708.—Edgeworth's Stadiometer.[52]—The general construction of this instrument is given in the inventor's specification of patent, from which the engraving, Fig. 325 is taken. The vernier plate of an ordinary theodolite is extended to a plate of about 10 inches in diameter. This is adjusted to level by means of parallel plate screws. The plate or plane table is divided on its edge ¼°. The part representing the limb of a theodolite is carried out from its axis by two arms only: upon these the standards RR of the telescope are mounted. These standards leave a striding space near the plate, into which any scale S of equal parts with a zero centre is introduced, which is intended to be used for the plotting, the striding space being so arranged that the fiducial edge of the scale shall pass exactly over the axis of the instrument. The standards unite in the same casting to form the horizontal axis bearing of the telescope. This axis permits the telescope to move in azimuth. The telescope carries a vertical arc divided to degrees, also a scale of centesimal differences of hypotenuse and base, with the ordinary clamp and tangent adjustment of a theodolite. It is also fitted with a level above it which is used in setting up the instrument. Stadia webs are placed in the diaphragm and are made adjustable to subtend upon the stadium a percentage of arc agreeing with the unit to which the land is measured. The inventor does not appear to have known the optical error of the system proposed for measuring distance, [art. 558]. Neither does this appear to have been recognised by others writing upon the instrument, who have generally followed the late J. F. Heather's description.[53]
Fig. 325.—Edgeworth's stadiometer.
Larger image
709.—To Use Edgeworth's Stadiometer.—After it is set up, a circular disc of paper of about an inch less diameter than the table is held down upon it by four spring clips. The telescope is directed consecutively from object to object, the positions of which it is desired to take. It is clamped by the screw below the plate during the observation. The stadium is placed against the object and the distance taken by the subtense of the webs in the diaphragm, which may be exact if a constant be added after proper adjustment, [art. 558]. If the stadium be above or below the horizontal plane it is inclined by means of a sight-hole through it, as originally proposed by Green, so that the subtense is equal under all conditions. The horizontal distance is taken by the difference of hypotenuse and base, as shown on the vertical arc, so that the record of a complete observation appears for calculation as—
stadium reading + constant - altitude correction.
This distance is at once set off from the centre of the instrument by the scale on a line drawn upon the disc of paper, and observations are written against the line. In making a number of observations from one station two or more discs of paper may be employed to save confusion of lines and interference of descriptions. These papers are separately used in plotting as protractors by pricking holes through the stations defined in the field from the centre of the disc which represents the station of observation.
710.—The Sandhurst Protractor, Fig. 326, is a military protractor adapted especially for topographical delineation, which is commonly used with the plane table. It is different from many instruments of its kind in having only useful matter upon it. It is made of boxwood, upon which the protractor is cut, and has also one scale of 6 inches to a mile in yards, at the lower edge, the tens of which are carried across to make parallels of 90° in the manner of an ordinary military protractor. Over the back of the protractor is a scale which gives a standard for shading slopes of land upon topographical maps, Fig. 327, from 2° to 35°, also lines for contour shades. A small plummet, the cord of which is passed through a hole in the centre, from which the degrees are protracted, is supplied with the instrument. When the protractor is held up, degrees downwards, the cord of the plummet will pass over the degrees and indicate the angle at which it is held. By looking over the edge the angle of inclination of the land may be taken directly, as with a clinometer, or by looking along the edge by a second person reading the plummet the angles of altitude may be taken more exactly.
Fig. 326.—Sandhurst Sketching clinometer protractor.