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curves, and in all instances where important angular observations are taken; it may, indeed, be resorted to for many astronomical purposes. Fig. 62 is a drawing of one of Elliott's improved transit theodolites, and the most perfect and complete instrument of the kind yet produced; when in use it is fixed, by means of a female screw under the lower part, to a three-legged triangular stand, which affords a firm support even on the most uneven ground; this tripod is so constructed that it shuts up in a round staff for carriage with a couple of rings.

L and V are two circular plates, L being the horizontal graduated "limb," and V the" vernier plate," both turn horizontally upon the vertical axis immediately underneath them; this axis consists of two parts, external and internal; the external belongs to the graduated limb, and the internal to the vernier plate; the edge of the limb is chamfered off and silvered to receive the graduations; on diametrically opposite sides of the vernier plate, that is, 180° apart, are the verniers V, and the 'chamfered inclinations of the verniers and that of the limb form one continued inclined plane. The graduations on the limb and vernier depend on the diameter of the plates; on this subject the reader is referred to what has been said under the head of "Vernier."

At P and P are the "parallel plates," held firm and set parallel to each other by four mill-headed screws, p, p, pairs of which are opposite to each other, so that there are only three seen in the drawing; these, by being turned, act opposite to each other, and by their means the instrument is set up level for taking either horizontal or vertical angles.

There is a "clamping screw," hidden from view in the drawing, by means of which a collar, embracing the axis between the parallel plates and the horizontal limb, may be tightened; this clamping screw serves to fix the horizontal limb in any direction that may be desired; but as this motion is only effected by hand, and as it is requisite that such direction should be fixed with minute exactitude, there is a "slow motion screw," S, by means of which the whole instrument may be turned any small quantity after having been fixed by the clamping screw; this slow motion screw is attached to the upper parallel plate. Similar contrivances are applied to the vernier plate, which has also a clamp C, and a screw for slow motion, T, called the "tangent screw;" only the best instruments are supplied with this "slow motion" movement, which is far superior to any other, as by its means the most gentle and steady motion possible is imparted to the instrument.

In the centre of the vernier plate is a compass, and there

G

are also two small spirit-levels l, l, on this plate, by means of which the observer is guided in setting the instrument level; these two spirit-levels are supplied with adjusting screws at the ends. A small microscope is attached to the limb for reading the vernier, and so as to slide round the instrument.

We must now call the reader's attention to the two frames, Y, Y, which support the remaining portion of the instrument, viz., the telescopé and its spirit-level above, and the vertical circle M, called the "vertical limb," with its two verniers v, with their microscopes, for measuring altitudes or vertical angles; these are supported on the frames by the pivots of the horizontal axis resting in the "y's," so called from a rough resemblance to the shape of that letter. It is this system of construction which gives the transit theodolite one of its very great advantages over the old style of instrument, as when it is required to reverse the ends of the telescope, it is merely necessary to make the telescope revolve vertically on its axis, instead of having to perform the troublesome operation of opening clips by the removal of pins every time it is done.

At either end of the spirit-level are capstan-headed screws, to adjust it parallel to the optical axis of the telescope, or line of collimation, by tightening the one and loosening the other. The vertical limb is also provided with clamp and tangent screw for slow motion, as at O.

For the purpose of adjusting to distinct vision, the object glass is thrust outwards by means of a milled-head at the side of the telescope, but which is concealed in our drawing from its being on the other side; this is to adapt the glass to the sight of the observer, and to his distance from the object observed; distinct vision, or the proper focus, is obtained when, by moving the milled-head the object observed appears clear and sharply defined; when it does so happen that the milled-head alone does not give this, move the eye-piece in the eye-piece tube.

When looking through the telescope the sight embraces a considerable field of view; but when making observations we only seek for one point in it, which is considered as the centre; it becomes, consequently, necessary to place some object in the focus of the eye-piece, and when making observations this object must coincide, or "bisect" that on which we are observing. The fixed point required in the telescope is obtained by the intersection of lines of spider's web, attached to a diaphragm, as seen at Fig. 63, where these threads are shown as attached to a ring of smaller diameter than that of the telescope, and which is fixed to it by four small screws acting in opposite pairs, so that if one is eased and the other tightened, the ring is moved right

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or left, or up and down accordingly as one pair or the other is acted upon, and with these of course the intersection of the threads; when this portion of the instrument is in adjustment, it coincides with the axis of the telescope, and the line of collimation is then correct. In our drawing of the theodolite these . screws are lettered d, d. Sometimes there are only three such

screws.

A plummet and line are supplied with theodolites; this being attached to a hook placed under the centre of the instrument, guides the observer so that he may plant the centre of the instrument exactly over the centre of his station, which is a matter always to be carefully attended to. There are also two extra eyepieces, the one called the "diagonal eye-piece," for astronomical observations, by the use of which the observer avoids an uncomfortable position in making such observations; the other has fewer glasses and a higher magnifying power, but inverts the object, so that in looking through it, objects appear turned upside down, and an object to the right appears to the left, but this one becomes very soon accustomed to, and I would advise the beginner to use this eye-piece from the first, and condemn the other; there will also be found in the box a small magnifyingglass, a turn-screw, and a small pin for turning the capstanheaded screws.

