Chapter IV · (continued 2)

Fig. 85.—Compact cheap form of dumpy level.

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247.—Sight Vanes.—Two sight vanes are placed above the telescope, either as loose fittings or to hinge down upon the level tube. One vane has a vertical narrow slit and cross hair; the other has a window with a vertical horse-hair placed in its centre. This arrangement gives sight of distant landmarks in line with the direction of the telescope, upwards or downwards, beyond its field of view. A slider, fixed upon the window sight, reads at its upper edge into divisions cut on the vane, by means of which an approximate rate of forward inclination of the land may be taken. This sighting arrangement adds about half a pound weight to the instrument. It was useful with object-glasses of small field of view, but is useless with good modern glasses of wide angle.

248.—Lower-class Levels.—A level is often required by an architect or a contractor for works of limited area, where it is quite unnecessary to go to the expense of a civil engineer's level of refined manufacture. In such cases the level may only be used occasionally and under favourable circumstances, so that extreme solidity is not demanded, neither is distant view in the telescope required. The level generally made for such work is a simple dumpy, without cross bubble, compass, or any extra fittings, and with one eye-piece only.

249.—The instrument Fig. 85 illustrates the author's newest design for a simple level. It has a light form of tripod. The legs clamp directly between angle plates—these are not quite so portable or so neat as cylindrical legs, but they are easily made, very firm, and will bear considerable wear and keep in order. A still cheaper form is made with smaller telescope and turned legs for the tripod.

Fig. 86.—Contractor's or builder's level.

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250.—The illustration Fig. 86 represents the cheapest form of level with a tripod stand that has been constructed, which contains the important factor of a telescope. The telescope has a sliding fitting, which is moved by a knob outside, this being made more quickly than a rack and pinion fitting. The level tube is solidly supported in collars. The adjustment is in one direction only, so that the bubble must be set and examined at the time of reading the staff. The instrument is supported on a sprang, jointed at one end and held by a milled-headed screw at the other. Any shakiness of the thread of screw there may be is taken up by a stiff German silver spring between the sprang and the limb. It is sometimes made with a ball and socket joint for first adjustment, but this renders it nearly as costly as a superior level. The tripod head is of simple construction. The legs are oak or ash, and are clamped on the head by bolts. This simple tripod is fairly firm in use. The level is good enough for ordinary building works, laying short drains, etc., within limited areas. It is much more accurate than any form of open sighted level without telescope. Sir George Leach has recently made a modification of this old form of level by placing a pendulum to rock the axis to cross level position, which is a refinement, although rather a costly one.

Fig. 87.—Sighted reflecting pocket level.

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251.—Sighted Pocket Level.—This consists of a tube, which is generally drawn of square section. A pin-hole sight is made in the closed end of the tube, Fig. 87, at E. The field end of the tube is left open. The sight is taken by looking through the centre of the pin-hole across the edge of the reflector R. A level with a small bubble is placed or inserted in the top of the tube at B. The metal casing of this is cut away on the upper and under sides to render the bubble visible from the interior of the tube by means of the reflector R, which occupies one half vertical section of the interior of the tube. This is placed at 45° to the axis. The reflector is fixed upon an inner tube so that it may be withdrawn to be cleaned. When the level is set horizontally, a distant object in the direct sight line is seen through half the tube, and simultaneously the reflection of the bubble in the other half appears. A line engraved upon R indicates when the bubble is central, and when these coincide the distant object and the eye are level. The instrument is about 4 inches long, and weighs about 8 oz. in its case.

Fig. 88.—Pocket telescopic level.

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252.—Pocket Telescopic Level.—In the above-described pocket level, where it is made short, the average middle-aged man will not have sufficient accommodation of vision to be able to see the bubble and the screen sharply defined simultaneously with the distant object to which the level is to be taken. In Captain Barrie's[5] level these objections are avoided by making the reflector and bubble form part of a telescope, Fig. 88. An achromatic glass of short focus is used, and the eye-piece is of long focus so as to bring the bubble to focus in the centre of the mirror, which is made of curved form to decrease the apparent size of the bubble. The image of the bubble does not give by bisection a very definite index. The author has found that this level may be much improved by placing a point in the telescope at the mutual foci of the object-glass, eye-piece and the bubble. The appearance of the mirror and point is shown at B. The point is shown by a dot at P. The curved mirror R. The dotted line shows the path of reflection from the bubble. This level will work with very fair accuracy as a hand instrument. Size, about 4½ inches by ¾ inch. Weight in case, about 8 oz.

