Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/397

 raise the radial bar till the level, duly adjusted on its axis, marks the horizontal line. When this has been done, and the needle has settled, the degrees on the compass are to be read off between the zero farthest from the quadrant, and the point of the needle nearest to that zero. This gives the magnetical bearing of the plane of the dip, that is, of a plane at right angles to the line of stretch on that side on which the plane of the stratification sinks below the plane of the horizon. The instrument may then be lifted from the rock, and the angle of the dip read off upon the quadrantal arch, being the angle which the plane of the stratification makes with the plane of the horizon. The bearing of the line of stretch is deduced from the bearing of the plane of the dip, as being at right angles to it. A correction for the magnetical variation gives the true position.

The roof of a cavern or of a mine, and sometimes the overhanging of a rock, may present an under surface for observation; but the instrument can scarcely be employed without an assistant to hold the plate firmly pressed upwards.

For an observation of this kind, the level and the compass-box must be inverted upon their axis, so as to have their faces turned towards the plate. The level is to be brought to the horizontal line as in the former case; but the quadrant must then be carried downwards through an arch of 90° and clamped by the screw. The radial bar is also to be drawn dawn till the level becomes horizontal. The magnetical bearing is read of upon the under side of the graduated plate in the compass.

The origin of the instrument is as follows:—A few years ago Mr. Grifths, of Dublin, when making a mineralogical tour in the Highlands of Scotland, employed for measuring the dip of strata an instrument, which differed little from a common road