a good view of His Holiness; but I am sure it was very disagreeable to him, and that he would much rather have walked like the rest. He passed into the church out of my sight, dismounted, and I (having also entered) next saw him approach one of the altars on the right, where he knelt and silently prayed for some minutes. He was then borne onward to his throne at the further end, and the service commenced.

The singing of the Mass was very good. The Pope's reading I did not hear, nor was I near enough even to see him, except fitfully. I think there were more than five thousand persons present, including a thousand priests and a thousand soldiers. There would doubtless have been many more, but for the fact that a smart shower occurred just before and at the hour (5 o'clock), while no public notice had been given that the Pope would officiate.

In the evening, St. Peter's and its accessories were illuminated—by far the most brilliant spectacle I ever saw. All was dark and silent till, at the first stroke of the bell, light flashed from a hundred thousand burners, and the entire front of the Church and Dome, up to the very summit of the spire, was one magnificent galaxy, while the double row of gigantic pillars or columns surrounding the square was in like manner radiant with jets of flame. I thought the architecture of St. Peter's Rome's greatest glory when I had only seen it by daylight, yet it now seemed more wondrous still. The bells rang sweetly and stirringly throughout the evening, and there was a like illumination on the summit of the Pincian Hill, while most of the shops and dwellings displayed at least one row of burning candles, and bonfires blazed brightly in the streets, which were alive with moving, animated groups, while the square of St. Peter's and the nearest bridges over the Tiber were black with excited thousands. To-night we have fire-works from the Pincian in honor of St. Peter, which would be thought in New England an odd way of honoring an Apostle, especially on Sunday evening; but whether Rome or Boston is right on this point is a question to be pondered.

P. S. Monday.—I did not see the Fire-Works last evening, but almost every one else in Rome did, and the unanimous verdict pronounces them admirable—extraordinary. Great preparations had been made, and the success must have been perfect to win so general and hearty a commendation. The display was ushered in by a rousing salute of artillery; but this was not needed to assemble in and around the Piazza del Popolo all the population of Rome that could be spared from their homes. The Piazza is the great square of Rome, in front of the Pincian Hill, whence the rockets, wheels, stars, serpents, &c., were let off. The display was not concluded till after 10 o'clock.

This day I have devoted to famous private galleries of Paintings and Sculpture, having been again disappointed in attempting to gain a sight of the Apollo Belvedere and Picture Gallery of the Vatican. The time for opening these treasures to the public has lately been changed from 10 A. M. to noon, and they are only open regularly on Mondays; so that I was there a little before noon to be ready; but after waiting (with many others) a full hour, in front of an inexorable gate, without being able to learn why we were shut out or when the embargo would cease, I grew weary of the uncertainty and waste of time, and left. A little past 1 (I now understand), the gate was opened, but too late for me, as I did not return, and leave Rome for Florence to-morrow. Had the simplest notice been given that such a delay would take place, or had the officers at the gates been able to give any information, I should have had different luck. "They manage these things better in France."