The length of the building, from Seventh to Ninth streets, is 410 feet, and its width, from F to G streets, 275 feet. Its original design was made by Mr. William P. Elliot, at that time surveyor of the City of Washington. The plan was largely executed by Mr. Mills, architect of Public Buildings; while the grand northern portico has been consummated under the superintendence of Mr. Edward Clark, the present architect of the Capitol. We enter the eastern door of the basement-story, into a spacious passage running from east to west, the whole length of the building. Through it, large-wheeled machines can be drawn. On each side of this hall are rooms for the deposit of fuel, large and heavy models and department offices. In the centre springs a semi-circular stone staircase, with three flights of steps, which ascend to the second, third and last story. The corridor in the first story is like the one that we entered below, and on each side of the hall, doors open into commodious apartments for the accommodation of the commissioners, examiners, clerks, etc. Ascending the stone staircase, we come to the Model Room—_par excellence_, the Exhibition Room of the nation. For architectural simplicity and space, and the purpose for which it was designed, it is unsurpassed in the whole world. Standing here, we look down a vista two hundred and seventy-four feet in length, and its perspective is enchanting to the sight. A double row of stone columns supports a succession of brick arches, finely proportioned, and corresponding in depth with the rooms below. The floor is paved with tessellated stone, and the light streams in from numerous windows on each side. The models and other articles are arranged in glass cases on each side of the room, leaving ample space in the centre for promenading. There are two rows of cases, one above the other—the upper row being placed within a light gallery of iron, reached by iron stairways, and extending entirely round the east, north and west halls. The ceiling is supported by a double row of pillars, which also act as supports to the galleries, and both the walls and ceiling are finished in marble and frescoes. Entering, we find a large register, with pens and ink, at the right of the door, in which we may record our name and the date of our visit, if we please. [Illustration: THE MODEL ROOM, PATENT OFFICE.—WASHINGTON. This room contains the fruits of the inventive genius of the whole nation. More than 160,000 models are here deposited. ]