In a line directly opposite the back of the house, and closing up the view at the end of the gardens, stands the monument which is being erected to Washington. This, when finished, is to be a circular colonnaded building, 250 feet in diameter, and 100 feet high, from which is to spring an obelisk 70 feet wide at the base, and 500 feet high, so that, when completed, the whole will be as high as if our monument in London were placed on the top of St. Paul's. At present nothing but its ugly shaft is built, which has anything but a picturesque appearance, and it is apparently likely to remain in this condition, as it is not allowed to be touched by any but native republican hands, here a rather scarce commodity. It is being built of white stone, one of the many kinds found in this country. By the by, we omitted to state, in describing the Capitol, that the balustrades of the staircases, and a good deal of ornamental work about the building, are of marble, from a quarry lately discovered in Tennessee, of a beautiful darkish lilac ground, richly grained with a shade of its own colour; it is very valuable, costing seven dollars per cubic foot. From the President's house we went to the Observatory, which, though unpretending in its external appearance, is said to be the finest in the world next to the one at St. Petersburgh; so at least says the Washington Guide Book, for I like to give our authority for what we ourselves should not have supposed to be the case. Mr. Erskine introduced himself, and then us, to Lieutenant Maury, who is at the head of it, and is well known as a writer on meteorological subjects. He is a most agreeable man, and we talked much about the comet, meteoric stones, &c.; we asked him what he thought of Professor Silliman's notion about the comet's tail being an electric phenomenon, but he seemed to think little was known on the subject. He said this comet had never been seen before, and might never return again, as its path seemed parabolic, and not elliptical; but he said that what was peculiarly remarkable about it was the extreme agitation observed in the tail, and even in the nucleus, the motion appearing to be vibratory. With regard to meteoric stones, he said the one we saw at New Haven, though of such a prodigious size, being 200 lbs. heavier than the one in the British Museum, was a fragment only of a larger stone. We asked permission to go to the top of the observatory, and at a hint from papa, I expressed the great desire I had to see Venus by daylight, through the great telescope; whereupon, he sent for Professor B----, and asked him to take us up to the observatory, and to direct the great telescope to Venus. We mounted accordingly, and I was somewhat alarmed when the whole room in which we were placed, began to revolve upon its axis.