A sliding lower plate is provided for accurately centring the instrument, the levelling screws are adjustable for wear, and the tribrach is fitted with quick-setting spherical joint.
This instrument will also be found of great utility in mining work, to mark out the general line of the outcrop when a lode has been discovered. This, by the ordinary method, entails a considerable amount of labour and careful work both in the field and office, and then only points at intervals are obtained, not a continuous line. With this instrument the line of sight may be made to revolve in any plane, so that if it be set up in line with the strike at the spot where the lode has been discovered and the gradient drum adjusted to the angle of dip, it is evident that the line of sight will be wholly in that plane, and a continuous line of outcrop may be pegged out on a flat or undulating country and can be produced to any length required by taking the instrument to a fresh station.
Fig. 268.—Stanley's gradioplane.
Larger image
The above illustration, Fig. 268, shows the gradioplane fitted with a horizontal circle to the telescope for subdividing the grades of the gradienter drum, and when thus fitted forms the most exact instrument for setting out or ascertaining gradients that has been devised.
604.—Abney's Clinometer.—This very popular little instrument, the invention of Captain Abney, Fig. 269, embraces the same form of sighted level with reflector as that shown in section, [Fig. 87], p. 142, but the level instead of being fixed in line with the tube is placed above it upon an axis which forms the centre of a divided arc. The axis with the bubble is turned to any angle by means of a light milled-edged wheel placed in front of the arc. It carries an index which reads on the arc the angular position of the level to the centre of the instrument by a vernier to 10′. There is also a scale placed upon the arc giving gradients from 1 in 1 to 1 in 10. As the bubble of the level in its course passes the centre over the axis its reflection is made to become coincident with the sight line through the tube only when it is quite level. Therefore whatever the inclination of the tube, the bubble may be brought level by turning the milled head until it appears centrally in the sight axis of the tube, and the angle at which this occurs can be clearly read afterwards upon the arc. The size of the instrument in its case is 5 by 2½ by 1½ inches; weight, 8 oz.
Fig. 269.—Abney's clinometer..
Fig. 270.—Troughton's clinometer.