Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/396



The clinometer may be used to determine the position of any plane surface to which the plate can be applied, so as to admit of observation with the quadrant. The upper surface of a rock is that which presents itself most frequently, and the mode of observing it may be thus described.

As the surface of a rock afforded by a strata-seam seldom approaches to a perfect plane, a part of it must be chosen, which appears to give the mean, or average position of the stratification. if the surface be smooth, the quadrant may be fixed upon the plate, before the plate is set down upon the rock. If the surface be rough, the plate must first be set down, and adjusted by the eye, till its surface appears to coincide with the mean position of the stratification; the quadrant is then to be fixed on. In fixing the quadrant upon the plate, the clamp should be thrust upon the edge, before the button of the central axis is inserted. No regard need be paid to the situation of the zeros of the plate.

The radial bar must be set home upon the base; and while one hand presses the plate firm against the rock, the other must be applied to the head of the screw g, and bring the quadrant round till the level, properly adjusted by turning it upon its axis, is horizontal. Its axis will now be parallel to the line of stretch, that is, to a line determined by the intersection of the plane of the stratification with the plane of the horizon. The division on the plate, to which the index of the base points, is then to be noted, and the quadrant carried upwards to the same division in the next quarter of the plate; when it will have described an arch of 90°, and of course be now in a vertical plane, at right angles both to the plane of the stratification and to the plane of the horizon. After clamping the quadrant to the plate in this position by turning the screw g, the hand is shifted to the compass in order to