altitude is taken with an artificial horizon (the shade being applied to the end of the telescope), that angle may be measured nearly as well as any other; for the rays incident on the index glass will pass through the transparent half of the horizon glass without much diminution of their brightness. "The advantages of this instrument, when compared with the sextant, are chiefly these: the observations for finding the index error are rendered useless, all knowledge of that being put out of the question, by observing both forwards and backwards. By the same means the errors of the dark glasses are also corrected; for if they increase the angle one way, they must diminish it the other way by the same quantity. This also perfectly corrects the errors of the horizon glass, and those of the index glass very nearly. But what is of still more consequence, the error of the center perfectly corrected by reading the three branches of the index; while this property, combined with that of observing both ways, probably reduces the errors of dividing to one-sixth part of their simple value. Moreover, angles may be measured as far as one hundred and fifty degrees, consequently the sun's double altitude may be observed when his distance from the zenith is not less than fifteen degrees; at which altitude the head of the observer begins to intercept the rays of light incident on the artificial horizon; and, of course, if a greater angle could be measured, it would be of no use in this respect.

"This instrument, in common with the sextant, requires three adjustments: first, the index glass perpendicular to the plane of the circle. This being done by the maker, and not liable to alter, has no direct means applied to the purpose; it is known to be right when, by looking into the index glass, you see that part of the limb which is next you reflected in contact with the opposite side of the limb as one continued arc of a circle: on the contrary, when the arc appears broken where the reflected and direct parts of the limb meet, it is a proof that it wants to be rectified. The second is, to make the horizontal glass perpendicular. This is performed by a capstan screw, at the lower end of the frame of that glass; and is known to be right when by a sweep of the index the reflected image of any object will pass exactly over, or cover the image of that object seen directly. The third adjustment is for making the line of collimation parallel to the plane of the circle. This is performed by two small screws, which also fasten the collar into which the telescope screws to the upright stem on which it is mounted; this is known to be right when the sun and moon, having a distance of one hundred and thirty degrees, or more, their limbs are brought in contact, just at the outside of that wire which is next to the circle, and then examining if it be just the same at the outside of the other wire: its being so is the proof of adjustment."

Another instrument of Troughton's construction upon the principle of the sextant is the dip sector, for measuring the dip of the horizon. Any person who is thoroughly acquainted with the sextant will find no difficulty in using it, after a few words of explanation from the maker.

THE TRANSIT INSTRUMENT.

The reflecting instruments, which we have just described, from their portability and the promptitude and facility with which they may be used in all situations, and upon all occasions, are most useful instruments to the surveyor. The sextant or reflecting circle, with an artificial horizon, and a

good chronometer, forms, in fact, a complete observatory, with which the latitudes and longitudes of places may be determined to a great degree of accuracy; while to the navigator a reflecting instrument is indispensable; all other instruments requiring to be supported upon a stand perfectly at rest*, while the sextant and similar instruments are held in the hand, and perform their duty well on the deck of a rolling. ship. In permanent observations, however, the capital angular instruments are placed permanently in the plane of the meridian, and the measurements sought for by their aid are the exact times at which the observed objects pass the meridian, and their angular altitudes or zenith distances, when upon the meridian. The instrument with which the first of these measurements are obtained is called a transit instru

ment, transit telescope, or merely a transit. Transits of portable dimensions, besides their use in small or temporary observatories, are also found serviceable to the surveyor, for determining, with the greatest possible accuracy, the true north point, and thence setting out a line in any required direction; and to the scientific traveller, for determining the longitude of any place from astronomical observations, and for adjusting his time-keepers with greater accuracy than can be obtained by his sextant or reflecting circle. The annexed figure represents a portable transit.

TT is a telescope formed of two parts, connected by a spherical center-piece, into which are fitted the larger ends of two cones, the common axis of which is placed at right angles to the axis of the telescope, to serve as the horizontal axis of the instrument. The two small ends of these cones are ground into two perfectly equal cylinders, called pivots. The pivots rest upon angular bearings or Ys. The Ys are supported upon the standards E and w, of which E may be called the eastern, and w the western standard; and one of the Ys is fixed in a horizontal groove, on the western standard, so that, by means of the screw s, one end of the axis may be pushed a little forwards or backwards, and a small motion in azimuth be thus communicated to the telescopet. The standards, E

* In observatories the instruments are supported by stone walls, or pillars, which pass below the floors, without touching them, or any part of the building, and are consequently independent of any tremor, communicated to the floor or walls of the buildings. It was considered that the passage of a railway through Greenwich Park would impair the observations at the Royal Observatory, by communicating a tremor to the ground.

The large transits in permanent observations have their Ys placed in two dove-tailed grooves, one horizontal, and the other vertical. By means

and w, are fixed by screws upon a brass circle, o o, and steadied by oblique braces, B B, which spring from the cross-piece, c.

