room.

two windows are, is an outside wall of the building. These two windows are made double, and covered with The clockwooden shutters. Thus both the sun's rays and the storms of winter are guarded against. If the clocks were hung on the stone wall which partly bounds the room, the turning of the dome, which rests on this wall,

FIG. 78.-MAIN FLOOR OF THE CHAMBERLIN OBSERVATORY.

might jar them a trifle. Therefore the great pier in the center of the tower is utilized. Stout beams are built into the pier, and project through the thin partition into the clock-room; the beams do not touch the partition, for in that case the vibrations of the floor, as people walk about on it, would shake the clocks.

The west wing contains the study of the astronomer. The study.

He does not care for the heat of the long summer afternoons, if the instruments are protected from it.

In the basement arrangements are made for the heatThe basement. ing plant, which, if one has plenty of money to spend, is a hot-water system; also for a photographic dark-room, battery-room, janitor's quarters, storeroom, and workshop. A good carpenter's bench and a small kit of tools are needed. If the observatory is a large one, a lathe and other machines for working metals are a part of the equipment. Quite a little of the basement is occupied by the piers on which the instruments rest.

Foundations and floors.

The upper story.

No heat.

The foundations of these are sunk pretty deep, the depth depending upon the character of the soil. A gravel bed makes an excellent foundation; rock or hard clay is also satisfactory, except that they readily transmit vibrations arising from the passage of railroad trains within half a mile, or heavy traffic in a neighboring street. The floors must not touch any of the piers, for, in that case, the vibrations caused by human footfalls will be communicated to the piers and thus to the instruments.*

In the upper story of the observatory the principal room is the dome-room, the home of the great telescope. On a level with the floor of this room is an extensive balcony from which one can glance at all parts of the sky. Two or three small rooms adjoin, where various attachments of the telescope are kept, and where the observer may occasionally warm himself on a bitter night.

It is not practicable to heat the dome-room, for the

*Before the telescope of the Chamberlin Observatory was installed, the floor of the dome-room was shored up on the great pier, so that it might not sag when the various parts of the telescope were laid on it, preparatory to being put together. After the telescope was mounted, the props were forgotten for a time, and every star under observation danced about in a most disheartening manner, as people walked about the room. remembered and knocked out. The trouble ceased at once. In a few days the props were which had been so shaken weighs 320 tons. The stone pier

heated air would escape through the slit, when the dome shutter was rolled off. Nor is it allowable to experiment in this direction, because a current of warm air rising in front of the large glass would cause the stars to appear blurred and to dance about in such fashion that no satisfactory views of them could be had.

An

Domes more than thirty-five feet in diameter are built of iron. They are made as light as is consistent with a proper degree of rigidity, and are covered with heavy galvanized iron. Great care is taken to mount them in such a manner that they will rotate with ease. astronomer whose strength has been exhausted by turning an unmanageable dome is in no physical condition. to manipulate a delicate instrument, the smallest reading of which corresponds to a distance of r of an inch.

Domes.

Where a good current of electricity and a small electric motor are available, the observer has but to touch a A rising floor. push button, and the dome revolves. For very large telescopes, the floor of the dome-room is made of iron, and is raised or lowered by powerful machinery, which may be started and stopped by pressing a button.

Some of the astronomer's

Some of the astronomer's tools are so important and so common that we must examine them. The great tools. telescope which was described in the last chapter is much too cumbersome to be used in the most refined investigations for determining the right ascensions and declinations of "fundamental stars. The instrument used for this purpose is comparatively small, extremely rigid, and so mounted that it can view a celestial object only when the latter is near the meridian.

Fig. 79 shows that the instrument consists of a telescope, which is perpendicular to a horizontal axis. The axis points east and west and terminates in two cylindrical steel pivots, each of which rests in a wedge-shaped

The graduated circles.

metal bearing called a V,* from its resemblance to that letter. These bearings are fastened very securely to two substantial piers, generally of stone.

Upon the axis are mounted two circles, one, at least, of which carries a band of silver, on which fine marks, technically called "divisions" or "graduations" have been cut with the utmost accuracy. If each division

Measurement of angles.

utes constitute one twelfth of a degree, there are 12 x 360, or 4,320 graduations on the circle.

If the tele

scope, which is now pointing upward, were turned so as to point downward, the graduated circle would turn with it, and the angle through which the telescope was turned could be measured by means of a suitable fixed pointer placed close to the silver graduations. If the pointer were opposite the 10° mark on the circle when the telescope was pointing directly upward, it would be opposite the mark for 190° when the telescope pointed *This bearing is commonly referred to as a "wye."

straight down, the circle having been turned just half
way round.
Instead of one pointer there are usually
four; the silver graduations are so fine that they cannot
be well seen without a magnifying glass; the pointer
must therefore be very fine, and the spider is called.
upon to furnish it.

microscopes.

The spider-web, which is to serve as a pointer, is placed inside of a microscope, which is sighted at the The reading silver circle. To insure great accuracy in reading the circle four microscopes are frequently employed. They are shown in Fig. 79, being fastened to a metal drum, which rests on top of one of the piers. On looking through one of the microscopes one sees the spiderweb, and also the magnified divisions on the circle. At the outer end of each microscope a little box is placed; this contains a measuring instrument called a micrometer. If the spider-web does not appear to coincide with one of the graduations on the circle, its distance from the nearest graduation is measured with the micrometer. The silver circles are usually read to the nearest tenth of a second of arc. If such a circle be ninety inches in circumference, a tenth of a second is only of an inch.

Standing upon the horizontal axis of the instrument is a metal frame which supports a delicate level, the sensitiveness of which is astonishing. Suppose that two points on the level tube, one eighth of an inch apart, are in the same horizontal plane at a given instant. If by some movement of the instrument one of the points is raised a millionth of an inch above its neighbor, the level bubble will move.

A peep through the eyepiece of the telescope reveals a forest of black lines; at night, when lit up by a special lamp, they appear as a system of golden wires. In Fig.

The level.

The reticle.