Fig. 89.—Reflecting level.

Fig. 90.—The same construction in protecting case.

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253.—Reflecting Level.—This simple level, Fig. 89, the invention of Colonel Burel, is one of the most portable. When it is used with a fair amount of care it will give good approximate results. It consists of a piece of parallel glass, which has half the surface silvered to form a reflector. It is suspended in such a manner that the glass hangs vertically by gravitation. The position of the mirror to the plain glass may be that shown in the engraving, or horizontally if preferred. The mirror, Fig. 89, is inserted in a solid metal frame suspended from a gimbal, which permits it to hang perfectly free to the action of gravitation. The centres of suspension are made with slightly-rounded knife-edges. A ring at the upper part of the instrument is placed over the thumb or finger to support the instrument when in use. A stout pin passes through a prolongation of the lower part of the frame, screwed or otherwise, which permits adjustment by filing to bring the mirror when it is suspended exactly into a vertical plane. The instrument, fitted into a neat case, weighs from 5 oz. to 9 oz.

254.—In using the Reflecting Level, it is held upon the thumb at about arm's length, and adjusted by raising or lowering the arm until the reflection of the pupil of the eye seen in the mirror is exactly bisected by the line cut by the mirror against the clear glass. The distant object seen in front, that cuts this sight line and the image of the pupil of the eye, will then be in true level position with the eye of the observer, provided the air is still, so that the mirror is not deflected from verticality. From the natural unsteadiness of the hand there is some little difficulty of getting this level quite free from oscillation. This may be obviated, or nearly so, by clutching a picket or staff with the hand and suspending the level from the thumb projected out for the purpose, or by resting the hand against a tree or other firm support. Capt. A. H. East, R.A., has suggested to the author a very capital device which he employs for hand instruments. This is to place the handle of a stick (or umbrella) in the waistcoat pocket, to clutch the body of the stick with the hand which holds the instrument, and to steady it with the other hand. In this manner the two arms and the stick form a tripod of surprising steadiness.

255.—Reflecting Level in Case.—In windy weather much greater exactness may be secured by placing the pendulous level, just described, in a tubular case, Fig. 90. The case is made of double tubes, so that the aperture cut on one side may by a half turn of the outer tube close and protect the instrument when out of use. The transparent side of the inner case is sometimes closed by thin glass tube of its own internal diameter. It is much better if made with two vertical sides glazed with parallel glass. When this form of instrument is used, it may be, if required, made to fit on the top of a light staff. The eye is then brought with much greater certainty to the point of bisection on the edge of the mirror, and much greater accuracy is thus attained in levelling with it.

256.—Water Levels.—The antique form of level, composed of two vials fixed on the ends of a tube and partly filled with water, by which a level is sighted in looking over the surface of the water, is still used to a limited extent in rural districts on the Continent; but the spirit level in some simple form is fast superseding it. The same principle of level, but with long tube, has been found convenient for the surveyor in measuring through close buildings, Fig. 91.

Fig. 91.—Tubular water level with open vials.

Fig. 92.—Browne's standard water level.

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257.—Browne's Water Level[6] is found to be a convenient instrument for levelling in close towns. It consists of a pair of glass tubes of about 2 feet in length, placed in a casing tube for protection. The casing tube is divided into inches and parts, or the scale is a detached piece of painted wood, or any rod or rule. A cock at the bottom admits the water to flow to level in the pair of tubes, one of which is shown, Fig. 92. There is a handle at the top which unscrews to fill the level, and a small air cock. It is easily seen that the water finds its level, and the difference of reading of the two standards is the difference of level of the surfaces upon which they are placed. By closing the cocks the level is made portable. In this position it does not matter how high the centre of the pipe is placed—for instance, in crossing over a wall—as the water will still find its level when the cocks are released by syphoning the water from the one side or the other. It is a very convenient and exact level for laying drain pipes in open weather, and for making foundations for heavy machinery, etc., but of course it will not stand frost.

Platelayers' levels and mechanics' levels generally are deferred to consider with useful hand tools and apparatus employed by surveyors in the final chapter.