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 tous.

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, revo.ves; 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" telescope, in honor of the Rev. Mr. Craig, under whose direction, and at whose expense, it was com etructed. 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. Transit Instrument is a telescope used for observing the transits of celestial objects across the meridian, for the pur pose 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 cast and west abutinents, between which it is placed. On the left is seen, attached to the mason work, a graduated cicle; 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 ten is 10', the index must be moved 10° from 0, as the degrees on the circle and the ale:tude of the object will correspond.

70% What is a transit instrument? Size? How mounted? Describe parts as showe 1x the cat How set the instrument for the altitude of a star? What cordation the of?

TRANSIT INSTRUMENT, WASHINGTON, D. C.

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.

701 Where located? How mounted? By whom nad?? 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 instruinent 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 altitude of the object. The Mural Circle and a transit instrument, as now combined, are called n Meridian Circle.

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

2. The old Mural Circle is now being rapidly superseded by the Meridian Cirolo iz the best observatories,

706. A Comet Seeker is a refracting telescope with a large Aperture and short focal distance. As comets cannot be found by their right ascension and declination, but often have to be searched up, by sweeping around the heavens with a telescope, before they became visible to the naked eye. it is important tc have telescopes that will cover considerable sp ce—that is, of

A COMET SEKKER.

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wide aperture and short focal distance. Such a telescope was made by Mr. Fitz for Miss Mitchel, of Newport, R. I.

Miss Mitchel is an amateur astronomer, and has the honor of having discovered a num. ber of new comets.

707. An Astronomical Clock is a clock adapted to keep exact sidereal time. Taking the vernal equinox in the heavens as the zero point, and reckoning 24 hours eastward to the same point again, the time-reckoning 15° to an hour-when an object crosses the meridian, will always represent the right ascension of the object. Hence right ascension is usually given in hours, minutes, and seconds; or in time by the astronomical clock, set by the vernal equinox.

Professor Mitchel, we believe, made some valuable improvements in astronomical clocks. A very fine instrument of this kind is located in the Dudley Observatory, at Albany, N. Y.

REFLECTING TELESCOPES.

708. The Reflecting Telescope is one in which the light is converged to a focus by means of a concave metallic reflector or speculum. Like the Refractors, they may be constructed with very little mounting; though for convenience in use, it is neccs. sary to place the reflector in a tube.

The student should fully understand the difference between the two kinds of telecopes, viz.: refractors and reflectors. In one respect they are alike, as they both con. verge the rays of light to a focus; but they do it by widely different processes, as the following pages will show.

706. What is a comet sceker? Why necessary? 707. What is an astronomica! 708. Describe a reflecting telescope. Simplest form?

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