A telescope in its home. astronomer presses the button, the motor fulfils his bidding. Fig. 75 shows a large telescope ready for work. The pillar goes through the floor without touching it, and rests on a stone pier below. Near the bottom of the pillar are two hand-wheels, by means of which the telescope can be moved quickly into any desired position. Above them is a box containing clock-work which indicates the right ascension and declination of any object at which the telescope is pointing. Through a glass door in the uppermost section of the pillar one may see the driving clock. The declination axis is behind the tube. The observing platform, which slides up and down along an inclined runway, is shown at the left. The overarching iron dome rests upon antifriction wheels, which are on top of the stone wall. CHAPTER IX. THE ASTRONOMER'S WORKSHOP AND SOME OF HIS TOOLS. "Go to yon tower, where busy science plies In all their wanderings thro' the ethereal arch, -Holmes. AN astronomical observatory is conspicuous among surrounding structures by its unusual appearance. One or more domes surmounting it catch the eye at once. There are long narrow doors in the walls and shutters on the roof, which arrest attention. Fig. 76 is a representation of an observatory. First as to the site. The location is usually not a matter within the astronomer's control; he is fortunate if he is even allowed to plan the building, so as to adapt it to the purposes to which it is to be devoted. If he had his choice of location, he would be likely to choose a considerable elevation. An observatory. Its site. A mountain top would seem most suitable were it available; but experience shows that such is not usually A mountain. the case. The advantage is that the observer is above quite a thickness of atmosphere, so that the stars shine out more clearly, and faint objects are more distinctly visible. But the disadvantages are many. On a moun tain top the air is almost always in motion; warm currents rush up the sides of the mountain, and cooler air descends. The expansion of the warm and vaporladen air, which comes from below, chills it, and pro Obtrusive currents of air. The environs of a city. FIG. 76.-THE YERKES OBSERVATORY. duces mists, or even clouds, which hang about the summit. Even when no mist forms, whirling currents come between the telescope and the celestial object toward which it is pointed. The light from the object, in passing through these changing currents, is bent hither and thither, so that the object appears to dance, and to be distorted; no satisfactory view of it is possible. Furthermore the wind shakes the telescope itself, and renders accurate observations out of the question. It is generally admitted that an ideal site is an elevated plateau; the farther it is from a mountain range the better; a dry atmosphere is also considered advantageous. Where circumstances limit the location to the neighborhood of some city, a study of the prevailing winds is made, so that the evil of the city's smoke may be minimized. A spot of ground embracing a few acres, so that other buildings may not be built too near the observatory, and commanding a fair sweep of the horizon, is sought. It is advisable to avoid proximity to railroads, because of the earth tremors caused by the passage of heavy trains. When the site has been chosen and the instrumental equipment determined upon, the building is so planned as to furnish a suitable home for the instruments, and working quarters for the astronomer. The building shown in the illustration (Fig. 77) faces southward because the large telescope under the dome is chiefly used for observing objects in the south, east, or west, and is not often pointed northward. Were the building turned around, the observer would have to look over some portion of the roof most of the time. From the roof, which has been heated during the day, arise currents of warm air which would disturb telescopic vision. The building. FIG. 77.-THE CHAMBERLIN OBSERVATORY. To avoid these as much as possible the wings of the building are set back. The transit The meridian circle, the instrument next in importance, is now to be provided for. Shall it be in the room. east wing or in the west? If it is put in the west wing, The clocks. Temperature and humidity. Special supports. which is heated up by the afternoon sun, observations in the early evening will be vitiated by the currents of warm air rising all about it. The east wing, on the other hand, is largely protected from the sun in the afternoon, being in the shadow of the rest of the building. This instrument is therefore installed in the east wing; a continuous slit is cut in the roof and in the north and south walls, so that the telescope may survey the entire meridian from the north point of the horizon up to the zenith, and down to the south point. When the instrument is not in use the slit is closed by doors. The clocks are next to be suitably housed. Shall they be put in the west wing? By no means. For the heat of the afternoon sun would cause them to change their rates. Fine clocks are supposed to be so constructed that changes in the temperature will not cause them either to gain or to lose. But no clock has yet been made which will not change its rate under variations of temperature. Why, then, shall they not be placed in the deep basement underneath the tower, below the surface of the ground, where the thermometer will probably not vary 5° a day, in ordinary weather? In that location there will be another foe to fight; for a cellar, even though it be surrounded by a stone wall two feet thick and have a cement floor, is damp. The delicate mechanism of the clocks will suffer from this cause. The clocks must not stand on the floor or be hung upon wooden partitions. Special piers must be built to support them, unless there is some other adequate provision for them. In order to avoid changes of temperature a portion of the round tower is partitioned off, on the main floor. The space shown in Fig. 78 is so selected that no wall of the clock-room, except a very short length, where |