After the destruction of Ramsden's sector, Colonel Colby applied to the Astronomer Royal for his advice as to the best form of instrument for determining latitude in the field. The construction given by Mr. Airy, and executed by Mr. Simms, differs in many respects from any which preceded it. The description, with explanatory figures, is to be found in the Monthly Notices of the Royal Astronomical Society,' vol. v., p. 1SS. The vertical axis, which 13 cast in one piece, and strongly framed, carriee at its back three levels, one above the other, which being read off at the moment the star is bisected, determine the position of the axis with respect to the zenith. The telescope-frame with the eye- and object-end is cast in another solidly braced piece, and is held at its middle on a centre in front of the vertical axis. This second frame moves freely for a few degrees on each side the zenith. The divided arcs are graduated on the vertical axis near its top and bottom, and there are four micrometer microscopes, one at each side of the object and eye-end, the tubes of which are bored in the solid telescope-frame. There is a wire-micrometer in the focus of the tele scope. A stop to the axis enables the observer to turn the instrument exactly half round by touch, and almost instantaneously. The observe. Hans are made thus :—The instrument being pretty nearly in the meridian, and the axis vertical, the telescope is set nearly for the star, and the microscopes are read oft Before the star reaches the centre of the field, the observer bisects it with the micrometer-wire, noting the time, while the assistant reads off both ends of each level. Time whole instrument is then turned half round, and the star is again observed, the bisection being new performed by the tangent-screw of the tele scope-frame, the time is again noted, the assistant reads off the levels as before, and, finally, time arcs above and below are read off by the micrometer microscopes. In this way the double zenith distance of a star, free from all error of collimation or of the vertical axis, may be obtained in a few minutes. This instrument performs very satisfactorily. The telescope is one of 31 feet focal length, and the instrument bears the same relation to a mural circle that the ordinary sector does to a quadrant.
When Troughton first proposed the mural circle as the best form for a meridian declination instrument, great doubt was thrown on the practicability of observing by reflexion with sufficient nicety, and in that case, as the mural circle does not reverse, a supplementary instru ment was wanted to show the position of the zenith or horizon. Partly on this account, but chiefly to settle the constants of aberration and precession with the greatest precision, Troughton planned a zenith tube, consisting of a telescope of 25 feet focal length, without any sector, and in which the variations of zenith distance of es Draconis and close zenithal stars were to he measured by a micrometer-screw. The telescope rests on its lower end, continued beyond the focus, on a piece which hats adjustments for verticality, and a collar below the object-glass is pressed by a spring into a Y bearing. The wires at the focus are moved by a micrometer-screw, and the star and wires are seen through a diagonal four-glass eye-piece. The plumb-line hangs within the tube, and is viewed above and below by micrometer microscopes. Inatead of adjusting the plumb-line before each obser vation, it is bisected by. the micrometers after the observation, and
a correction applied which is deduced from .the upper and lower read. ings. Mr. Airy having had some reason to suspect that the wire twisted on reversing the instrument, gave a double suspension to the plumb-line, and made the instrument reversible on a star in the same night, by using a stop as in the ordnance sector. The observatioua with the zenith tube are printed yearly in the Greenwich Observations.
The zenith sector has not been much used upon the continent siuce the great surveys made in the middle of last century for ascertaining time figure of the north. In the French are from Dunkirk to For mcntera, time latitudes were observed by the repeating circle, and in soma of the stations there is reason to suspect that error has been committed. More recently, the transit in the prime vertical has been employed in Germany and Russia for ascertaining differences of lati tude, and as it would seem with great success. [Teaesre.] A prime vertical transit was constructed by Repeold for the imperial observatory of Pulkowa, of which a most favourable account has been given by Professor Struve. While admitting the excellence of this kind of instrument for telescopes of moderate size, we do not see how they can equal, far less surpass, the zenith sector when made reversible and of the proper magnitude.
Some years ago Mr. Babbage proposed a conetruetion for a zenith sector (' Memoirs of the Astronomical Society,' vol. ii., p. 101) which might perhaps be applied in the following mrmer :—Conceive a parallel ruler to bo placed upright, one of the bars being made into a vertical axis with the necessary adjustments, and the other carrying a telescope. It is clear that if the bands were equal the telescope would continue parallel to itself whether the ruler be open or shut. But if one of the bands is a little longer than the other, then a very large angular motion of the band will give a small angular motion to the telescope-bar, and as the measurement of the former angle can be easily made with tolerable accuracy, the latter angle can be computed with great exactness.* Exquisite workmanship would no doubt be required to make such an instrument answer, but we think that for this and other differential purposes Mr. Babbage's suggestion is de serving of attention, ;especially where telescopes of limited size' are used.
The adjustments of a zenith sector or zenith tube will differ accord ing to the construction of the instrument. Where it is not reversible, the time of the transit of a star near the zenith must be got from observations with another instrument, and the star made to pass the meridian-wire at the calculated time by the proper adjusting screws. When this is done and the telescope secured, a star must be made to pass along the declination-wire (this should be carried by a micrometer screw) by twisting the wire-cell, when the adjustment for a fixed zenith telescope Is complete. If the telescope rest on a cross axis and carries a sector, the cross axis must be made horizontal, the transits of stars towards the extremities of the arc must be observed, and the azimuthal deviation ascertained [Tansisrr] and corrected ; or, the time at which an extreme star should pass being known, the cross axis at top and fixed arc below must be turned so as to make the star pass at the right time.