the instrument is taken off by a counterpoise acting by levers which push a slider upwards against the pivot. To support the upper pivot, an iron triangle is established on the three-rayed pier. On each side of this, is erected another iron triangle, whose plane is vertical, and whose sides unite in a vertex which forms one of the angles of a corresponding triangle above. This upper triangle supports three radial bars, which carry at their point of union the Y in which the upper pivot plays. The bars of the lateral triangles, which are apparent in the drawings, pass the holes in the floor without touching it.

The frame, revolving in azimuth and carrying the instrument with it, consists of a top and bottom connected by vertical cheeks, all of cast-iron. The supports of the four microscopes for reading off the azimuth on the lower circle are cast in the same piece with these vertical cheeks.

The vertical circle carrying the telescope is 3 feet in diameter, and, like the azimuth circle, is made of hard gun metal. The aperture of the object-glass is 3 in. The top and bottom of the instrument each carries two levels, parallel to the plane of the horizontal axis, used in observations of azimuth; and two levels are fixed on one of the vertical cheeks parallel to the plane of the vertical circle, used in observations of zenith distance.

The ladder

The dome over the instrument is cylindrical, with double sides, between which the air passes freely. Its diameter is 10 feet. The drawing represents the instrument as in use. revolves in azimuth, round the central pier, to facilitate which motion, rollers are placed under it. A metal frame is attached to the vertical cheek of the instrument, having its edges in a plane parallel to that of the vertical circle. The eye being directed along these to view the object, the instrument is placed very nearly in the proper azimuth, and the telescope is then accurately directed to the object by the ring-finder. This frame is omitted in the drawing.

The results of the observations made with this instrument are stated to have fulfilled all the anticipations of the Astronomer Royal, as well as to the number of observations as to their excellence. The number of observations have exceeded those made with the meridional instruments in the proportion of about 16 to 9. Some have been made even within a day of conjunction.

50. The Northumberland equatorial — Cambridge Observatory. The late Duke of Northumberland, who filled during the latter part of his life the high and honourable office of Chancellor of the University of Cambridge, presented to that university this instrument, which, successively in the hands of the Astronomer Royal and Professor Challis, has contributed so effectually to the advancement of astronomical science.

The instrument, of which a perspective view is given in Plate X., together with a view of the building in which it is erected, consists of a refracting telescope of 193 feet focal length and 11 inches aperture equatorially mounted. The polar axis, as appears in the drawing, consists of a system of framing composed of six strong deal poles, attached at the ends to two hexagonal frames of cast-iron, the centres of which support the upper and lower pivots on which the telescope revolves. These poles at the middle are braced by transverse iron bands, and by a system of diagonal rods of deal abutting near the middle of the poles. These give stiffness to the entire framing of the polar axis, and maintain the hexagonal frames square to it. Efficient means are provided to give elasticity to the supports of the pivots and smoothness to the equatorial motion.

The tube of the telescope is made of well-seasoned deal, and attached to one side of it is a flat brass bar, 6 feet long, carrying a small graduated arc at right angles to it at one end, and turning at the other on a pin fixed in the telescope tube at a distance of 30 inches from the axis of revolution. This arc, which is called the declination sector, serves to measure small differences of declination, and is read by a micrometer microscope fixed to the telescope tube.

The hour-circle, which measures the equatorial motion, is 5 feet in diameter, and is so arranged that it can be clamped to the telescope, or disengaged from it, at pleasure. It has two indices with verniers, one fixed to the support of the lower pivot, and the other to the hexagonal frame. By setting the latter to a certain angle, determined by an observation of a star of known right ascension, the telescope can be directed to any proposed right ascension by means of the other index. Observations of right ascension can be made to I second of time. The outer rim of the circle is cut into teeth, which are acted on by an endless screw connected at pleasure by a brass rod with a large clock, by which a motion can be given to the telescope corresponding with the diurnal motion of the heavens.

The hour-circle is clamped to the frame of the axis by a tangentscrew-clamp fixed to the frame itself, by means of which, with the aid of a handle extending to the place of the observer, he can, when the endless screw is applied, give motion to the instrument through a limited space upon the hour-circle. The rate of motion given to the hour-circle by the clock is not affected by this moveThe hour-circle, therefore, going according to sidereal time, small differences of right ascension can be measured by reading off the angles pointed to by the movable index before and after the changes of position.

ment.

The dome which covers the instrument, and which, as well as the other details of its erection, was constructed under the direction

4

of the Astronomer Royal, who was then the Cambridge astronomer, is supported so as to revolve on free balls between concave channels, holdfasts of peculiar construction being provided to obviate the eventuality of the dome being dislodged or blown off by wind or any other unusual disturbance. The winch which acts on the machinery for turning the dome, is carried to the observer's chair, so that he can, while engaged in a long observation, turn the dome slowly without removing from his position.

The magnitude of the instrument, and the consequent extensive motion of the eye-piece, rendered it necessary to contrive adequate means by which the observer could be carried with the eye-piece by a common motion without any personal derangement which might disturb the observation. This is accomplished by means of an ingenious apparatus consisting of a frame, of which the upper edge is nearly a circular arc whose centre is the centre of the telescope, which frame traverses horizontally round a pin in the flcor exactly below the centre of the telescope, the observer's chair sliding on the frame. The observer can, by means of a winch placed beside his chair, turn round the frame on which the chair is supported, and by means of a lever and ratchet wheel he can raise and lower the chair on the frame. He has also means of raising and depressing the back of the chair so as to give it the inclination he may at the moment find most convenient.

