The American Almanac and Repository of Useful Knowledge for the Year

received at this observatory on the 4th of December, 1846; the great tube and its parallactic mounting did not reach us until the afternoon of the 11th of June, 1847. It then required nine days to prepare the stone pier to which the telescope was to be attached. It being necessary that the upper surface of the stone should have an exact inclination in order to secure for the hour axis of the instrument an angle of elevation equal to the latitude of the place, a triangular frame of wood was made, having one of its sides vertical and another horizontal, the former watched by a plumb line and the latter by a spirit level, for the guidance of the workmen; this was found to answer the purpose perfectly, it being easily set whenever a trial of the inclination was thought necessary.

A strong platform was raised round the pier, with apparatus for raising the weighty machinery into place; the boxes containing the different parts of the telescope, which altogether were estimated to weigh about four tons, were arranged on the floor; and the process of putting together was commenced on the morning of the 23d of June. No accident occurred, and no step had to be retraced during the work. On the evening of the next day, the telescope was directed upon celestial objects.

It may here be proper to give a description of the building, with its revolving dome and pier, which had been prepared for the reception of this "Grand Refractor."

The part of the observatory which has been appropriated to this instrument is a square tower of thirty-two feet on a side. The walls are of brick, resting on a granite foundation. The interior is gradually brought into a circular form of thirty-one feet diameter, surmounted by a granite circle, on which is laid an iron rail of ten inches width, hollowed in the middle to serve as a track for the eight-inch iron balls on which the dome rolls. The dome is thirty feet, interior diameter, with an opening five feet wide, extending beyond the zenith. The shutters to this opening are raised and closed by means of endless chains working in teethed pulleys, and are easily managed by a winch and pinions, geered into wheels of one foot diameter. They are perfectly weather proof. To the lower edge of the dome is affixed a grooved iron rail, similar to the one laid on the granite cap of the walls. Eight iron balls, which had been smoothly and truly turned, were placed at equal distances round the circle, and the dome gradually let down to rest upon them. Although this dome is estimated to weigh about fourteen tons, yet it can be turned through a whole revolution by a single individual, without any very great exertion, in thirty-five seconds.

The central pier, for the support of the telescope, is of granite, and is in form the frustum of a cone, of twenty feet diameter at the base, and ten feet at the top. It is forty feet high, and rests on a wide foundation of grouting, composed of hydraulic cement and coarse gravel, and is entirely detached from every other part of the building. Upon the top of the pier is laid a circular cap-stone, ten feet in diameter and two feet thick; on this

stands, by three bearings, the granite block, ten feet in height, to which the metallic bed-plate of the instrument is firmly attached by bolts and screws, without any cement whatever. Five hundred tons of granite were employed in the construction of the entire pier.

The object glass of the telescope is fifteen English inches in diameter, and has twenty-two feet eight inches focal length. Some of the eye-pieces are six inches long, making the whole length a little more than twenty-three feet English. There are eighteen different powers, ranging from 180 to 2000. The declination circle is twenty-six inches in diameter, divided on silver, and reads by four verniers to four seconds in arc. The hour circle is eighteen inches in diameter, divided on silver, reading by two verniers to one second in time. The movable portion of the telescope and machinery is estimated to weigh about three tons. It is, however, so well counterpoised in every position of the telescope, and the effects of friction are so nearly obviated by an ingenious arrangement of rollers and balanceweights, that the observer can direct the instrument to any part of the heavens, by a slight pressure of the hand upon the ends of the balance rods. While observing, a sidereal motion is given to the telescope by clock-work, regulated by centrifugal balls.

Since the erection of the telescope, there have been only a few favorable moments, when the state of the atmosphere would allow of an examination of the more severe test objects, with the whole aperture and under high powers. At these times the optical power of the instrument has been fully recognized. The components of the star y Coronæ, which Struve, with the Pulkova refractor, pronounces most difficult to separate, are seen in the Cambridge telescope, distinct and round, the dark space between them clearly defined. The same distinctness attends the separation of 22 Andromeda, with our telescope. Nearly all the best European instruments show this star as only elongated in a particular direction; in ours, on the morning of the 20th of July, it was well divided, and measures of distance were obtained with the filar micrometer, while the sun was shining on the object glass. The primary chromatic dispersion seems to be as nearly corrected as possible. This has been apparent when tried on Venus and the Moon.

The nebulæ are well shown by the telescope. That in Vulpecula, 27 Messier, it exhibits with multitudes of points of light or stars, star dust, as it is sometimes called, together with the transverse nebulous appearance spoken of by Sir John Herschel. Planetary and stellar nebulæ it shows beautifully. The companion of Antares, discovered by Prof. Mitchell with the Cincinnati Refractor, is quite conspicuous with a power of 700.

The great nebula of Andromeda is seen with a small, well-defined, central nucleus, unlike the gradual condensation of many of the other nebula. There are a great number of stars visible within the boundary of its light; thirty have been pretty well located. But, unless this nebula is of an

inconceivable depth, they can hardly be considered as belonging to it, for the greater part retains the primitive character assigned to it by Simon Marius, namely, as resembling the light of a candle shining through horn. This very interesting object has apparently yielded less to the increase of optical power than any other celestial object which is visible to the naked eye, although the elder Herschel considered it as one of the nearest of the large nebula.

In the neighborhood of a Lyræ, within a circle surrounding that star of less than seven minutes diameter, upwards of thirty stars have been counted. The ring nebula of Lyra is shown more elongated than it is represented in the drawings of it in the Philosophical Transactions, with numerous points of light in the interior; the north preceding portion of the ring is considerably fainter than the rest. The minute double stars, mentioned by Lord Ross as difficult objects with his twenty-seven feet reflector, are seen in our telescope. Upon the whole, there is sufficient reason to be satisfied with the optical character of this instrument, particularly when we consider, that since it has been mounted, there have been but a very few hours when the state of the atmosphere would allow of using to advantage so high a power as 700, with the full aperture of the object glass.

PINGRÉ'S PREDICTED COMET OF 1848.

By George P. Bond, Assistant at the Cambridge Observatory.

The two comets numbered (17) in Prof. Peirce's catalogue of comets, which was published in the last volume of the American Almanac, were suspected by Pingré to be the same comet, at different returns, on account of the close similarity of their orbits. The returns in 1264 and 1556, give a period of about 292 years, so that the next return should be in 1848. Pingre's opinion seems to have been generally approved by geometers, and has been subjected to a very thorough and rigid examination, by Mr. Hind, of England, who has given coördinates, by which the computation of the quarter of the heavens, in which it must be looked for at different seasons of the year, may be readily performed, and from which the following table has been computed. In this table, the right ascension and declination are given, which the comet must have at the time of its discovery, provided this time is less than ninety days before the date of its perihelion passage, or less than eighty days after this date. The column headed ▲ contains the distance of the comet from the earth, and that headed contains the brilliancy of the comet, supposing the unit of brilliancy to correspond to the unit of distance from the sun and the earth. At its former returns, the comet approached exceeding near the earth, so that it is quite improbable that it will be seen again under as favorable circumstances for the magnificent display of its brilliancy.