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by the plate-screws. This operation must be repeated with the other pair of plate-screws, and care must be taken that the screw represented by A in the sketch is never touched except for the purpose of making this adjustment.
In Troughton's instrument, the spirit level, being fixed to the telescope, has no separate means of adjustment, and the line of collimation must therefore be determined by its assistance. The telescope also, being bedded in a sort of frame, cannot be reversed end for end; the level is first adjusted by correcting half the error when turned round, by the screws which act upon the supports, and half by the plate-screws; the line of collimation is then made to agree with the corrected level by noting the height of the intersection of the cross wires on a staff about 200 or 300 yards distant. The instrument and the staff are then made to change places, and if the difference of level remains the same, the optical axis is already correct; if not, half the difference of the results must be applied to the observed height of the vane on the staff, and the cross wires adjusted to this height by means of the screws of the diaphragm at the eye-piece of the telescope.
A pool of water furnishes another easy mode of adjusting the line of collimation. A mark being set up at any convenient distance of exactly the same height above the surface of the water as the instrument adjusted for observation, the cross wires have only to be made to intersect each other at this point.
The adjustments of Mr. Gravatt's level (the best of the three) are nearly similar; and will be found described by himself, in Mr. Simms' little work, already quoted *.
The French water level is much used on the Continent, in taking sections for military purposes. It possesses the great advantage of never requiring any adjustment, and does not cost one-twentieth part of the price of a spirit level. From having no telescope, it is impossible to take long sights with this instrument; and it is not of course susceptible of very minute accuracy; but, on the other hand, no gross errors can creep into the section, as may be the case with a badly-adjusted spirit level or theodolite, the horizontal line being adjusted by nature without the intervention of any mechanical contrivance. As this species of level
* Also in page 137 of Mr. Bruff's “Engineering Field Work.”
is not generally known in England, the following description is given; which, with the assistance of the sketch, will enable any person to construct one for himself without further aid than that of common workmen to be found in every village *.
ab is a hollow tube of
brass about half an inch in diameter, and about three feet long, c and d are short pieces of brass tube of larger diameter, into which the long tube is soldered, and are for the purpose of receiving the two small bottles e and f, the ends of which, after the
bottoms have been cut off by tying a piece of string round them when heated, are fixed in their positions with putty or white lead -the projecting short axis g works (in the instrument from which the sketch was taken) in a hollow brass cylinder h, which forms the top of a stand used for observing with a repeating circle; but it may be made in a variety of ways so as to revolve on any light portable stand. The tube, when required for use, is filled with water (colored with lake or indigo), till it nearly reaches to the necks of the bottles, which are then corked for the convenience of carriage. On setting the stand tolerably level by the eye, these corks are both withdrawn †, and the surface of the water in the bottles being necessarily on the same level, gives a horizontal line in whatever direction the tube is turned, by which the vane of the levelling-staff is adjusted. A slide could easily be attached to the outside of c and d, by which the intersection of two cross wires could be made to coincide with the surface of the water in each of the bottles; or floats, with cross hairs made to rest on the surface
* The instrument from which the sketch was made was constructed for me by an ironmonger in Chatham; and I have tried it against a very good spirit-level, and found the results perfectly satisfactory. This water-level is, I find, now constantly used on the Ordnance Survey for interpolating horizontal contours at vertical intervals of 25 feet between the more correct contours, traced at greater distances apart by the spirit-level.
†These corks must be drawn carefully, and when the tube is nearly level, or the water will be ejected with violence.
of the fluid in each bottle, the accuracy of their intersection being proved by changing the floats from one bottle to the other: either of these contrivances would render the instrument more accurate as to the determination of the horizontal line of sight; though one of its great merits, quickness of execution, would be impaired by the first, and its simplicity affected by either of them. For detailed sections on rough ground where the staff is set up at short distances apart, it is well qualified to supersede the spiritlevel, and is particularly adapted to tracing contour lines: which operation will be described in its proper place.
A mason's level and boning-rods also answer very well for taking sections where no better instruments are at hand, and are used as described below.
