Operation.—The instrument being set over a centre peg with the telescope at the slope angle, turn the telescope to any other peg and adjust the cross web to line therewith (on line c f). Take the section depth cd (14 feet), to which add de (7′ 6″) = 21′ 6″. This multiplied by ·707, the natural sine of the slope angle (45°) will give the distance from the axis of the instrument to the slope line, thus: 21·50 × ·707 = 15·20 feet, and the point on the surface at g where 15·20 can be read on the staff is the position of the slope peg. This is similar to that described in the last paragraph, but if the telescope be now changed to the angle of the slope on the other side of the line ch, the peg i is instantly found by the same reading (15·20).
601.—The Use of the Inclinometer in Mining.—A lode having been discovered, it is required to mark out on the ground the general line of the outcrop. Hitherto the method employed has been to find the strike and drift of the lode and to level and survey the surrounding country and plot on a contour plan. Lines parallel to the strike and spaced according to the trigonometrically calculated bases are ruled in. The points of intersection of the contour line with that of the parallel line to the strike of the same height above datum will be a point of outcrop. The bearings of these points are read, and their distances scaled from the plan, the theodolite is then taken to the field, and the points found are marked out on the ground. This entails a considerable amount of labour and careful work both in the field and office, and then only points at intervals are obtained and not a continuous line.
The inclinometer, having an adjustable axis at right angles to the horizontal, enables the line of sight to be made to revolve in any plane. If at the spot where the lode has been discovered the instrument be set up in line with the strike, and the movable axis adjusted to the angle of dip, it is evident the line of sight lies wholly in that plane, and a continuous line of outcrop may be pegged out on a flat or undulating country, which can be produced to any length required by taking the instrument to a fresh station. This feature of the instrument is equally, if not more, important than its use for rapidly pegging out railway slopes.
602.—The Gradiometer.—This instrument, while performing all the duties of a first-class level, is designed also for taking vertical inclines at small fixed angles for railways, drainage works, steep incline levelling, etc., etc., and also telemetrical readings up to great distances.
In general construction, as regards telescope, stand, etc., it resembles a level, and when set at zero is equal in every way to one of the best, with the additional advantage that it may be used for rapid work without the trouble of setting up by the levelling screws, as the telescope may be levelled at any sight by means of the gradienter drum milled head. The gradiometric arrangement is effected by the telescope being mounted in trunnions, one pair being adjusted vertically; the amount of elevation or depression is indicated by a drum carrying an open extended scale graduated to read rise or fall, from 1 in 12 to 1 in 1,200, which may be conveniently and distinctly read without the use of a vernier.
The additional parts do not increase the bulk of the case and add very little to the weight.
Fig. 266.—Stanley's gradiometer.
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By its use a great saving of work is effected. For instance, for a town drainage in which it is desired to work out an inclination, say the levels indicate a fall of 10 feet between the extreme points: if the line the drainage is intended to take be measured, however angular or zigzag it may be, and the length of that line be divided by the amount of fall, this will give the gradient; say the line of streets measures 5,000 feet, this, divided by 10 feet, gives a gradient of 1 foot in 500. Therefore if the drum be set to that proportion, all the pipes may be laid directly without further setting. The gradients for any railway may be instantly found by merely turning the drum until the telescope sights, up or down the incline to be measured, a reading on the staff equal to the height of the instrument, and taking the reading of the drum at the position of the indicator.