Lord Webb Seymour on a Clinometer, 389 



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 va- 

 riation gives the true position. 



The roof of a cavern or of a mine, and sometimes the over- 

 hanging 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 

 doisonwards through an arch of 90" and clamped by the screw. 

 The radial bar is also to be drawn down till the level becomes ho- 

 rizontal. The magnetical bearing is read off upon the under side 

 of the graduated plate in the compass. 



The origin of the instrument is as follows : — A few years ago 

 Mr. Griffiths, 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 



