701. This is probably the best instrument in the United States. Its object-glass is 15 inches in diameter, with a focal length of 22 feet 6 inches. It has eighteen different powers, ranging from 103 to 2,000. It was made by Merz & Mahler, of Munich, Bavaria, and cost $19,842.

The cut shows the opening in the revolving dome of the observatory, and the observer In his chair at the eye-piece.

701. Comparative value? Size? Magnifying powers? By whom made? Cost of instrument?

702. This is the largest refracting telescope ever constructed The object-glass is two feet in diameter, with a focal distance of 76 feet. The tube is of heavy sheet iron, and shaped somewhat like a cigar. It is 13 feet in circumference in the largest place, and weighs about three tons.

This telescope is suspended from a brick tower, 65 feet high, 15 feet in diameter, and weighing 220 tons. The top of the tower, from which the telescope is suspended, revolves; and by a chain running over pulleys, and a weight and windlass, it is balanced, and raised or lowered. The lower end rests on a small carriage, that runs upon a circu.. lar railroad around the tower, at the distance of 52 feet from its center. By these means it is directed to almost any point in the heavens. It is called the "Craig" teiescope, in honor of the Rev. Mr. Craig, under whose direction, and at whose expense, it was conetructed. It is located at Wandsworth Common, near London.

Focal distance? T-be? How

702. Describe the Craig telescope. Object glass? mounted? Why called "Craig" telescope? Where located?

703. A Transit Instrument is a telescope used for observing the transits of celestial objects across the meridian, for the purpose of determining differences of right ascension, or obtaining correct time. They are usually from six to ten feet long, and are mounted upon a horizontal axis, between two abutments of mason-work; so that the instrument, when horizontal, will point exactly to the south. It will then take objects in the heavens, when they are exactly on the meridian.

The Transit Instrument and Mural Circle have been combined in one instrument, called a Meridian Circle, as shown on a subsequent page.

Let A D in the cut represent the telescope, and E and W the east and west abutments, between which it is placed. On the left is seen, attached to the mason work, a graduated circle; and on the eastern end of the axis of the telescope is seen an arm, n, extending to the circle, as an index. Now, suppose the index n to be at o, in the upper part of the circle, when the telescope is horizontal; then if the meridian altitude of the object to be taken is 10, the index must be moved 10° from 0, as the degrees on the circle and the altitude of the object will correspond.

703. What is a transit instrument? Size? How mounted? Describe parts as shown in the cut. How set the instrument for the altitude of a star? What coinbination spoken of?

704. This instrument is located in the National Observatory at Washington, D. C. It is mounted upon piers of granite, which rest firmly upon a foundation of stone, extending ten feet below the surface of the ground. The object-glass was furnished by Merz & Mahler, and the instrument was constructed by Ertel & Son, Munich. The entire cost was $1,480.

704. Where located? How mounted? By whom mad?? Cont?

705. This is a superior transit instrument, with a mural circle attached. It is located in the east wing of the Dudley Observatory, at Albany, N. Y., and rests upon piers of Lockport limestone, which rest upon a bed of sand and gravel, some ten feet below the floor of the cellar. Taken as a whole, it is probably the best transit instrument in the United States.

1. A Mural Circle is a large graduated circle, with a telescope crossing its center, used for the measurement of the altitudes and zenith distances of the heavenly bodies, at the instant of their crossing the meridian. They are usually fixed upon a horizontal axis, that turns in a socket firmly fixed in a north and south wall. The degrees, minutes, and seconds on the circle are read by means of microscopes, and indicate the altitudo of the object. The Mural Circle and a transit instrument, as now combined, are called a Meridian Cirole.

705. Where located? How mounted? Comparative importance? What is a Mural Circle! Use? How usually mounted? How combined? What called?