The northwestern tower, sixteen feet square and fifty feet high, was built for the reception of a Meridian Circle.

The extreme length of the edifice from the western tower to the east side is 117 feet; and the extreme depth is 112 feet. The three cuts which are given herewith exhibit the arrangement of rooms on each of the three stories. The basement, which is not here represented, contains a Janitor's apartment, and a metallurgical laboratory, in addition to the hot-air furnaces, store rooms, etc.

The Observatory occupies the two towers, each sixteen feet square, recently added to the edifice. In one of these is mounted an EQUATORIAL TELESCOPE; in the other, a MERIDIAN CIRCLE, with a SIDEREAL CLOCK; both telescope and circle being the recent gifts of Mr. Sheffield.

"10, Agricultural Lecture-room.

"11, Chemical Lecture-room.

"12, Private Chemical Laboratory.

"13, Study-Professor of Mineralogy and Metallurgy.

G, Study-Professor of Analytical and Agricultural Chemistry.

The Equatorial Telescope, ordered of Messrs. Alvan Clark & Sons, of Cambridgeport, in November, 1865, was, early in October last, mounted in the revolving turret at the top of the front tower, some eighty feet above the ground, where it commands a good horizon. It is supported by a freestone pier, six feet in height, which stands on a massive floor of masonry arched in from the side walls, just above the tower clock. Though it thus partakes of whatever motion the tower itself is subject to, from winds and other causes, no noticeable inconvenience has been experienced, or is anticipated, from this source. The floor of the room, which is of wood, immediately above the stone floor, rests only in the outer walls, and does not touch the pier.

The object-glass has a clear aperture of nine inches, and is nine feet ten inches in focal length. The tube, made of pine handsomely finished, and ten inches in diameter, is at once stiff and light. Seven Huygenian eye-pieces give powers ranging from 40 to 620. All but one of these fit also a diagonal eyetube containing a prismatic reflector. Another diagonal reflector-the first surface of an acute prism of glass-is used in observing the sun, the greater part

of whose light and heat is transmitted, while the image formed by the reflected rays is viewed without inconvenience, with the full aperture of the telescope.

The equatorial mounting is the German, or Fraunhofer's-the declination axis carrying a circle of twelve inches diameter, graduated on silver so as to read by two verniers to 10", and the polar axis carrying an hour circle of nine inches diameter, graduated to minutes of time, and reading by two verniers to five seconds.

Beneath the polar axis, in the curve of the U-shaped iron piece by which that axis is supported, is placed the driving clock. Its going is regulated by a half-second pendulum, and the intermittent motion of the scape wheel is changed into a smooth and equable motion for the telescope by the simple and ingenious device known as "Bond's Spring-Governor."

The performance of the telescope accords with the reputation of its makers. On favorable nights, it shows easily such test objects as Cygni, the companion of Sirius, the 6th star in the Trapezium of Orion, and, with more difficulty, Y 2 Andromedæ. The second and third have been seen with the aperture reduced to five inches.

There is used with the telescope a bi-filar position-micrometer, with four eyepieces, by Dollond.

A very simple observing chair enables the observer to change his position, quickly and easily, to any height required, without leaving his seat.

The revolving turret, resembling in form that of a "Monitor," rests, by a circular rail at its base, on eight grooved iron wheels, nine inches in diameter, the steel journals of which run in boxes of Babbitt's metal. It is turned by a crank, the pinion of which gears into a rack cast on the circular rail. The opening, three feet in width, extends entirely across, through the roof and sides, from base to base. It is closed by eight hinged shutters, so controlled by rods and levers as to be opened or shut with great facility.

The tower connected with the west wing was erected during the last summer, specially for the reception of the Meridian Circle purchased of the U. S Government and formerly used in the East room of the Washington Observatory. This instrument was mounted in September on the massive granite piers, which came with it, and the bases of which are imbedded in the upper part of a shaft of solid masonry, thirty-six feet in height, nine in diameter at the base, and seven at the top. This shaft rises, independently of the building throughout, from a foundation ten feet below the surface of the ground, and is surrounded, at a few inches distance, by a double casing made of tarred felt and matched sheathing boards. It is thus well protected against sudden changes of temperature.

