Notwithstanding the fact that a general description of the new Jefferson Physical Laboratory has recently appeared, the following details of the structure of the new building, taken from specifications in the hands of the architects, we believe will prove of interest. The total length of the building is 209 feet. There are two wings, one at each end of the building, 65 feet 4 inches in dimension each way; the central part is 72 feet in length and 46 feet in width. The westerly wing of the building is to be used for special students and professors conducting experiments of great delicacy, and requiring the greatest accuracy. Every precaution will be taken to ensure the complete isolation of the piers carrying the sensitive instruments. Iron in any shape will be rigidly excluded from the rooms where experiments in magnetism are conducted. The central portion will be used for recitation rooms for students and for cabinets containing instruments. The easterly wing will contain a lecture room, a laboratory, and cabinets for storing instruments. Above the basement the eastern wing will be divided into two floors by the lecture room and the laboratory. The distance from the lecture room floor to the laboratory floor will be 26 feet; this space corresponds to two stories of the western wing, to two stories of recitation rooms and to three stories of cabinets. The landings of the stairs will be so arranged as to communicate with these different rooms at their proper levels. To supply power, electric currents, etc., an engine house is to be constructed to the eastward. As the vibrations from this source might endanger the success of delicate experiments, care will be taken to completely isolate this building from the main building and all shafting will be carried on isolated piers. The foundations are to be built of the best Somerville slate laid in cement. The foundations of all the piers carrying sensitive instruments, and the foundations of the central tower of the western wing, will be entirely independent of the other foundations. The entire basement, excepting the boiler, coal and engine rooms, but including both the floor and roof of the constant temperature room, will be concreted two and one half inches thick with cement and gravel in two coats, of which the second coat will be pure cement put on before the first coat has entirely set. The floor of the fire, engine and coal rooms, will be made of hard brick set on a gravel bed and covered with cement. The floor of the laboratory and that part of the floor of the western wing that lies over the Rumford lecture room, and those portions of the floors of the central part of the building that lie over the two recitation rooms, will be deadened on the under floor by laying cement mortar, and covering the whole with stout manilla paper. The under floors are to be made of spruce plank, and he upper of hard pine; all the doors will be made of ash. In the rooms destined for experiments in magnetism, all the door frames, window frames, and all framing and construction whatever, will be put up with brass, bronze, copper, zinc, or bell metal nails, or screws, to the rigid exclusion of iron in any shape. The glass throughout the building will be of the best quality, double-thick German, ground on one side.