instruments and storage. The ground floor or basement affords space at the western end for photographic dark rooms and enlarging room, emulsion room, constant temperature room (including space for clocks), physical laboratory, and concave grating room; and at the eastern end for optical, instrument, and pattern shops.

The attic between the two small towers is 104 feet long and 12 feet wide. It is fitted up as a heliostat room. A portion of the roof near the northeast dome is mounted on wheels which run on steel rails. By a windlass this portion of the roof can be drawn to the southward far enough to allow the Sun's rays, at all seasons of the year, to fall upon a heliostat placed near the northern end of the room. A heliostat having a mirror of 24 inches aperture is being made in the shops of the observatory. The large attic rooms along the main axis of the building are so arranged that they can be used in conjunction with the heliostat room for the use of apparatus having lenses or mirrors of great focal length.

The spectroscopic laboratories have solid brick piers on concrete foundations. These are so arranged, with reference to the doors and windows, that the instruments mounted upon them can be used in conjunction with each other, or with instruments in the open air. One of these laboratories is especially arranged for bolometric work. The apparatus for these laboratories includes spectroscopes of various kinds, bolometers, galvanometers, interferential refractometers, induction coils, and a variety of subsidiary apparatus.

The concave grating room is designed to contain a concave grating of 21 feet radius, mounted in the usual manner. At present there are mounted here a 4-inch grating of 10 feet focus and a smaller one of 6 feet focus, both from the Kenwood Observatory. The physical laboratory adjoins the concave grating room, and the latter is so arranged that it can be used in conjunction with apparatus in the former. Both are provided with rolling wooden shutters so that the light can be effectually excluded.

At the Kenwood Observatory, Professor HALE found that many of the problems with which he had to deal, involving, as they did, new methods of research, required the construction of instruments of new and special design. While the principal instruments used there were obtained from BRASHEAR and from WARNER & SWASEY, it was found necessary to have a workshop in which nearly the entire time of an instrument maker was

employed in constructing pieces of apparatus required in the solar and spectroscopic work. This shop proved so indispensable to the Kenwood Observatory that it was decided to provide the Yerkes Observatory with the best facilities for mechanical and optical work. A room, 18 by 54 feet, for metal working, was selected on the ground floor of the observatory in the southeast quarter of the building, with smaller adjacent rooms to the east fitted up as a forge room and a pattern shop.

The machine tools used at Chicago were an engine lathe, a shaper, a small speed lathe, an 8-inch Rivett "Precision " lathe, and a Brown & Sharpe universal milling machine. These have been transferred to the new shops, and a planer, a drill press, a circular saw, and speed lathes added.

Two mechanicians are regularly employed in this shop. Several important machines and various pieces of apparatus are in process of construction. A friend of science in Chicago has provided the means of employing a third mechanician for the express purpose of constructing a machine for ruling gratings, according to plans by Professor WADSWORTH.

The optical shop (20 by 70 feet), with rooms fitted up for grinding, polishing and testing lenses and mirrors, is on the north side of the building, just across the hall from the machine shop. The walls of these rooms and the double windows are so constructed as to maintain a nearly constant temperature, a condition necessary for the most successful conduct of the work. Some large pieces of optical work have already been completed in this shop, and still more important ones are planned. A large grinding machine has been constructed under the direction of the observatory optician, Mr. G. W. RITCHEY, for the purpose of grinding and polishing a 60-inch mirror, to be used for stellar spectroscopic work. The work of rough grinding has already been undertaken.

The 40-inch telescope, with its dome and elevating floor, are the principal attractions of the Yerkes Observatory, viewed from a popular standpoint. These are the largest in the world. The dome is 90 feet in diameter, 60 feet high above the top of the tower upon which it rests, or 112 feet above the ground. Its framework is of steel, riveted together. This is covered, first, with a sheathing of wood, and next with roofing tin. It is supported upon 36 wheels, each 36 inches in diameter, and is turned by an endless cable which passes around the dome and is connected with the driving mechanism. The cable is driven

by an electric motor, controlled by a switch on the observing floor. Provision is also made for revolving the dome by hand. The wheels upon which the dome revolves have journals with roller bearings for relieving the friction, and are so constructed as to adjust themselves to possible inequalities of the track.

The observing slit is 13 feet wide, and extends from the horizon to a point 5 feet beyond the zenith. The shutters covering this opening are arranged to open simultaneously on either side, and remain parallel in all positions. Adjustable canvas curtains are placed within the opening to protect the telescope, in whatever direction it may be pointed, from the wind.

It is of interest to compare this dome with the one the next largest; namely, that of the Lick Observatory. The large dome at Mt. Hamilton has an outside diameter of 74 feet 4 inches, and an inside diameter of 71 feet, and weighs, including shutters and live ring, 99% tons. The live ring itself weighs 121⁄2 tons. This dome rises 41 feet 8 inches above the top of the supporting tower, and 76 feet 10 inches above the ground. The dome is supported on a live ring consisting of 21 conical rollers, each roller having three wheels. The base plate of the dome rests on the central wheel of each group, while the outside wheels rest upon the lower track. The two rails of this track are a part of a conical surface with its apex in the vertical axis of the dome, and in the plane with the tops of the rollers. The upper track is a plane surface. The outside wheels of the live ring are 30 inches in diameter, and the inside ones 281⁄2 inches. The three wheels of each roller were pressed on a steel spindle 31⁄2 inches in diameter, and the journals at the extremities of these spindles are provided with roller bearings for avoiding sliding friction. The framework of the dome is of steel construction, and it is covered with galvanized steel plates.

The observing slit is 9 feet 634 inches in the clear, and extends from near the horizon to a point 31⁄2 feet beyond the zenith. This opening is closed with double shutters, hinged at a point beyond the zenith, and supported on wheels resting on a track below. These shutters open simultaneously, but do not remain parallel. The dome is turned by a cable, operated by an hydraulic engine. It may also be turned by hand.

The elevating floor of the Yerkes Observatory is 75 feet in diameter, and rises through 22 feet. It is supported by wire cables, 90° apart. These cables pass over large drums, and are