The Meridian Circle has a five-foot telescope, with an object glass of 3-8 inches aperture, and 58 inches focal length. It has three Ramsden eye-pieces. A diagonal eye-piece in addition has been ordered, for more conveniently observing objects at high altitudes. At the focus is a system of one horizontal, and eleven vertical, spider-lines, together with a micrometer thread movable in declination only. The mean equatorial interval of the vertical threads is

14s 167.

The axis, thirty inches in length, terminates in steel pivots two inches in diameter, and to opposite faces of its central cube are bolted the two conical frusta forming the tube of the telescope. This tube is so constructed at the

ends that the object-glass and eye-tube are readily interchangeable. On the axis, within the piers, are two circles forty inches in diameter. They are graduated on silver, the one to read by a vernier to single minutes, the other by six micrometer microscopes, to single seconds. Four of the microscopes are mounted at the corners, and two at intermediate points on the opposite sides, of a square alidade frame, which is carried by the axis, and held in position by adjusting screws connected with the pier. Attached also to the alidade is a spirit level. Suitable counterpoises prevent undue pressure of the pivots on the Y's. For finding the nadir point, and the level and collimation errors, a collimating eye-piece and vessel of mercury are used. There is also a striding level for the axis; an observing couch; and a reversing apparatus traversing the floor on rails between the piers.

This instrument, as originally constructed by Ertel & Sons, of Munich, had a thirty-inch circle at each extremity of the axis, outside of the piers. These were subsequently replaced by the present forty-inch circles on the axis inside of the piers, by Wm. J. Young, of Philadelphia, who made other minor alterations. In the first and second volumes of the Washington Observations, this circle, in its original form, is fully described, and illustrated by plates. It has been put in adjustment, but not yet sufficiently used to test its performance.

The circle-room has a meridional opening from side to side, twenty inches in width, with roof-shutters, which are opened or shut by a single motion of a lever. The side shutters are ordinary doors.

A Sidereal Clock, by Appleton, London, the gift of William Hillhouse, Esq., of New Haven, is attached to the west wall.

The observatory possesses also a Pistor & Martin's Patent Sextant. The private instruments referred to as used by students in Practical Astronomy are: a superior portable Telescope, by Clark & Sons, of 4 inches aperture and five feet focal length; a Transit Instrument with three-foot telescope, and twelveinch circle reading by two verniers to 10"; a Sidereal Clock and an Eight-day Sidereal Chronometer. The telescope of the transit instrument has an objectglass, by Fitz, of 23 inches aperture, and a micrometer, so constructed as to be used with equal facility at all angles of position, without danger of disturbing the fixed system of threads. With this micrometer, and the spirit-level attached to the alidade of the circle, this instrument is practically a Zenith Telescope, and is used as such in observations for latitude by Talcott's method. The eyepiece constantly used is a diagonal one giving a power of 200, a power warranted by the excellence of the object-glass. The instrument is mounted on a heavy iron stand, cast in one piece, which is supported by a brick pier, four feet in height, with its foundation of masonry extending several feet below the surface of the ground.

The tower clock was made by Messrs. E. Howard & Co., of Boston. It has a wooden pendulum rod eight feet in length, with a zinc compensation-tube below the bob, specially ordered for this clock. The bob is of cast iron, and about twice as heavy as those usually furnished by the makers. The rate has thus far proved to be nearly uniform and quite satisfactory. The clock is set anew to zero, whenever its error amounts to half a minute. This has occurred but twice since August last. The hours are struck on a fine toned bell of 675 lbs. weight, suspended in a separate bell-tower on the main roof, some twentyfive feet from the clock.