THE ADJUSTMENTS.

When the instrument comes from a good maker, it will very rarely happen that anything is out of adjustment, but as derangements of the parts may accidentally occur, it is indispensable that the reader should know how to correct them; and under any circumstances it is desirable to examine a new instrument, and also from time to time afterwards.

The line of collimation is the first adjustment to attend to; it consists in making the intersection of the wires of the diaphragm coincide with the axis of the telescope. Set up the instrument on its tripod, and level it by means of the screws p, p, being guided by the bubbles of the spirit-levels on the vernier plates, which should occupy the middle of the tubes. Set the telescope on some well-defined portion of any object in the distance, and adjusting to distinct vision, turn the mill-head in the side of the telescope until it appears clear and sharp, and perfectly steady, whilst the eye is moved up and down, or right and left, before the eye-piece. In the mind's eye carefully mark the position of that on which the intersection of the wires bears. Loosen the two screws just visible in the drawing below the clamp and tangent screw O, of the vertical limb, open the clips at the top of the

"y's," lift out the telescope, and turn the axis end for end, when the bubble will be downwards. Now examine, through the telescope, if the intersection of the wires still coincides with exactly the same object as before; if so, the line of collimation is in adjustment; if not, then it must be made so; if too high or too low, the two screws, d, d, must be turned to correct half. the deviation, whilst the other half is corrected by depressing or elevating the telescope itself; if the observer has been careful in making his first observation, he will be able to estimate correctly this half of the deviation, or very nearly so; now restore the telescope to its former position, and if the coincidence with the object remain as corrected, then the line of collimation in altitude is correct; if not, exactly the same operation must be repeated until it is so; a similar proceeding will set this line of collimation in adjustment laterally, by acting upon the little side screws instead of the vertical ones.

To adjust the level over the telescope.

The former adjustment having been made with the vertical arc clamped, and the clips over the "y's" still open, bring the air-bubble of the level to the centre of its run, or of the glass tube, by turning the tangent screw O; now reverse the telescope as before, and very carefully, so as not to disturb the vertical limb; if the bubble now again settles in the centre of its run, then the level is in adjustment, that is, parallel to the corrected line of collimation; but if this be not so, correct half the deviation by the capstan-headed screws at the end of the level, and the other half by means of the tangent screw 0; the same as above, this operation must be repeated until this adjustment is made correct.

To make axis of the horizontal limb truly vertical.

Set the instrument as level as you can by means of the tripod legs, being guided by the spirit-levels on the vernier plate and that above the telescope; clamp the collar round the axis under the horizontal limb by means of the clamping screw, which, as before noticed, is concealed in the drawing, and the clamp C of the vernier plate being free, turn the vernier plate round, so as to bring the telescope over two of the parallel plate screws p, p, and bring the bubble of the level over the telescope to the centre of its run, by means of the tangent screw O. Now turn the vernier plate half way round, that is 180°, so that the telescope shall still be over the same pair of parallel plate screws, P, P, only turned end for end; if the bubble does not now continue in the

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centre of its tube, correct half the deviation by means of the parallel plate screws, and the other half by means of the tangent screw 0; this operation must be repeated until in both these positions of the telescope the bubble of the level remains in the centre of its run. Now turn the vernier plate one-quarter, or 90°, round, so that the telescope may be over the other pair of parallel plate screws, and by their means bring the bubble again to the centre of its run, and now see that it remains so whilst the vernier plate is carried right round the limb. This being so, and this level being the most sensible of the three the instrument is supplied with, the other two, that is, those on the vernier plate, may now be adjusted by means of their screws, that is, should they be out of adjustment, for the bubbles of these levels should now be in the middle of their tubes. To test the construction of the instrument itself on this point, tighten the clamp C, loosen the collar round the axis, and turn the instrument slowly round; if the bubble maintains the centre of its run during an entire revolution all is well, but if otherwise, there is a defect which can only be remedied by the maker.

The vertical limb.

All these adjustments being made, the zero on the vernier should coincide with the zero of the limb; if not, make it so by means of the screws.

THE ORDINARY THEODOLITE.

By the expression "ordinary theodolite" we designate the instrument constantly in use up to a very late period, and still employed by a great many. Its capabilities are perhaps equal to the transit theodolite above described, but it is certainly much less convenient, and involves loss of time. At Fig. 64 is a drawing of this instrument, in which similar parts are denoted by the same letters as those in Fig. 62; it will at once be perceived that the great difference is in the vertical limb M, the attachments of the telescope to the rest of the instrument, and the position of the telescope level, which is here underneath instead of above; O is the tangent screw of the vertical limb, but the clamp of this limb is at N. When it is desired to "reverse the telescope," that is, turn it end for end, the pins a, a, are taken out, the clips b, b, are opened, and the telescope is taken out of its "y's" (so called from their somewhat resembling the form of that letter), when the telescope may be reversed. Q is a microscope suspended to an arm moveable between the bars of the frame for reading off