being done, reverse the telescope in its y's; that is, turn it end for end, which must be done carefully that it may not disturb the vertical arc, and if the bubble resume its former situation in the middle of the tube, all is right; but if it retires to one end, bring it back one-half, by the screw, which elevates or depresses that end of the level, and the other half by the tangent-screw, o; this process must be repeated until the adjustment is perfect; but to make it completely so, the level should be adjusted laterally, that it may remain in the middle of the tube when inclined a little on either side from its usual position immediately under the telescope; this is effected by giving the level such an inclination, and, if necessary, turning the two lateral screws. If making the latter adjustment should derange the former, the whole operation must be carefully repeated.

The third adjustment is that which makes the azimuthal axis, or axis of the horizontal limb, truly vertical.

Set the instrument as nearly level as can be done by the eye, fasten the centre of the lower horizontal limb by the staff-head clamp, leaving the upper limb at liberty, but move it till the telescope is over two of the parallel platescrews, ; then bring the bubble of the level under the telescope to the middle of the tube, by the screw; now turn the upper limb half round, that is 180°, from its former position; then, if the bubble return to the middle, the limb is horizontal in that direction, but, if otherwise, half the difference must be corrected by the parallel plate-screws over which the telescope lies, and half by elevating or depressing the telescope, by turning the tangent-screw, o, of the vertical arc; having done which, it only remains to turn the upper limb forward or backward, 90°, that the telescope may lie over the other two parallel plate-screws, P, and by their motion set it horizontal.

Having now levelled the limb-plates by means of the telescope level, which is the most sensible upon the instrument, the other air-bubbles fixed upon the vernier plate may be brought to the middle of their runs by merely giving motion to the screws which fasten them in their places.

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The vernier of the vertical arc may now be attended to; it is correct, if it points to zero, when all the foregoing adjustments are perfect; and any deviation in it is easily rectified by releasing the screws by which it is held, and tightening them again after having made the adjustment, or, what is perhaps better, note the quantity of deviation as an index error, and apply it, plus or minus, to each vertical angle observed. This, however, necessitates making a note of the error.

This deviation is best determined by repeating the observation of an altitude or depression, in the reversed positions, both of the telescope and the vernier plate: the two readings will have equal and opposite errors, one half of their difference being the index error. Such a method

of observing angles is decidedly the best, since the mean of any equal number of observations taken with the telescope reversed in its y's must be free from the effects of any error that may exist in the adjustment of the vernier, or zero of altitude.

The theodolite, as constructed in the manner we have described, is not inconveniently heavy, as the diameter of the horizontal limb seldom exceeds six inches; but when the diameter is increased, the other parts must be made proportionably large and strong, and the instrument becomes weighty and cumbersome to carry from station to station. The object of increasing the dimensions is, to enable the instrument to furnish more accurate results, by applying a telescope of greater power, and by a more minute subdivision of the graduated arcs. With the increase of size, a small variation takes place in the construction, and in the manner of attaching the supports to the horizontal limb, to afford the means of adjusting the horizontal axis, and making the vertical arc move in a vertical plane.

The transit theodolite is now the favourite instrument, though we do not consider that its adjustments are made under such favourable conditions as in the cradle theodolite, which we have just described; the great length of the standards, we also fear, is rather against it. Its great advantage, however, lies in the principle of construc

H

tion implied by its name of transit; in setting out work, this is unquestionably a great recommendation, because, when the telescope requires to be turned end for end, it is merely necessary to make it revolve on its vertical axis, instead of having recourse to the troublesome operation of opening the clips in the cradle instrument, by the removal of pins every time this has to be done.

The illustration on p. 99 represents a transit theodolite of the usual construction.

The methods of adjustment are so similar to what we have already observed, that it would be needless repetition to give them again.

We may, however, make a few observations as to the line of collimation. Set up the instrument in the manner we have already described, and, by means of the parallel plates, bring the bubbles of the levels on the vernier plate to the proper centres of their tubes: do this carefully and as patiently as possible.

Select some well-defined object in the distance, and by means of the eyepiece and milled head of the telescope, adjust for distinct and steady vision; the way in which this is done has already been described. Mark exactly where the cross-hairs of the diaphram intersect on the object referred to.

Now loosen the clamp of the horizontal limb and turn the instrument bodily round. This will bring the telescope to bear at the eye end on the object of observation; therefore, reverse it by turning it end for end. Clamp the horizontal limb, and perfect the bisection of the object by means of the tangent screw s; now see that the bubbles of the levels are in the centres of their runs, which they should be, if the instrument has been turned properly out of hand. If there is any error in the collimation line, it may be corrected by means of the screws at d. Before doing this, however, release the vernier plate by loosening the clamp, c; turn the instrument half round, by the vernier plate, reverse the telescope as before, bring the cross-hairs to bear on the object, and perfect the bisection by means of the tangent screw, T, after having clamped c. See to the bubbles, and examine if the distant object is well

COLLIMATION OF THE TRANSIT THEODOLITE.

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bisected, now that the instrument has been made to turn round exactly 180°; if not you may begin the necessary correction.

Now loosen the clips at the top of the standards, which hold down the axis of the telescope, and turn it half over

laterally, so that the long level attached to the telescope will be downwards, and see if the collimation continues correct. All this, however, should be examined before the instrument is used in practice, and, if found incorrect, it should be sent back to the maker: this should be particularly looked to before going abroad; otherwise, the want

of adjustment may be discovered at a very awkward

moment.

The object of the above examinations is to see to the levels of the vernier plate as well as to the line of collimation. If the whole instrument is in adjustment, the centre of the diaphragm will intersect the distant point, after the telescope has been turned over laterally in its standards, just the same as it did before: if otherwise, the correction, by means of the screw d, will depend on whether the correction is required laterally by the side screws, or vertically by means of the top and bottom screws. course it is half the error which has to be corrected.

The telescope is in the next place brought back to its former or normal position, and again the correction is effected if any is required. This is to be repeated until the intersection is correct, whilst the telescope is supported on the standards in both of the above positions. It is understood that, as regards the vertical intersection, the clamp of the vertical circle, or limb, M, is fast, and remains undisturbed the whole time. With regard to the verniers of the vertical arc coinciding with zero and 180°, when the bubble of the level of the telescope is in the centre of its run, we should only have to repeat what we have said on the same subject relative to the cradle theodolite.

ECKHOLD'S OMNIMETER.

This important surveying instrument (made by Elliott Brothers), one form of which is shown in fig. 1, measures distances and altitudes with an extraordinary degree of accuracy and great economy of time and labour : it accomplishes, without being moved, the work of theodolite, Îevel, and chain, and may be used as a transit theodolite.

This instrument, fig. 1, has a powerful microscope ab, permanently fixed at right angles to a telescope cd; both microscope and telescope move on the same axis o, at right angles to one another in every position; the line through the centre of cd is always perpendicular to the line through the centre of a b.

The microscope is directed to read the divisions of a