70 DEEP BOREHOLE SURVEYS AND PROBLEMS 



2. Slight rotation of the core on fracture in hard beds. 



3. Fragile cores. 



Thurmann's Stratameter. — The apparatus (Fig. 36) con- 

 sists of a shell with a compass device. Above and 

 below the shell are bronze rods; the one above is about 83 

 cm. long and one below about 73 cm. long, as shown. 

 The iron rods are equally distant from the needle, i.e., 

 1 m., but this distance is not sufficient to cut out the disturb- 

 ing influences in the vicinity. The device consists of a 

 stratameter base A, the interior joint cap B and an external 

 casing C. Cast on to A is a petroleum container D, in 

 the floor of which the small pressure compensation tube R 

 is screwed and in which a leather plug P moves like a piston. 

 Over this, held by a safety bar St, is the compass box 

 with the magnetic needle N. This is covered by a glass 

 plate which can be screwed off. Under the needle lies 

 the horizontal arm H of the arresting cone K on the base 

 of the box which is held by the spring-pressed nut Mi 

 and the spiral spring F. The rod of the cone extends up 

 through the joint nut and a leather disc S placed over the 

 rod prevents the penetration of borehole mud. A pressure 

 or blow on the rod forces it down, thus pushing aside 

 the arresting cone on to the erect arm of lever H. Its lower 

 arm raises the needle from its seat, pressing it against the 

 glass cover plate and holding it fast. 



Mud and rinsing water cannot enter because the internal 

 compass box and the space about the tube R and the cone K, 

 beside being spring compressed, is full of petroleum. The 

 external casing is now fitted and the apparatus let into 

 the hole. The procedure from here on is exactly as with 

 Heine's apparatus. 



It will be seen that Thurmann's apparatus is very much 

 like Heine's in form and manipulation. It has all of 

 Heine's advantages over Gothan's apparatus and it exhib- 

 its some small improvements on Heine's apparatus. 



A special advantage is that the interior of the apparatus 

 is provided with a protective filling of petroleum against 

 the entry of rinsing water. In percussive boring — assuming 



