3·25 feet 0·78 × 100 = 416·6 feet.
The above illustrations are for readings taken approximately level. If there be much elevation or depression the angle must be read and the difference of hypo and base calculated and the stadia rod or staff must be inclined so that its face is at right angles to the line of sight from telescope. This can be done by the rod man inclining the staff or rod until the shortest reading is given if a staff be used, or the longest measurement is recorded by the gradienter screw head if a stadia rod be used. It is better in this case to have the staff fitted with a director (see art. 561), so that the person holding the staff may sight into the telescope of the instrument, thus ensuring the staff being exactly at right angles to the line of sight.
No constant should be added with either this, Bakewell's, or omnimeter measurements, as the angles are taken from the centre of the instrument. This gradienter screw has the same fault as mentioned for the two foregoing, viz., that all readings are taken by two movements of the instrument.
CHAPTER XIII.
INSTRUMENTS CONSTRUCTED ESPECIALLY FOR OFFERING FACILITY OF TAKING INCLINES—INCLINOMETER—THEODOLITE—GRADIOMETER—CLINOMETERS—ABNEY'S, TROUGHTON'S, DE LISLE'S, STANLEY'S, BARKER'S, BURNIER'S, WATKINS'—CLINOMETER SIGHTS—RULE CLINOMETERS—ROAD TRACER.
Certain instruments are constructed specially with the object of taking inclines, where this is the predominant work to be performed with them. They form an important branch of surveying instruments, and for their special kind of work present many time-saving capabilities.
593.—Lister's Inclinometer Theodolite.[35]—This instrument is the invention of Mr. James Lister, C.E. It was originally designed to set out upon the surface of land the widths of slopes or batters by pegs, as required in the execution of railway, canal, and other earth works. In general construction it resembles a theodolite as before described, [arts. 370] to 391, with the addition of an extra vertical axis to the telescope piercing the horizontal axis at right angles, Fig. 257. In this construction the telescope upon the horizontal axis can be set by the vertical supplementary axis to any inclination, so that if the vertical axis be set to the slope of a railway cutting, any number of points or pegs may be set out continuously with the same setting by direct observation through the telescope across any irregularity or inclination of the land surface. In this operation an immense amount of labour is saved over the ordinary system of pegging by calculation with the aid of a theodolite, where each peg requires a separate setting of the instrument. When the inclinometer theodolite is used for surveying purposes, the telescope is fixed by a spring catch which places it firmly true to the reading of the ordinary vertical arc.