On one end of the axis is fixed, so as to revolve with the axis, a vertical circle, v v; and a double index bar, furnished

with a spirit level, 7 l, to set it horizontal, carries two verniers, nn, adapted to the vertical circle, and showing the angle of elevation of the telescope. The index-bar is fixed in its position by the clamping screw, c, and can be fixed upon of the latter one end of the axis may be raised or depressed; but in the portable transit the same object is attained by turning one of the foot screws upon which the entire instrument rests.

either the eastern or western standard, at pleasure, while the telescope, with its attached circle, can also be lifted out of, and have its position reversed in, the Ys. The pivot, which does not carry the vertical circle, is pierced, and allows the light from a lamp to fall upon a plane speculum, fixed, in the spherical center piece, on the axis of the telescope, and inclined to this axis at an angle of 45°. The light is thus thrown directly down the telescope, and illuminates the wires of the diaphragm, placed in the principal focus of the telescope. Of these wires, one is horizontal; and a vertical wire, intersecting it in the center of the field of view, gives, by its intersection with it, the collimating point. There are, then, other vertical wires arranged in pairs equidistant from the central vertical wires, so that we have either three, or five, or seven vertical wires, the most common number being five. The lamp has a contrivance for regulating the quantity of light thrown into the telescope, by turning a screw, so that the light from a small star may not be overpowered by the superior light of the lamp.

The requisites of a good instrument, are- 1stly, that the telescope be of the best quality, which is to be tested by the methods already given (pp. 86-88); 2ndly, that the feet screws act well and remain steady; 3rdly, that all the screws, by which the instrument is put together, are turned home, and remain so, after the instrument has been shaken by carriage; 4thly, that the length of the axis be just sufficient to reach from one Y to the other, without either friction or liberty; 5thly, that the lamp be held so as not to require adjustment for position; 6thly, that the screws of adjustment of the diaphragm, and Ys, be competent to give security of position to the parts adjusted by them; 7thly, that the metallic parts be free from flaws in casting, and that the pivots be formed of hard bell metal and incapable of rusting.

The principal adjustments of the transit are three :

1st. To make the axis on which the telescope moves horizontal. 2nd. To make the line of collimation move in a great vertical circle, by setting it perpendicular to the horizontal axis.

3rd. To make it move in that vertical circle, which is the meridian.

To make the Axis Horizontal.-Apply to the pivots the large level, LL, which is supplied with the instrument for this purpose, and is either constructed to stand upon the pivots, in which case it is called a striding level, or of the form shown at page 96, in which case it is suspended from the pivots, and is called a hanging level. Bring the air bubble

to the center of its run, by turning the foot screw, f. Turn the level end for end, and, if the air bubble retains its position, the axis is horizontal, but, if not, it must be brought back half by the foot screw, f, and the other half by turning the small screw at one end of the level. Repeat the operation till the bubble retains the same position in both positions of the level, and the axis will be horizontal.

To adjust the Line of Collimation in Azimuth.-Direct the telescope to some distant, small, and well-defined object, and bisect it by one extremity of the middle vertical wire, giving the telescope the azimuthal motion necessary for this purpose by turning the screw s. By elevating or depressing the telescope, examine whether the object is bisected by every part of the middle vertical wire; and if not, loosen the screws which hold the eye-end of the telescope in its place, and turn the end round very carefully till the error is moved. Lift the transit off the Ys, and reverse it, so that the end of the axis, which was upon the eastern Y, may now be upon the western, and vice versá; and, if the object is still bisected by the central vertical wire, the collimation in azimuth is perfect; but, if not, move the center of the cross wires half way towards the object by turning the small screws which hold the diaphragm, and, if this half distance has been correctly estimated, the adjustment will be accomplished. Again, bisect the object by the center of the cross wires by turning the azimuthal screw s, and repeat the operation, till the object is bisected by the center of the cross wires in both positions of the instrument, and the adjustment will be known to be perfect *.

To adjust the Transit to the Meridian.-The line of collimation by reason of the previous adjustment describes a vertical circle, and, therefore, bisects the zenith, which is one point in the meridian. If, then, we can make it also bisect another point in the meridian, it will move entirely in the meridian. Compute from the tables in the Nautical Almanack, the time of Polaris coming to the meridian, and at the computed time bisect the star by the middle vertical wire, and the transit will be very nearly adjusted to the meridian.

To make the great vertical circle described by the line of collimation more nearly coincident with the meridian, let the intervals between the successive passages of Polaris across

* The horizontal motion given to the Y, by the azimuthal screw s, forms, evidently, no part of the adjustment for collimation, but only enables us to examine if the adjustment has been made with sufficient exactness.