A horizontal line is obtained by driving two pickets (1 and 2) into the ground, and applying a large mason's level to their heads, which should be previously cut square. The pickets 2 and 3, 3 and 4, &c., can be levelled in the same manner, as far as may be necessary, to obtain a correct horizontal line for a short distance; but if any considerable
length is required, two
boning - rods, of about three feet long, with a cross-piece at the top, are placed on the heads
of any two of the pickets already levelled, and the vane of a staff raised or depressed at any required point, till it is on a level with the tops of the boning-rods. The reading of the staff will give the respective depths below the level of the heads of the rods, the heights of which must be subtracted. Boning-rods are chiefly used in laying out slopes in military works, and for setting up profiles to direct working parties. A slope of 5 to 1, for instance, is laid out by measuring 5 feet from a towards b, and driving the head of the picket at the end nearest b, one foot lower than that at a; the heads of boning-rods, of equal height, placed on the tops of these pickets, are evidently on a slope of 5 to 1.
The last description of instrument used for levelling is the French
Reflecting Level," invented by Colonel Burel; a description of which, is given in the second volume of "Professional Papers of the Royal Engineers."
The principle upon which this instrument acts is implied by its name. In a plane mirror the rays are reflected as though they diverged from a point behind the mirror, situated at precisely the same distance in rear of its surface, as the object itself is in front. If the mirror be vertical, the eye and its image are on the same horizontal line; and any object coinciding with these is necessarily on the same level. It appears then only requisite to ensure the verticality of a small piece of common looking-glass set in a frame of wood or metal, to be able without further assistance to trace contour lines in every direction, or to take a section on any given line. The mirror AB, described in the paper alluded to, is only one inch square, fixed against a vertical plate of metal weighing about 1lb., and suspended from a ring m, by a twisted wire n, so that it may hang freely, but not turn round on its axis of suspension. It can either be used for sketching in the field, being held by this ring at arm's length; or fixed, for greater accuracy, in a frame which fits upon the top of the legs of a theodolite, with a bar of metal like a bent lever, pressing so slightly against it from below, that it may check any tendency to oscillation, and at the same time not prevent the mirror from adjusting itself vertically by its own weight. The accompanying sketch will render this description more intelligible.
The required verticality of the plane of the mirror is thus ascertained: a level spot of ground is chosen, where it is suspended in its frame (or any temporary stand) 40 or 50 yards from a wall, and the prolongation of the line of sight from the eye to its image, coinciding with a fine silk thread across the centre of the mirror, is marked on the wall, which is visible through a small opening p, in the metal frame. The mirror is then turned round, and the observer, placed between it and the wall, with his back to the latter, notes the spot where the image of his eye
coincides with the reflected wall above or below the former mark. The mean distance between these two points is assumed and marked; and, by turning the screw r, the centre of gravity of the mirror is altered until the image of the eye coinciding as before with the silk thread agrees also with this central mark on the wall. It would perhaps be a better plan to send an assistant some distance behind the mirror with a levelling-staff, the vane of which could be raised or lowered to coincide with the line of sight; on reversing the mirror (the staff remaining stationary) the vane would be again moved, until its refleted zero mark is cut by the thread on a level with the image of the eye, and finally, the mirror adjusted by the screw, to the mean between these two heights; this method admits, apparently, of greater nicety than a chalk mark on a rough wall.
The reflecting-level is not generally known in this country; but for many purposes it is superior to any other description of instrument, particularly for tracing contour lines on the ground in a military sketch. It is peculiarly simple in its construction; is easily managed, easily adjusted, is not liable to have this adjustment deranged, or to be injured by a fall; is from its size, more portable than any other instrument, and can be used either held at an arm's length, or at a distance of several feet; in which position, the length of the line of sight ensures the greatest accuracy.
The levelling-staff, a necessary accompaniment to each of the species of levelling instruments described, was formerly made with a sliding vane to move up and down a staff graduated to feet and decimals, or feet and inches: this was effected by a string and pulley, or the staff itself was made in two or three pieces, each of the upper pieces sliding in a groove in the one next below it. For any height less than the length of the first piece (generally about 6 feet) the vane was slid up or down with the hand; but for a greater height, the second piece, with the vane at the top, was moved up bodily till the centre of the vane was cut by the line of the optical axis of the instrument, when the height was read on another scale graduated downwards from the top on the side of the lower joint of the staff. A description of staff was however introduced some years ago by Mr. Gravatt, and has been since improved upon, on which the divisions (in feet and decimals) are