51. The Greenwich great equatorial.-This instrument, which was completed in the beginning of the year 1860, was erected from designs by the Astronomer Royal, by Messrs. Ransomes and Co. of Ipswich, the general optical work being performed by Messrs. Troughton and Simms, of London. It consists of a telescope with an object-glass by Merz of 12 inches aperture, and about 18 feet focal length, mounted according to the principle, known as the English form of equatorial mounting.

No novelty is introduced into the construction of the polar axis, except that the declination axis is so far advanced in front of the polar axis, and the upper part of the polar frame is so cut away that the telescope commands the meridian without interruption to a short distance beyond the pole. Each cheek of the polar axis is constructed in the form of a skeleton prism, the pillars being braced by a series of diagonal tension bars and transversal thrusting bars; these are of wrought iron. The upper and lower ovals which carry these are of cast iron. On the spindle of the lower oval, the hourwheel, 6 feet in diameter, on which the clock movement acts, turns freely; this wheel can be clamped when necessary to the oval or to the foundation-plate.

The declination circle, attached to the declination axis, is read by two microscopes placed in such a position, that though they

view opposite graduations on a 5-feet circle, the eye-pieces are only a few inches apart. For the illumination of the microscopes the light enters a hole in the side of the eye-tube, when it is reflected downwards by diagonal plates of transparent glass; it then falls upon the limb, whose surface is turned to a concave or dished conical form, so that the axis of the microscope is perpendicular to the portion of the limb under view; the light, therefore, which has been thrown down that axis is again reflected up the axis to the eye. There is a 5-feet clamp circle attached to the opposite cheek of the polar axis, whose clamp-screw and slow-motion are acted on by long handles near the eye end of the telescope.

For convenience of setting, and for reading small differences of polar distances, a radial bar is fixed on one side of the telescope, which turns on a pin near the centre of motion, its graduation being near the eye end of the telescope; this radial bar is bridled by a graduated sliding rod, of which the distant end is carried by a pin in one cheek of the polar axis.

The instrument is provided with a clock movement, which is a beautiful specimen of the application of mechanism for driving smoothly so heavy a mass. From a self-supplying tank placed on the upper story of the building, a sufficient fall of water is obtained for working a reaction machine, which revolves four times in a second. This, acting through two worms, drives the hour-circle. The regulation is effected by the contrivance called Sieman's chronometric governor, acting upon a pendulum having an uniform conical motion.

The limits of this work do not permit a lengthened detail of all the peculiarities of this instrument, especially as, in many respects, the general appearance and many of its parts are very similar to what is already described in the account of the Northumberland equatorial. However, as a specimen of astronomical engineering it is considered unique; and the adoption in its construction of every modern instrumental improvement, together with its great stability, renders it one of the most important instruments of its class to be found in any country.

CHAPTER III.

THE GENERAL ROTUNDITY AND DIMENSIONS OF THE EARTH.

52. The earth a station from which the universe is observed. The earth is, in various points of view, an interesting object of scientific investigation. The naturalist regards it as the habitation of the numerous tribes of organised beings which are

the special subject of his observation and inquiry, and examines curiously those properties and qualities of soil, climate and atmosphere, by which it is fitted for their maintenance and propagation, and the conditions which govern their distribution over its surface. The geologist and mineralogist regard it as the theatre of vast physical operations continued through periods of time extending infinitely beyond the records of human history, the results of which are seen in the state of its crust. The astronomer, rising above these details, regards it as a whole, examines its form, investigates its motions, measures its magnitude, and, above all, considers it as the station from which alone he can take a survey of that universe which forms the peculiar object of his study, and as the only modulus or standard by which the magnitudes of all the other bodies in the universe, and the distances which separate them from the earth and from each other, can be measured.

53. Necessary to ascertain its form, dimensions, and motions.—But since the apparent magnitudes, motions, and relative arrangement of surrounding objects severally vary, not only with every change in the position of the station of the observer, but even with every change of position of the observer on that station, it is most necessary to ascertain with all attainable accuracy the dimensions of the earth, which is the station of the astronomical observer, its form, and the changes of position in relation to surrounding objects to which it is subject.

54. Form globular. The first impression produced by the aspect presented by the surface of the earth is that of a vast indefinite plane surface, broken only by the accidents of the ground on land, such as hills and mountains, and by the more mutable forms due to the agitation of the fluid mass on the sea. Even this departure from the appearance of an extensive plane surface ceases on the sea out of sight of land in a perfect calm, and on certain plains of vast extent on land, such as some of the prairies of the American continents.

This first impression is soon shown to be fallacious; and it is easily demonstrated that the immediate indications of the unaided sense of vision, such as they are, are loosely and incorrectly interpreted, and that, in fact, even that small part of the earth's surface which falls at once within the range of the eye in a fixed position does not appear to be a plane.

Supposing that any extensive part of the surface of the earth were really a plane, let several stakes or posts, of equal height, be erected along the same straight line, and at equal distances, say a mile apart. Let these stakes be represented by ss, s' s', s'' s", &c., fig. 16, and let a stake of equal height, o o, be erected at the station of the observer. Now if the surface were a plane, it is evident that