458 On the Rate of Increase of Underground Temperatures. 



walls, prevented the upper disc from turning, but offered little 

 resistance to its rising or falling. The lower disc, on the con- 

 trary, was permitted to turn. Both discs were carried by the 

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

 the discs nearer together, and rotation in the other direction 

 brought them farther 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 rais- 

 ing or lowering. In order to prevent the boring-rods from 

 becoming unscrewed one from another, when rotated ba 

 it was necessary to fasten them together by clamps, a rather 

 tedious operation in working at great depths. 



In taking observations at othe: 

 of these plugs were ernploj^ed, o 

 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 disc being supported by rods extend- 

 ing to the bottom of the bore, while the upper disc 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 obser in another place (Sudenberg), 



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

 india-rubber bags, filled with water, and pressed, not by screw- 

 ing, but by the weight of the rods, were employed with much 

 satisfaction. 



All the methods of plugging employed by Herr Dunker in- 

 volved the use of the iron rods belonging to the bori _ 

 tus, and therefore would be inapplicable (except at great 

 expense) after the operation of boring is finished and the 

 apparatus removed. 



It seems desirable to contrive, if possible, some plug that can 

 be let down and raise. 1 by a wire. In the first report of your 

 Committee, it was suggested that two bags of sand, one above 

 and the other below the thermometer, should be a» 

 purpose. Bags of sand, however, would be liable to rub off 



les of the bore, and thus to becom- 

 in drawing up. Mr. Lebour has devised a plug wh 

 of small diameter during the process of lowering a 

 but can be rendered large and made to fit the bore, when at the 

 proper depth, bv letting down up. n ii a sliding ,veight suspen- 

 -. \\ 1 , . ,__ -♦, r . - i ; 



probably be found effectual. 



