308 RETORT— 1870. 



Herr Bunker's paper gives a very full account of the different kinds of plug 

 employed at Sixrenberg. 



For stopidiig Ihe mouth of the advance-bore the plug had a tapering shape, 

 and was of hard wood, strengthened by two iron rings, one at each end, and 

 covered with a layer of tow 5 lines thick, outside of which was thick and 

 strong linen, nailed above and below to the wood through a leather strap. 

 It was lowered into its place by means of the iron rods used for boring ; and, 

 when in position was pressed home by a portion of the weight of the rods. 

 The plug carried the thermometer suspended from it. Its extraction was 

 commenced by means of a screw on the beam of the boring-machine, in order 

 to avoid a sudden jerk, which might have broken the thermometer. The force 

 which was found necessary for thus starting the plug, as well as the impres- 

 sion observed upon it when withdrawn, showed that it had fitted tight. To 

 insure a good fit, the top of the advance-bore had been brought to a suitable 

 shape, and its inequalities removed, by means of a revolving cutting-tool. 

 Herr Bunker remarks that this plan is adapted to a soft material like rock- 

 salt, but that in ordinary hard rock it would be better to make the bottom of 

 the main bore flat, and to close the advance-bore by an elastic disk pressed 

 over it. The method of observation by advance-bores can only be employed 

 during the sinking of the bore, a time when it is difficult to avoid error arising 

 from the heat generated in boring. The expense of making an advance- 

 bore at each depth at which an observation is required is also an objection to 

 its use. 



Another kind of plug devised by Herr Bunker, and largely used in the 

 observations, consisted of a bag of very stout india-rubber (9 millimetres 

 thick) tilled with water, and capable of being pressed between two wooden 

 disks, one above and the other below it, so as to make it bulge out in the 

 middle and fit tightly against the sides of the bore. On the suggestion of 

 bore-inspector Zobel, the pressure was applied and removed by means of 

 screwing. Two steel springs fastened to the upper disk, and appearing, in 

 Herr Bunker's diagram, very like the two halves of a circular hoop distorted 

 into an oval by pressing against its waUs, prevented the upper disk from 

 turning, but offered little resistance to its rising or falling. The lower disk, 

 on the contrary, was permitted to turn. Both disks were carried by the 

 iron boring-rods. Eotation of these in one direction screwed the disks 

 nearer together, and rotation in the other direction brought them further 

 apart. The india-rubber bag could thus be made to swell out and plug the 

 bore when it was at the desired depth, and could be reduced to its original 

 size for raising or lowering. In order to prevent the boring-rods from be- 

 coming unscrewed one from another, when rotated backwards, it was neces- 

 sary to fasten them together by clamps, a rather tedious operation in working 

 at great depths. 



In taking observations at other points than the bottom, two of these plugs 

 were employed, one above and the other below the thermometer. 



In some of the experiments, the apparatus was modified by using linen 

 bags filled with wet clay, instead of india-rubber bags filled with water; 

 and, instead of screwing, direct pressure was employed, the lower disk being 

 supported by rods extending to the bottom of the bore, while the upper disk 

 could be made to bear the whole or a portion of the weight of the rods above 

 it. Some successful observations were obtained with both kinds of bag ; but 

 the water-bags were preferred, as returning more easily to their original size 

 when the pressure was removed, and consequently being less liable to injury 

 in extraction. In some observations since taken in another place (Suden- 

 berg), Herr Bunker states (in the private letter above referred to) that 



J 



