conveniently and steadily, having a brass pillar movable by a tooth and wheel adjustment-by which it may be set at any height required-with vertical brass joint and horizontal motion, and rack and pinion motion to the tubes; the length of the tube is five feet, the aperture is three inches, having terrestrial and celestial eye-pieces. The construction of the various parts may be understood by reference to the one previously described. Price, $150.00.

Fig. 133.

B PIKE J'294 BROADWAY N.Y.

Another form of these splendid astronomical instruments is represented in the above cut, Fig. 133, of a larger construction than the preceding ones, and is mounted on a stand of a new and highly approved construction. It possesses the advantage of supporting the telescope in two places, which renders it extremely steady, a property of great importance when viewing celestial objects with high magnifying powers. It possesses, likewise, the advantage of enabling the observer to continue seated at the same height from the floor, although the telescope be raised to any altitude, the elevation being

entirely at the object end, although it be changed from the horizon to the zenith.

In the other constructions, where the centre of motion is nearly in the middle of the tube, it is at times inconvenient to stoop to the eye-end of the telescope when the altitude of the object is considerable; this new and improved construction of a stand remedies this inconvenience, which, together with its unusual steadiness, recommends it highly to astronomical observers. The frame-work is composed of bars of hard wood, firmly braced, and screwed together in a very durable manner, and is mounted on three castors; these castors may, by the motion of a lever, be so arranged that the stand may rest on the floor more steadily without the rollers, when the proper position of the telescope has been obtained. For the horizontal and vertical motion of the telescope, the arrangements are very complete; the former being a toothed wheel and arch; the latter having a small wheel moving a larger wheel, at the ends of the axis of which there are cog wheels working in links, forming an endless chain, or band, for drawing up one part of the frame, and elevating the telescope. The tubes are of brass, five and a half feet in length, the aperture of the object glass four inches, having two terrestrial and two celestial eye-pieces. Price, $275.00 to $350.00.

The Transit Instrument.-(Fig. 134, next page.)—The transit is a meridional instrument, employed, in conjunction with a clock or chronometer, for observing the passage of celestial objects across the meridian, either for obtaining correct time, or determining their difference of right ascension; the latter of which, in the case of the moon and certain stars near her path, that differ but little from her in right ascension, affords the best means of determining the difference of longitude between any two places where corresponding observations may have been made. Such being more especially the use of the portable transit instrument, it forms a valuable accession to the apparatus of the scientific traveller, who, remaining a short time at any station, is enabled thereby to adjust his time-keepers both with ease and accuracy, and to obtain the best data for finding his longitude. It also may be employed very successfully in determining the latitude.

The following figure represents this instrument as con

Fig. 134.

structed by Mr. Troughton, when the telescope does not exceed twenty inches, or two feet focal length. The telescope-tube, A A, is in two parts, and connected together by a sphere, B, which also receives the larger ends of two cones, C C, placed at right angles to the direction of the telescope, and forming the horizontal axis. This axis terminates in two cylindrical pivots, which rest in Y's fixed at the upper end

of the vertical standards, D D. One of the Y's possesses a small motion in azimuth, communicated by turning the screw, a; in these Y's the telescope turns upon its pivots. But, that it may move in a vertical circle, the pivots must be precisely on a level with each other, otherwise the telescope will revolve in a plane oblique (instead of perpendicular) to the horizon. The levelling of the axis, as it is called, is therefore one of the most important adjustments of the instrument, and is effected by the aid of a spirit-level, E, which is made for this purpose to stride across the telescope, and rest on the two pivots.

The standards, DD, are fixed by screws upon a brass circle, F, which rests on three screws, b c d, forming the feet of the instrument, by the motion of which the operation of levelling is performed. The two oblique braces, G G, are for the purpose of steadying the supports, it being essential for the telescope to have not only a free but a steady motion. On the extremity of one of the pivots, which extends beyond its Y, is fixed a circle, H, which turns with the axis while the double vernier, e e, remains stationary in a horizontal position, and shows the altitude to which the telescope is elevated. The verniers are set horizontal by means of a spirit-level, f, which is attached to them, and they are fixed in their position by an arm of brass, g, clamped to the supports by a screw at h. The whole of this apparatus is movable with the telescope, and when the axis is reversed, can be attached in the same manner to the opposite standard.

Near the eye end, and in the principal focus of the telescope, is placed the diaphragm, or wire-plate, which, in the theodolite or levelling telescope, need only carry two cross wires, but in this instrument it has five vertical and two horizontal wires. The centre vertical wire ought to be fixed in the optical axis of the telescope, and perpendicular with respect to the pivots of the axis. It will be evident, upon consideration, that these wires are rendered visible in the daytime by the rays of light passing down the telescope to the eye; but at night, when a very luminous object, as the moon, is observed, they cannot be seen. Their illumination

is therefore effected by piercing one of the pivots, and admitting the light of a lamp fixed on the top of one of the standards, as shown at I; which light is directed to the wires by a reflector placed diagonally in the sphere B; the

reflector having a large hole in its centre, does not interfere with the rays passing down the telescope from the object, and thus the observer sees distinctly both the wires and the object at the same time; when, however, the object is very faint (as a small star), the light from the lamp would overpower its feeble rays. To remedy this inconvenience, the lamp is so constructed, that by turning a screw at its back, or inclining the opening of the lantern, more or less light may be admitted to the telescope, to suit the circumstances of the case.

The telescope is furnished with a diagonal eye-piece, by which stars near the zenith may be observed without inconvenience.

Of the adjustments.-Upon setting the instrument up, it should be so placed that the telescope, when turned down to the horizon, should point north and south as near as can possibly be ascertained. This of course can be but approximate, as the correct determination of the meridian can only be obtained by observation, after the other adjustments are completed.

The first adjustment is that of the line of collimation. Direct the telescope to some small distant well-defined object (the more distant the better), and bisect it with the middle of the central vertical wire; then lift the telescope very carefully out of its angular bearings, or Y's, and replace it with the axis reversed; point the telescope again to the same object, and if it be still bisected, the collimation adjustment is correct; if not, move the wires one half the error, by turning the small screws which hold the diaphragm near the eye-end of the telescope, and the adjustment will be accomplished; but as half the deviation may not be correctly estimated in moving the wires, it becomes necessary to verify the adjustment by moving the telescope the other half, which is done by turning the screw a; this gives the small azimuthal motion to the Y before spoken of, and consequently to the pivot of the axis which it carries. Having thus again bisected the object, reverse the axis as before, and if half the error was correctly estimated, the object will be bisected upon the telescope being directed to it; if not quite correct, the operation of reversing and correcting half the error, in the same manner, must be gone through again, until, by successive approximations, the object is found to be bisected in both positions of the axis; the adjustment