40 FIELD GEOLOGY. 



2. The dip. 



3. The horizontal breadth of outcrop. 



Clinometer. It matters little by what kind of in- 

 strument the dip of strata is ascertained, so long as it 

 indicates with a fair approach to accuracy the amount 

 of inclination. All clinometers must possess a gradu- 

 ated arc, and either a pendant that hanging perpendicu- 

 larly, or a spirit-level lying horizontally, shall show the 

 number of degrees that its base deviates to either hand 

 from a horizontal plane. A simple form of clinometer 



Fig. 10. 



is shown in fig. 10 ; but as portability is a great deside- 

 ratum, and as the pocket compass, which should be the 

 geologist's constant companion, possesses a graduated 

 circle, the one instrument may well be made to answer 

 the two purposes. This is effected by the addition of 

 the parts shown by the dotted lines in fig. 1, and of a 

 small pendant swung from the point which carries also 

 the magnetic needle. When the two instruments are 

 thus combined, the circumference must be divided not 

 only into cardinal and intermediate points, but also into- 

 degrees numbered to the right and left of zero. This 

 point is just over the centre of the clinometer base, the 

 figures run up to 90 on either side, and then back to 



